SHORT-CIRCUIT ISOLATOR
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a short-circuit isolator.
2. Description of the Related Art
As a conventional short-circuit isolator, there is known "short-circuit
detection means for detecting the short circuit of a line when an input line
voltage of the line is equal to or lower than a predetermined threshold line
voltage" (see, for example, Japanese Patent Application Laid-open No. Sho
63-19097 (pages 3 and 5, FIG. 1)).
In the technology described in Japanese Patent Application Laid-open
No. Sho 63-19097, when the voltage of the line is equal to or lower than the
predetermined threshold line voltage, the short circuit is detected and a
short-circuit section is disconnected. When the voltage of the line is higher
than the predetermined threshold line voltage, the recovery from short circuit is
detected and the disconnected section is reconnected.
However, the threshold line voltage for detecting the short circuit is
equal to the threshold line voltage for detecting the recovery from short circuit,
and hence it takes time to reconnect the section disconnected by the short-circuit
detection means after the actual recovery from short circuit.
SUMMARY OF THE INVENTION
The present invention has been made to solve the problem as described
above. An object of the present invention is to provide a short-circuit isolator
capable of earlier detection of recovery from short circuit of a disconnected line
and reconnecting the line.
According to the present invention, there is provided a short-circuit
isolator for detecting short circuit of a signal line, including: switch means for
switching a connection state between a primary signal line and a secondary
signal line; short-circuit determination means which is connected to a primary
wiring of the switch means through current limiting means and connected to a
secondary wiring of the switch means, for detecting short circuit when a voltage
of the secondary signal line is lower than a short-circuit detection threshold and
detecting recovery from the short circuit when the voltage of the secondary
signal line is higher than a short-circuit recovery threshold which is a voltage
value lower than the short-circuit detection threshold after the short circuit is
detected; and switch control means for turning off the switch means when the
short circuit is detected by the short-circuit determination means and turning on
the switch means when the recovery from the short circuit is detected by the
short-circuit determination means.
According to the present invention, there is also provided a short-circuit
isolator to be connected to a signal line which is applied with a voltage from
both sides, for detecting short circuit of the signal line, including: first switch
means and second switch means which are connected in series between a
primary signal line and a secondary signal line; first short-circuit determination
means which is connected, through current limiting means, to a connection side
wiring for connecting the first switch means to the second switch means, and
which is connected to a primary wiring for the first switch means which is
opposite from the connection side wiring, for detecting short circuit when a
voltage of the primary signal line is lower than a first short-circuit detection
threshold and detecting recovery from the short circuit when the voltage of the
primary signal line is higher than a first short-circuit recovery threshold which is
a voltage value lower than the first short-circuit detection threshold after the
short circuit is detected; first switch control means for turning off the first switch
means when the short circuit is detected by the first short-circuit determination
means and turning on the first switch means when the recovery from the short
circuit is detected by the first short-circuit determination means; second
short-circuit determination means which is connected to the connection side
wiring through current limiting means and connected to a wiring of the second
switch means which is opposite from the connection side wiring, for detecting
short circuit when a voltage of the secondary signal line is lower than a second
short-circuit detection threshold and detecting recovery from the short circuit
when the voltage of the secondary signal line is higher than a second
short-circuit recovery threshold which is a voltage value lower than the second
short-circuit detection threshold after the short circuit is detected; and second
switch control means for turning off the second switch means when the short
circuit is detected by the second short-circuit determination means and turning
on the second switch means when the recovery from the short circuit is detected
by the second short-circuit determination means.
The short-circuit isolator according to the present invention further
includes: third switch means for switching a connection state of a signal line
extended from the connection side wiring for connecting the first switch means
to the second switch means; third short-circuit determination means which is
connected to the connection side wiring of the third switch means through
current limiting means and connected to a wiring which is opposite from the
connection side wiring, for detecting short circuit when a voltage of the
extended signal line is lower than a third short-circuit detection threshold and
detecting recovery from the short circuit when the voltage of the extended signal
line is higher than a third short-circuit recovery threshold which is a voltage
value lower than the third short-circuit detection threshold after the short circuit
is detected; and third switch control means for turning off the third switch means
when the short circuit is detected by the third short-circuit determination means
and turning on the third switch means when the recovery from the short circuit is
detected by the third short-circuit determination means.
In the short-circuit isolator according to the present invention, the
short-circuit determination means includes a Zener diode which is connected to
a wiring on one end side of the switch means through the current limiting means
and connected to a wiring on another end side of the switch means and a
capacitor connected in parallel to the Zener diode.
In the short-circuit isolator according to the present invention, the switch
control means includes a switching element to be turned on in accordance with a
current flowing through one of the Zener diode and the capacitor.
According to the short-circuit isolator of the present invention, the
threshold voltage for detecting the recovery from short circuit is set to the
voltage lower than the threshold voltage for detecting the short circuit, and
hence the recovery from short circuit may be detected earlier. Therefore, for
example, when the short-circuit isolator according to the present invention is
applied to a signal system of a fire alarm system, a period from the recovery
from short-circuit of the signal line to the restart of operation of the signal
system may be shortened, and hence an operation stop time caused by the short
circuit may be shortened.
The short-circuit isolator according to the present invention may be
connected to a loop signal line applied with a voltage from both sides.
The short-circuit isolator according to the present invention may be
connected to a signal line extended from the loop signal line.
The short-circuit isolator according to the present invention may include
a simple circuit in which a Zener diode and a capacitor are provided in
combination, and thus may be manufactured at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is an overall configuration diagram illustrating a fire alarm system
including a short-circuit isolator (SCI) according to a first embodiment of the
present invention;
FIG. 2 is a block diagram illustrating the fire alarm system in the first
embodiment of the present invention;
FIG. 3 is a circuit diagram illustrating the SCI according to the first
embodiment of the present invention;
FIG. 4 is an overall configuration diagram illustrating a fire alarm system
including an SCI according to a second embodiment of the present invention;
FIG. 5 is a circuit diagram illustrating the SCI according to the second
embodiment of the present invention;
FIG. 6 is an overall configuration diagram illustrating a fire alarm system
including an SCI according to a third embodiment of the present invention; and
FIG 7 is a circuit diagram illustrating the SCI according to the third
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
In a first embodiment, description is given of an example of the case
where a short-circuit isolator (hereinafter, referred to as SCI) according to the
present invention is applied to a fire alarm system including a so-called R-type
fire alarm control panel, which transmits/receives a transmission signal (pulse
signal) to cause sounder equipment to generate an alarm when a fire is detected
by a fire detector or the like.
[Fire Alarm System]
(Overall Configuration)
A fire alarm system in the first embodiment is described.
FIG. 1 is an overall configuration diagram illustrating the fire alarm
system in the first embodiment. FIG 2 is a configuration block diagram
illustrating the fire alarm system in the first embodiment. A fire alarm control
panel FA is connected to various types of terminal equipment through a signal
lineSG
The signal line SG is divided into a plurality of branch paths. The
short-circuit isolators SCI1 to SCI3 (hereinafter may be referred to simply as
SCIs) are connected to the roots of the respective branch paths. FIG 1
illustrates three paths, that is, branch paths Rl, R2, and R3. However, the
number of branch paths is not particularly limited.
The branch path Rl of the signal line SG is connected to an analog
photoelectric detector SE11, an analog heat detector SE12, an addressable
manual call point SE13, a zone alarming control relay Cll, and a fire and smoke
control relay Dll. In the first embodiment, the equipment (except SCIs)
connected to the fire alarm control panel FA through the signal line SG may be
collectively referred to as terminal equipment.
The analog photoelectric detector SE11 is a kind of smoke detector and
transmits an analog value corresponding to detected smoke to the fire alarm
control panel FA.
The analog heat detector SE12 is a kind of heat detector and transmits an
analog value corresponding to a detected ambient temperature to the fire alarm
control panel FA.
The addressable manual call point SE13 is a so-called fire manual call
point, and includes a manually-operated push button for a person to discover a
fire. When the push button is turned on, a fire signal is transmitted to the fire
alarm control panel FA.
The zone alarming control relay Cll is connected to a zone bell Clll
serving as controlled equipment.
The zone bell Clll is a bell that sounds.
The fire and smoke control relay Dll is connected to a fire door Dill, a
smoke ejector D112, a shutter D113, and a hanging wall D114. In FIG. 1, the
above-mentioned fire and smoke control equipment as controlled equipment is
connected one by one, but a plurality of the fire and smoke control equipment
may be connected.
The branch paths R2 and R3 are connected to various types of terminal
equipment.
The terminal equipment connected to the signal line SG is
communicated with the fire alarm control panel FA through the signal line SG
and applied with power supply voltages through the signal line SG.
(Operation of Fire Alarm System)
An example of an operation of the fire alarm system is described.
When smoke is detected by the analog photoelectric detector SE11
connected as the terminal equipment or when heat is detected by the analog heat
detector SE12, detection information is transmitted as state information to the
fire alarm control panel FA through the signal line SG.
The fire alarm control panel FA collects the state information transmitted
from the analog photoelectric detector SE11 or the analog heat detector SE12.
When fire information (detection information which exceeds predetermined
threshold) is included in the collected state information, a fire alarm is generated.
To be specific, the fire alarm control panel FA transmits a control signal to the
zone alarming control relay Cll to sound the zone bell Clll, to thereby inform
people about fire. In addition, the fire alarm control panel FA transmits a
control signal to the fire and smoke control relay Dll to operate the fire door
Dill, the smoke ejector D112, the shutter D113, and the hanging wall D114, to
thereby prevent the spread of fire.
When a branch path of the signal line SG is short-circuited, the
short-circuited branch path is disconnected by the SCI connected to the root
thereof in order to prevent the other branch paths of the signal line SG from
being affected by short circuit. Note that the SCIs are described later.
(Communication Operation)
Communications between the fire alarm control panel FA and the
terminal equipment are described.
The fire alarm control panel FA communicates with the terminal
equipment through a pulse signal which is a combination of a high-level voltage
(V1H) and a low-level voltage (V1L).
(1) Normal Communication Method
The fire alarm control panel FA communicates with the respective
terminal equipment in order to collect the state information of the respective
terminal equipment (see FIG. 1) connected to the signal line SG.
The fire alarm control panel FA may use the following three types of
methods for the respective terminal equipment to collect the state information of
the terminal equipment and to control the terminal equipment.
(1-1) Point Polling
The fire alarm control panel FA transmits a state information request
command to each group of a plurality of terminal equipment in order to collect
the states of the plurality of connected terminal equipment. In contrast to this,
each of the terminal equipment returns state information to the fire alarm control
panel FA at a timing based on its own address in response to the state
information request command. The fire alarm control panel FA repeats
communication with each group to collect the state information of all the
terminal equipment.
(1-2) Selecting
The fire alarm control panel FA may specify an address corresponding to
desired terminal equipment and transmit a predetermined control command to
control the corresponding terminal equipment, or transmit, for example, the state
information request command to desired terminal equipment to collect the state
information from the terminal equipment. The terminal equipment having the
specified address returns a control result to the fire alarm control panel FA in
response to the control command or returns the requested state information.
(1-3) System Polling
The fire alarm control panel FA may transmit a common control
command to all the terminal equipment to control the respective terminal
equipment. Examples of the control command in system polling include a fire
recovery command (command for recovering, to normal monitoring state,
detector or relay which has output the fire signal) and a zone alarming stop
command (command for stopping zone bell which is sounding).
(2) Collection of Information of Abnormal State
When fire information is included in the state information collected from
terminal equipment, for example, the analog photoelectric detector SE11 by
point polling, the fire alarm control panel FA transmits a control signal by
selecting to a relay (zone alarming control relay or fire and smoke control relay)
corresponding to the terminal equipment which has transmitted the fire
information, based on a database stored in a storage section 13, to operate a zone
bell and a fire and smoke control device. In a case where the state information
request command is transmitted by point polling to the terminal equipment
registered in the database stored in the storage section 13 of the fire alarm
control panel FA, when terminal equipment does not reply in response to the
state information request command, a no-response failure alarm is generated by
a display and operation section 12.
[Block Diagram of Fire Alarm System]
A detailed configuration of the fire alarm system is described with
reference to FIG. 2.
For the purpose of description, FIG. 2 illustrates the fire alarm control
panel FA, the SCI, and terminal equipment. The SCI is described later with
reference to FIG. 3.
(Fire Alarm Control Panel)
The fire alarm control panel FA includes a control section 11, the display
and operation section 12, the storage section 13, and a transmitting and receiving
section 14.
The control section 11 controls the operation of the entire system
including the display and operation section 12 and the transmitting and receiving
section 14 based on control programs stored in advance in the storage section
13.
The display and operation section 12 includes: display means such as a
screen and a lamp, for displaying the fire information detected by the terminal
equipment or the states of the respective terminal equipment; and operation
means such as a touch panel and a button, for operating the fire alarm control
panel FA and the respective terminal equipment.
The transmitting and receiving section 14 is controlled by the control
section 11 to transmit signals to the respective terminal equipment and receive
signals transmitted from the respective terminal equipment.
The storage section 13 stores programs and various data to operate the
control section 11.
(Terminal Equipment)
FIG. 2 illustrates a detector, for example, a smoke detector or a heat
detector, as an example of the terminal equipment. The detector includes a
control section 21, a power supply section 22, a sensor 23, and a transmitting
and receiving section 24.
In the detector, the sensor 23 detects a temperature and a concentration
of smoke and the transmitting and receiving section 24 transmits detected
information to the fire alarm control panel FA.
(Configuration of SCI)
A configuration of the SCI according to the first embodiment is
described.
FIG. 3 is a circuit diagram illustrating the SCI according to the first
embodiment. In FIG. 3, signal lines of the fire alarm control panel FA are
connected to A-side (primary) terminals SA+ and SA- and signal lines of the
terminal equipment are connected to B-side (secondary) terminals SB+ and SB-.
The SCI includes a switch SWB for connecting the A-side terminal SA+
to the B-side terminal SB+, a constant voltage circuit 31, a resistor RB3 which is
current limiting means, a short-circuit monitor circuit B, and switch control
means B. The short-circuit monitor circuit B corresponds to short-circuit
determination means in the present invention.
The constant voltage circuit 31 includes one end side connected to a
primary wiring for connecting the switch SWB to the A-side terminal SA+ and
the other end side connected to the short-circuit monitor circuit B through the
resistor RB3 which is the current limiting means and a diode DB2. An anode
of the diode DB2 is provided on the constant voltage circuit 31 side.
The short-circuit monitor circuit B is connected to the primary wiring
connected to the switch SWB through the constant voltage circuit 31, and
connected to a secondary wiring for connecting the switch SWB to the B-side
terminal SB+.
In the short-circuit monitor circuit B, an anode of a diode DB1 is
provided on the constant voltage circuit 31 side. A cathode of the diode DB1 is
connected in series to cathodes of a capacitor CB and a Zener diode ZB which
are connected in parallel. Anodes of the capacitor CB and the Zener diode ZB
are connected in series to resistors RBI and RB2.
The switch control means B includes a transistor QB. A base of the
transistor QB is connected to an intermediate point between the resistors RBI
and RB2 of the short-circuit monitor circuit, and an emitter thereof is connected
to one end of the resistor RB2 which is opposite from the other end thereof
which is connected to the base. When a current flows into the short-circuit
monitor circuit B, the current flows through the resistor RB2 to generate a
voltage between both the ends of the resistor RB2, and hence the base and
emitter of the transistor QB are biased to turn on the transistor QB. Therefore,
the switch SWB is on/off-controlled. The switch control means B may be a
field effect transistor (FET) or an analog switch.
[Operation of SCI]
The operation of the SCI according to the first embodiment is described.
(Normal State)
The operation in the normal state (state in which short circuit does not
occur) is described. In an initial state in which the fire alarm control panel FA
is not powered on, the switch SWB is in the off-state.
When the fire alarm control panel FA is powered on, a signal voltage is
applied to the A-side of the signal line SG to start communication.
When the signal voltage is applied to the A-side of the signal line SG, the
signal voltage is applied between the A-side terminals SA+ and SA- of the SCI.
The signal voltage is also applied to the constant voltage circuit 31.
A constant voltage is applied from the constant voltage circuit 31 to the
terminal equipment connected to the B-side terminals SB+ and SB- and the
short-circuit monitor circuit B through the resistor RB3 and the diode DB2.
The constant voltage applied from the constant voltage circuit 31 is a voltage
lower than the low-level voltage of the signal applied from the fire alarm control
panel FA to the signal line SG (A-side).
When the constant voltage is applied to the short-circuit monitor circuit
B, a charge current bypasses the Zener diode ZB and flows into the capacitor CB
because the capacitor CB is not initially charged. Therefore, immediately
when the constant voltage is applied, a current flows through the resistors RBI
and RB2, and hence the transistor QB is turned on. When the transistor QB is
turned on, the switch SWB is turned on.
When the switch SWB is turned on, the A-side and B-side of the signal
line SG are connected to each other, and hence the fire alarm control panel FA is
connected to the terminal equipment. Therefore, the signal voltage is supplied
to the terminal equipment connected to the B-side to start communication.
In contrast to this, when the signal voltage is applied to the B-side, the
voltage is directly applied from the signal line SG to the short-circuit monitor
circuit B. When the applied voltage exceeds a Zener voltage of the Zener
diode ZB, the Zener diode ZB is turned on. Therefore, even after the capacitor
CB is completely charged and the charge current does not flow thereinto, the
current continues to flow into the resistors RBI and RB2, and hence the
transistor QB continues to be in the on-state. The B-side of the signal line SG
is applied with the signal voltage and thus becomes a voltage higher than the
constant voltage of the constant voltage circuit 31. Thus, a power supply
voltage is not supplied from the constant voltage circuit 31 to the short-circuit
monitor circuit B by the off operation of the diode DB2.
(Short-Circuit State)
An operation in a case where short circuit occurs on the B-side of the
signal line SG is described.
When the B-side is short-circuited, a voltage between the B-side
terminals SB+ and SB- reduces. When the voltage reduces to a predetermined
voltage (hereinafter, referred to as short-circuit detection voltage), the Zener
diode ZB of the short-circuit monitor circuit B is turned off, and hence a current
does not flow into the resistors RBI and RB2. Then, the transistor QB is
turned off and the switch SWB is turned off, and hence the A-side wiring is
disconnected from the B-side wiring. That is, the Zener voltage of the Zener
diode ZB is a first short-circuit detection threshold in the present invention.
The A-side wiring is disconnected from the B-side wiring, and hence the A-side
wiring is not affected by the short circuit of the B-side wiring. The capacitor
CB is discharged.
In a case where a complete short circuit does not occur, when the voltage
between the B-side terminals SB+ and SB- is lower than the Zener voltage of the
Zener diode ZB, a current does not flow into the resistors RBI and RB2. Then,
the transistor QB is turned off, and hence the switch SWB is turned off.
After the A-side wiring is disconnected from the B-side wiring, the
signal voltage is not applied to the B-side of the signal line SG, and hence the
B-side of the signal line SG becomes a voltage lower than the constant voltage
of the constant voltage circuit 31. In such a state, when the constant voltage
from the constant voltage circuit 31 is applied between the B-side terminals SB+
and SB- through the resistor RB3 and the diode DB2, the recovery from short
circuit may be monitored. In this case, the B-side terminals SB+ and SB- are
short-circuited, and hence a short-circuit current flows therethrough. However,
in order to reduce the short-circuit current to a value by which the A-side is not
affected, a resistance value of the resistor RB3 which is the current limiting
means is adjusted.
In order to monitor the recovery from short circuit, the constant voltage
circuit is provided to reduce the voltage applied between the terminals of the
signal line which are located on the short-circuit-monitored side (B-side
terminals SB+ and SB- in the first embodiment) to a value equal to or lower than
the low-level voltage. The constant voltage circuit is provided, and hence the
change in short-circuit current depending on changes in high-level voltage and
low-level voltage of the signal voltage is suppressed. However, when the
change in short-circuit current which is caused by the changes in voltages is
within a range in which the influence on a wiring which is not short-circuited is
allowed and a change in current supplied to the terminal equipment at the time
of recovery from short circuit is allowed, the constant voltage circuit is not
necessarily provided.
The current limiting means may include an element through which a
constant current may flow, for example, a constant current diode.
(Recovery from Short-Circuit State)
An operation in a case where short circuit occurs to disconnect the
A-side wiring and the B-side wiring from each other and then recovery from the
short circuit is made is described.
When the short circuit of the B-side is eliminated, the constant voltage is
applied from the constant voltage circuit 31 to the terminal equipment connected
to the B-side terminals SB+ and SB- and the short-circuit monitor circuit B
through the resistor RB3 and the diode DB2.
When the constant voltage is applied to the short-circuit monitor circuit
B, the charge current bypasses the Zener diode ZB and flows into the capacitor
CB because the capacitor CB is discharged by short circuit. Therefore,
immediately when the constant voltage is applied, a current flows into the
resistors RBI and RB2 through the capacitor CB, and hence the transistor QB is
turned on. When the transistor QB is turned on, the switch SWB is turned on.
That is, the voltage capable of turning on the transistor QB by the current
flowing through the capacitor CB is a first short-circuit recovery threshold for
detecting the recovery from short circuit in the present invention.
When the switch SWB is turned on, the A-side and B-side of the signal
line SG are connected to each other, and hence the fire alarm control panel FA is
connected to the terminal equipment. Therefore, the signal voltage is restarted
to be supplied to the terminal equipment connected to the B-side to restart
communication.
An operation of recovery from short circuit in a case where the capacitor
CB connected in parallel to the Zener diode ZB as in the short-circuit monitor
circuit B in the first embodiment is not provided is described. In this case, the
Zener voltage of the Zener diode ZB is a threshold voltage for detecting short
circuit and recovery from the short circuit.
In the case where the capacitor CB is not provided, even when the B-side
of the signal line SG is recovered from short circuit, the voltage applied to the
short-circuit monitor circuit B is low during the charging of capacitor
components such as a stray capacitor formed on the B-side of the signal line SG
and a backup capacitor provided in an inner portion of the terminal equipment,
and hence the Zener diode ZB is not turned on. Therefore, the transistor QB is
maintained in the off-state and thus the switch SWB is not turned on.
When the voltage applied to the short-circuit monitor circuit B increases
with the progress of charging of the capacitor components connected to the
B-side, the Zener diode ZB is turned on and thus the transistor QB is turned on.
Then, the switch SWB is turned on. When such a series of operation is
performed, the A-side and B-side of the signal line SG are electrically connected
to each other, and hence the fire alarm control panel FA is connected to the
terminal equipment.
As described above, in order to prevent the A-side from being affected
by short circuit on the B-side, the current flowing into the B-side is limited by
the resistor RB3. Even when the signal line SG is recovered from short circuit,
the switch SWB is not turned on during the charging of the capacitor component
provided in the inner portion of the terminal equipment. Therefore, it takes
some time to charge the capacitor component provided in the inner portion of
the terminal equipment located on the B-side because the current flowing into
the B-side is limited. That is, it takes time to reconnect the fire alarm control
panel FA to the terminal equipment.
When it takes some time to reconnect the fire alarm control panel FA to
the terminal equipment after the recovery from the short circuit, the following
problems may occur.
For example, when it takes time to apply the signal voltage to the
detector which is the terminal equipment, smoke or heat cannot be detected
before the terminal equipment is activated, and hence the detection of fire may
be delayed. Further, the short circuit may be detected by the short-circuit
monitor circuit B erroneously operated because of the influence of noise to
disconnect the A-side and B-side of the signal line SG from each other. In such
a case, when the recovery takes time, an interruption time of communication
between the fire alarm control panel FA and the terminal equipment lengthens.
Therefore, although the terminal equipment is actually connected, the fire alarm
control panel FA may determine that the terminal equipment is disconnected and
generate the no-response failure alarm.
When a period in which the voltage is not applied to the terminal
equipment extends, the terminal equipment is reset.
In order to activate a microcomputer (not shown) which is a part of the
control section of the terminal equipment located on the B-side after several tens
of milliseconds from the recovery from the short circuit, a rising current rapidly
increases. Unless the switch SWB is turned on and thus the current is supplied
to the terminal equipment before the rapid increase of the rising current, the
current cannot be sufficiently supplied to the terminal equipment because the
current is limited by the resistor RB3. Therefore, the microcomputer cannot be
normally activated and the terminal equipment may be erroneously operated.
In order to avoid the terminal equipment from being erroneously operated, it is
necessary to limit the number of connected terminal equipment to be able to
achieve normal activation with a limited current.
As described above, in the SCI according to the first embodiment, when
the secondary voltage becomes lower than the Zener voltage (short-circuit
detection voltage) of the Zener diode ZB, the short circuit is detected. In
addition, when the voltage of the signal line SG becomes the predetermined
voltage (short-circuit recovery voltage) lower than the short-circuit detection
voltage, the recovery from the short circuit is detected. Therefore, a period
from the time when the signal line SG is recovered from the short circuit to the
time when the recovery from the short-circuit is detected by the short-circuit
monitor circuit B may be shortened. Thus, a period from the time when the
signal line SG is actually recovered from the short circuit to the time when the
connection between the primary side and secondary side of the SCI is recovered
may be shortened.
Therefore, for example, in the fire alarm system as described in the first
embodiment, a period of disconnection between the fire alarm control panel FA
and the terminal equipment, which is caused by the short circuit, may be
shortened, and hence time except fire monitoring time may be reduced, and
hence fire may be more reliably detected. Unnecessary no-response failure
alarm may be suppressed from being generated. A larger number of terminal
equipment may be connected. The terminal equipment may be suppressed
from being erroneously operated.
Second Embodiment
In a second embodiment, a case where the SCI according to the present
invention is applied to a fire alarm system in which a fire alarm control panel FA
and terminal equipment are connected through a loop signal line is described as
an example.
FIG. 4 is an overall configuration diagram illustrating the fire alarm
system in the second embodiment. The fire alarm control panel FA is
connected to the terminal equipment through the loop signal line. The fire
alarm control panel FA may perform voltage application and signal transmission
from a terminal A and a terminal B. During a normal state, each of voltage
application and signal transmission is performed from one of the terminals (for
example, terminal A). During an abnormal state, each of the voltage
application and signal transmission is performed from the other terminal
(terminal B) simultaneously with the one of the terminals.
Short-circuit isolators SCI1 to SCI6 are connected to a path of a signal
line SG wired in loop. The terminal equipment is connected between the
respective SCIs.
(Configuration of SCI)
Next, a configuration of the SCI according to the second embodiment is
described.
FIG. 5 is a circuit diagram illustrating the SCI according to the second
embodiment. In FIG. 5, signal lines on the terminal-A side of the fire alarm
control panel FA are connected to A-side (primary) terminals SA+ and SA- and
signal lines on the terminal-B side of the fire alarm control panel FA are
connected to B-side (secondary) terminals SB+ and SB-.
As illustrated in FIG 5, the SCI according to the second embodiment
includes: the constant voltage circuit 31 used in common; and two circuit groups
which are provided on the A-side and the B-side and symmetric with respect to
the constant voltage circuit 31, the two circuit groups each having the same
configuration as the SCI including the short-circuit monitor circuit, the switch
control means, and the current limiting means as described in the first
embodiment described above.
To be more specific, switches SWA and SWB are connected in series
between the A-side and B-side of the signal line. A connection side wiring for
connecting the switches SWA and SWB in series is connected to the constant
voltage circuit 31. The switch SWA is connected to the A-side terminal SA+
through a primary wiring. The primary wiring is connected to a short-circuit
monitor circuit A. The short-circuit monitor circuit A is connected to the
connection side wiring through current limiting means RA3. Switch control
means A for controlling an on/off-state of the switch SWA is connected to the
short-circuit monitor circuit A. The switch SWB is connected to the B-side
terminal SB+ through a secondary wiring. The secondary wiring is connected
to a short-circuit monitor circuit B. Switch control means B for controlling an
on/off-state of the switch SWB is connected to the short-circuit monitor circuit
B.
The short-circuit monitor circuit A corresponds to first short-circuit
determination means in the present invention. The switch control means A
corresponds to first switch control means in the present invention. The
short-circuit monitor circuit B corresponds to second short-circuit determination
means in the present invention. The switch control means B corresponds to
second switch control means in the present invention.
The short-circuit monitor circuits A and B in the second embodiment
have the same circuit configuration as the short-circuit monitor circuit B
described above in the first embodiment.
[Operation of SCI]
Next, the operation of the SCI according to the second embodiment is
described. Even when a power supply voltage is supplied from any one of the
A-side and the B-side, the SCI according to the second embodiment may be
operated. Even when any one of A-side and the B-side is short-circuited, the
short-circuited side may be disconnected.
(Normal State)
First, the operation in the normal state (state in which short circuit has
not occurred) is described. In an initial state, the switches SWA and SWB are
in the off-state. In this case, assume that a signal voltage is applied from the
terminal-A side of the fire alarm control panel FA.
When the fire alarm control panel FA is powered on, the signal voltage is
applied to the A-side of the signal line SG to start communication.
When the signal voltage is applied to the A-side of the signal line SG, the
signal voltage is applied between the A-side terminals SA+ and SA- of the SCI.
When wirings connected to the A-side terminals SA+ and SA- of the SCI are
normal, a charge current bypasses a Zener diode ZA and flows into a capacitor
CA because the capacitor CA is not initially charged. Then, the current flows
through resistors RA1 and RA2, and hence the transistor QA is turned on.
When the transistor QA is turned on, a switch SWA is turned on, and hence the
connection side wiring for the switches SWA and SWB is connected to the
A-side terminal SA+ of the SCI. After that, the signal voltage is applied to the
constant voltage circuit 31.
In the short-circuit monitor circuit A, the voltage is continuously applied
from the signal line SG. When the applied voltage exceeds a Zener voltage of
the Zener diode ZA, the Zener diode ZA is turned on. Therefore, even after the
capacitor CA is completely charged and the charge current does not flow
thereinto, the current continues to flow into the resistors RA1 and RA2, and
hence the transistor QA continues to be in the on-state.
When the signal voltage is applied to the constant voltage circuit 31, a
constant voltage is applied to the short-circuit monitor circuit B through the
resistor RB3 and the diode DB2. The constant voltage applied from the
constant voltage circuit 31 is a voltage lower than the low-level voltage of the
signal applied from the fire alarm control panel FA to the signal line SG
(A-side).
When the constant voltage is applied to the short-circuit monitor circuit
B, a charge current bypasses the Zener diode ZB and flows into the capacitor CB
because the capacitor CB is not initially charged. Therefore, immediately
when the constant voltage is applied, a current flows through the resistors RBI
and RB2, and hence the transistor QB is turned on. When the transistor QB is
turned on, the switch SWB is turned on.
When the switch SWB is turned on, the A-side and B-side of the signal
line SG are connected to each other, and hence the fire alarm control panel FA is
connected to the terminal equipment. Therefore, the signal voltage is also
supplied to the terminal equipment connected to the B-side to start
communication.
On the other hand, when the signal voltage is applied to the B-side, the
voltage is directly applied from the signal line SG to the short-circuit monitor
circuit B. When the applied voltage exceeds a Zener voltage of the Zener
diode ZB, the Zener diode ZB is turned on. Therefore, even after the capacitor
CB is completely charged and the charge current does not flow thereinto, the
current continues to flow into the resistors RBI and RB2, and hence the
transistor QB continues to be in the on-state. The B-side of the signal line SG
is applied with the signal voltage and thus becomes a voltage higher than the
constant voltage of the constant voltage circuit 31. Thus, a power supply
voltage is not supplied from the constant voltage circuit 31 to the short-circuit
monitor circuit B by the off operation of the diode DB2.
Through such a series of operation, the A-side and B-side of the signal
line SG in the short-circuit isolator SCI1 are connected to each other. Next, the
signal voltage is applied to the short-circuit isolator SCI2 and the A-side and
B-side of the signal line SG in the short-circuit isolator SCI2 are connected to
each other in the same manner. Therefore, the A-side and B-side of the signal
line SG in each of the short-circuit isolators SCI1, SCI2, SCI3, ... are connected
to each other, and hence the signal voltage is input to all the terminal equipment
and the terminal B of the fire alarm control panel FA.
(Short-Circuit State)
An operation in a case where short circuit occurs on the B-side of the
signal line SG is described.
When the B-side is short-circuited, a voltage between the B-side
terminals SB+ and SB- reduces. When the voltage reduces to a predetermined
voltage (hereinafter referred to as short-circuit detection voltage), the Zener
diode ZB of the short-circuit monitor circuit B is turned off, and hence a current
does not flow into the resistors RBI and RB2. Then, the transistor QB is
turned off and the switch SWB is turned off, and hence the A-side wiring is
disconnected from the B-side wiring. That is, the Zener voltage of the Zener
diode ZB is a second short-circuit detection threshold in the present invention.
The A-side wiring is disconnected from the B-side wiring, and hence the A-side
wiring is not affected by the short circuit of the B-side wiring. The capacitor
CB is discharged. The switch SWA located on the A-side is maintained in the
on-state.
The fire alarm system in the second embodiment has the loop signal line.
Therefore, when short circuit occurs and thus the signal voltage is not input to
the terminal B of the fire alarm control panel FA, the fire alarm control panel FA
starts voltage application and signal transmission from the B-side terminal.
Then, an SCI which is located closest to a short-circuited portion and on
the B-side terminal of the fire alarm control panel FA detects the short circuit in
the same manner. That is, the Zener voltage of the Zener diode ZA of FIG. 5 is
the first short-circuit detection threshold in the present invention to detect the
occurrence of short circuit.
For example, when short circuit occurs between the short-circuit
isolators SCI4 and SCI5 illustrated in FIG. 4, the short-circuit isolators SCI4 and
SCI5 detect the short circuit and performs the wiring disconnection operation in
the same manner. The signal voltage application and signal transmission to the
short-circuit isolators SCI1 to SCI4 disconnected by the short circuit are
performed from the A-side terminal of the fire alarm control panel FA. The
signal voltage application and signal transmission to the short-circuit isolators
SCI6 and SCI5 are performed from the B-side terminal of the fire alarm control
panel FA.
In FIG. 5, after the A-side wiring is disconnected from the B-side wiring,
the signal voltage is not applied to the B-side of the signal line SG, and hence
the B-side of the signal line SG becomes a voltage lower than the constant
voltage of the constant voltage circuit 31. Accordingly, when the constant
voltage from the constant voltage circuit 31 is applied between the B-side
terminals SB+ and SB- through the resistor RB3 and the diode DB2, the
recovery from short circuit may be monitored. In this case, the B-side
terminals SB+ and SB- are short-circuited, and hence a short-circuit current
flows therethrough. However, in order to reduce the short-circuit current to a
value by which the A-side is not affected, a resistance value of the resistor RB3
which is the current limiting means is adjusted.
In order to monitor the recovery from short circuit, the constant voltage
circuit is provided to reduce the voltage applied between the terminals of the
signal line which are located on the short-circuit-monitored side (A-side and
B-side terminals SA+ and SA- and SB+ and SB- in the second embodiment) to a
value equal to or lower than the low-level voltage. The constant voltage circuit
is provided, and hence the change in short-circuit current depending on changes
in high-level voltage and low-level voltage of the signal voltage is suppressed.
However, when the change in short-circuit current which is caused by the
changes in voltages is within a range in which the influence on a wiring which is
not short-circuited is allowed and a change in current supplied to the terminal
equipment at the time of recovery from short circuit is allowed, the constant
voltage circuit is not necessarily provided.
The current limiting means may include an element through which a
constant current may flow, for example, a constant current diode.
(Recovery from Short-Circuit State)
An operation in a case where short circuit occurs to disconnect the
A-side wiring and the B-side wiring from each other and then recovery from the
short circuit is made is described.
When the short circuit of the B-side is eliminated, the constant voltage is
applied from the constant voltage circuit 31 to the terminal equipment connected
to the B-side terminals SB+ and SB- and the short-circuit monitor circuit B
through the resistor RB3 and the diode DB2.
When the constant voltage is applied to the short-circuit monitor circuit
B, the charge current bypasses the Zener diode ZB and flows into the capacitor
CB because the capacitor CB is discharged by short circuit. Therefore,
immediately when the constant voltage is applied, a current flows into the
resistors RBI and RB2, and hence the transistor QB is turned on. When the
transistor QB is turned on, the switch SWB is turned on. That is, the voltage
capable of turning on the transistor QB by the current flowing through the
capacitor CB is a second short-circuit recovery threshold for detecting recovery
from the short circuit in the present invention. Similarly, the voltage capable of
turning on the transistor QA by the current flowing through the capacitor CA is
the first short-circuit recovery threshold for detecting recovery from the short
circuit in the present invention.
When the switch SWB is turned on, the A-side and B-side of the signal
line SG are connected to each other, and hence the fire alarm control panel FA is
connected to the terminal equipment. Therefore, the signal voltage is restarted
to be supplied to the terminal equipment connected to the B-side to restart
communication.
The case where the B-side is short-circuited is described above as an
example. Even when the A-side is short-circuited, the disconnection of the
signal line SG after the detection of short circuit and the reconnection of the
signal line SG after recovery from the short circuit may be performed in the
same manner.
As described above, the SCIs according to the second embodiment are
connected to the loop signal line. Even when short circuit occurs in any of the
primary wiring and the secondary wiring, the short circuit may be detected.
When the voltage of the signal line SG becomes the predetermined short-circuit
recovery voltage lower than the short-circuit detection voltage, recovery from
the short circuit is detected, and hence the same effect as in the first embodiment
may be obtained.
The second embodiment describes the fire alarm system in which the
terminal equipment is connected to the loop signal line. During the normal
state, each of the voltage application and signal transmission is performed from
one of the terminals (for example, terminal A). During the abnormal state,
each of the voltage application and signal transmission is performed from the
other terminal (terminal B). The present invention may also be applied to a
configuration in which each of the voltage application and signal transmission is
performed always from both side terminals.
Third Embodiment
The first embodiment describes the example of the directional SCI
capable of detecting the short circuit occurring in the one-side (secondary)
wiring of the SCI and recovery from the short circuit. The second embodiment
describes the example of the SCI capable of detecting the short circuit occurring
in both side wirings on the loop signal line which may be applied with voltages
from both sides and the recovery from the short circuit. In a third embodiment,
an SCI capable of detecting short circuit occurring in a wiring extended from a
loop signal line which may be applied with voltages from both sides and
recovery from the short circuit is described. The number of extended wirings
is not limited to one and a plurality of extended wirings may be provided. In
the third embodiment, an SCI which has four terminals and may detect short
circuit in four directions is described as an example.
FIG. 6 is an overall configuration diagram illustrating a fire alarm system
in the third embodiment. The fire alarm control panel FA is connected to the
terminal equipment through the loop signal line. The short-circuit isolator
SCI2 is connected to four signal lines SG. Four connection portions of the
short-circuit isolator SCI2 are denoted by A, B, C, and D.
The operation of the SCI2 according to the third embodiment is
described.
FIG. 7 is a circuit diagram illustrating the short-circuit isolator SCI2
according to the third embodiment. The short-circuit isolator SCI2 illustrated
in FIG. 7 includes the constant voltage circuit 31 used in common, and four
circuit groups which are provided on the A-side, the B-side, the C-side, and the
D-side with respect to the constant voltage circuit 31 and each have the same
configuration as the SCI described above in the first embodiment. The
short-circuit isolator SCI2 according to the third embodiment is a circuit which
operates even when a power supply voltage is supplied from any one of the
SA-side and the SB-side, and disconnects a short-circuit side even when any one
of the SA-side, the SB-side, the SC-side, and the SD-side is short-circuited.
As illustrated in FIG. 7, the short-circuit isolator SCI2 according to the
third embodiment includes the constant voltage circuit 31 used in common, and
the plurality of circuit groups each having the same configuration as the SCI
including the switch, the short-circuit monitor circuit, the current limiting means,
and the switch control means as described above in the second embodiment.
To be more specific, the upper circuit and the lower circuit which are
illustrated in FIG. 7 each have the same configuration as the circuit described
above in the second embodiment. The upper circuit and the lower circuit are
connected to each other through a wiring for connecting the connection side
wiring for the switches SWA and SWB to a connection side wiring for switches
SWC and SWD. That is, short-circuit monitor circuits A to D are connected to
be supplied with the constant voltage from the constant voltage circuit 31.
Next, the operation of the SCI according to the third embodiment is
described.
(Normal State)
The operation in the normal state (state in which short circuit has not
occurred) is described. In an initial state, the switches SWA, SWB, SWC, and
SWD are in the off-state. In this case, assume that a signal voltage is applied
from the terminal-A side of the fire alarm control panel FA.
When the fire alarm control panel FA is powered on, a signal voltage is
applied to the A-side of the signal line SG to start communication.
When the signal voltage is applied to the A-side of the signal line SG, the
signal voltage is applied between the A-side terminals SA+ and SA- of the SCI.
When wirings connected to the A-side terminals SA+ and SA- of the SCI are
normal, the Zener diode ZA in the short-circuit monitor circuit A is turned on,
and hence a current flows through the resistors RA1 and RA2 and the transistor
QA is turned on. When the transistor QA is turned on, the switch SWA is
turned on, and hence the connection side wiring for the switches SWA and SWB
is connected to the connection side wiring for the switches SWC and SWD.
After that, the signal voltage is applied to the constant voltage circuit 31.
After the signal voltage is applied to the constant voltage circuit 31, the
same operation as in the first embodiment is performed, and hence the
transistors QB, QC, and QD are turned on and then the switches SWB, SWC,
and SWD are turned on. Therefore, the signal voltage is applied to the terminal
equipment connected to the signal lines SG located on the B-side, the C-side,
and the D-side and communication between the fire alarm control panel FA and
the respective terminal equipment starts.
(Short-Circuit State)
An operation in a case where short circuit occurs on the C-side of the
signal line SG is described.
When the C-side is short-circuited, a voltage between the C-side
terminals SC+ and SC- reduces. When the voltage reduces to a predetermined
voltage, the Zener diode ZC of the short-circuit monitor circuit C is turned off,
and hence a current does not flow into the resistors RC1 and RC2. Then, the
transistor QC is turned off and the switch SWC is turned off, and hence the
C-side wiring is disconnected from all of the A-side wiring, the B-side wiring,
and the D-side wiring. The C-side wiring is disconnected from all the wirings,
and hence the other wirings are not affected by the short circuit of the C-side
wiring. A capacitor CC is discharged. The switches SWA, SWB, and SWD
continue to be in the on-state.
After the A-side wiring is disconnected from the C-side wiring, the
signal voltage is not applied to the C-side of the signal line SG, and hence the
C-side of the signal line SG becomes a voltage lower than the constant voltage
of the constant voltage circuit 31. Accordingly, when the constant voltage
from the constant voltage circuit 31 is applied between the C-side terminals SC+
and SC- through the resistor RC3 and the diode DC2, the recovery from short
circuit may be monitored. In this case, the C-side terminals SC+ and SC- are
short-circuited, and hence a short-circuit current flows therethrough. However,
in order to reduce the short-circuit current to a value by which the A-side is not
affected, a resistance value of the resistor RC3 is adjusted.
In order to monitor the recovery from short circuit, the constant voltage
circuit is provided to reduce the voltage applied between the terminals of the
signal line which are located on the short-circuit-monitored side (A-side, B-side,
C-side, and D-side terminals SA+ and SA-, SB+ and SB-, SC+ and SC-, and
SD+ and SD- in the third embodiment) to a value equal to or lower than the
low-level voltage. The constant voltage circuit is provided, and hence the
change in short-circuit current depending on changes in high-level voltage and
low-level voltage of the signal voltage is suppressed. However, when the
change in short-circuit current which is caused by the changes in voltages is
within a range in which the influence on a wiring which is not short-circuited is
allowed and a change in current supplied to the terminal equipment at the time
of recovery from short circuit is allowed, the constant voltage circuit is not
necessarily provided.
The current limiting means may include an element through which a
constant current may flow, for example, a constant current diode.
(Recovery from Short-Circuit State)
An operation in a case where short circuit occurs to disconnect the
A-side wiring and the C-side wiring from each other and then recovery from the
short circuit is made is described.
When the short circuit of the C-side is eliminated, the constant voltage is
applied from the constant voltage circuit 31 to the terminal equipment connected
to the C-side terminals SC+ and SC- and the short-circuit monitor circuit C
through the resistor RC3 and the diode DC2.
When the constant voltage is applied to the short-circuit monitor circuit
C, the charge current bypasses the Zener diode ZC by the capacitor CC because
the capacitor CC is discharged by short circuit. Therefore, immediately when
the constant voltage is applied, a current flows into the resistors RC1 and RC2,
and hence the transistor QC is turned on. When the transistor QC is turned on,
the switch SWC is turned on. That is, the recovery from short circuit may be
detected based on a voltage lower than the Zener voltage of the Zener diode ZC
which is a threshold voltage for detecting short circuit.
When the switch SWC is turned on, the A-side and C-side of the signal
line SG are connected to each other, and hence the fire alarm control panel FA is
connected to the terminal equipment. Therefore, the signal voltage is restarted
to be supplied to the terminal equipment connected to the C-side to restart
communication.
The case where the C-side is short-circuited is described above as an
example. Even when the A-side, the B-side, and the C-side are short-circuited,
the disconnection of the signal line SG after the detection of short circuit and the
reconnection of the signal line SG after recovery from the short circuit may be
performed in the same manner.
As described above, the SCIs according to the third embodiment are
connected to the loop signal line and the signal lines extended from the loop
signal line. Even when short circuit occurs in any of the paths of the connected
signal lines, the short circuit may be detected. When the voltage of the signal
line SG becomes the predetermined short-circuit recovery voltage lower than the
short-circuit detection voltage, recovery from the short circuit is detected, and
hence the same effect as in the first embodiment may be obtained.
In each of the first to third embodiments described above, the so-called
R-type fire alarm system for detecting a fire in accordance with signals is
described as an example. The SCI according to the present invention may be
also applied to another system including a power supply line in addition to the
fire alarm system.
WHAT IS CLAIMED IS:
1. A short-circuit isolator for detecting short circuit of a signal line,
comprising:
switch means for switching a connection state between a primary signal
line and a secondary signal line;
short-circuit determination means which is connected to a primary
wiring of the switch means through current limiting means and connected to a
secondary wiring of the switch means, for detecting short circuit when a voltage
of the secondary signal line is lower than a short-circuit detection threshold and
detecting recovery from the short circuit when the voltage of the secondary
signal line is higher than a short-circuit recovery threshold which is a voltage
value lower than the short-circuit detection threshold after the short circuit is
detected; and
switch control means for turning off the switch means when the short
circuit is detected by the short-circuit determination means and turning on the
switch means when the recovery from the short circuit is detected by the
short-circuit determination means.
2. A short-circuit isolator to be connected to a signal line which is
applied with a voltage from both sides, for detecting short circuit of the signal
line, comprising:
first switch means and second switch means which are connected in
series between a primary signal line and a secondary signal line;
first short-circuit determination means which is connected, through
current limiting means, to a connection side wiring for connecting the first
switch means to the second switch means, and which is connected to a primary
wiring for the first switch means which is opposite from the connection side
wiring, for detecting short circuit when a voltage of the primary signal line is
lower than a first short-circuit detection threshold and detecting recovery from
the short circuit when the voltage of the primary signal line is higher than a first
short-circuit recovery threshold which is a voltage value lower than the first
short-circuit detection threshold after the short circuit is detected;
first switch control means for turning off the first switch means when the
short circuit is detected by the first short-circuit determination means and
turning on the first switch means when the recovery from the short circuit is
detected by the first short-circuit determination means;
second short-circuit determination means which is connected to the
connection side wiring through current limiting means and connected to a wiring
of the second switch means which is opposite from the connection side wiring,
for detecting short circuit when a voltage of the secondary signal line is lower
than a second short-circuit detection threshold and detecting recovery from the
short circuit when the voltage of the secondary signal line is higher than a
second short-circuit recovery threshold which is a voltage value lower than the
second short-circuit detection threshold after the short circuit is detected; and
second switch control means for turning off the second switch means
when the short circuit is detected by the second short-circuit determination
means and turning on the second switch means when the recovery from the short
circuit is detected by the second short-circuit determination means.
3. A short-circuit isolator according to claim 2, further comprising:
third switch means for switching a connection state of a signal line
extended from the connection side wiring for connecting the first switch means
to the second switch means;
third short-circuit determination means which is connected to the
connection side wiring of the third switch means through current limiting means
and connected to a wiring which is opposite from the connection side wiring, for
detecting short circuit when a voltage of the extended signal line is lower than a
third short-circuit detection threshold and detecting recovery from the short
circuit when the voltage of the extended signal line is higher than a third
short-circuit recovery threshold which is a voltage value lower than the third
short-circuit detection threshold after the short circuit is detected; and
third switch control means for turning off the third switch means when
the short circuit is detected by the third short-circuit determination means and
turning on the third switch means when the recovery from the short circuit is
detected by the third short-circuit determination means.
4. A short-circuit isolator according to any one of claims 1 to 3, wherein
the short-circuit determination means comprises:
a Zener diode which is connected to a wiring on one end side of the
switch means through the current limiting means and connected to a wiring on
another end side of the switch means; and
a capacitor connected in parallel to the Zener diode.
5. A short-circuit isolator according to claim 4, wherein the switch
control means comprises a switching element to be turned on in accordance with
a current flowing through one of the Zener diode and the capacitor.

Provided is a short-circuit isolator capable of earlier detection of
recovery from short circuit of a disconnected line and reconnecting the line.
The short-circuit isolator includes: a switch (SWB) for switching a connection
state between a primary signal line and a secondary signal line; a short-circuit
monitor circuit (B) which is connected to a primary wiring of the switch (SWB)
through a resistor (RB3) as a current limiting section and connected to a
secondary wiring of the switch (SWB), for detecting short circuit when a voltage
of the secondary signal line is lower than a short-circuit detection threshold and
detecting recovery from the short circuit when the voltage of the secondary
signal line is higher than a short-circuit recovery threshold which is a voltage
value lower than the short-circuit detection threshold after the short circuit is
detected; and a switch control section (B) for turning off the switch (SWB) when
the short circuit is detected by the short-circuit monitor circuit (B) and turning
on the switch (SWB) when the recovery from the short circuit is detected by the
short-circuit monitor circuit (B).