Residual Current Devices
The present invention relates to residual current devices (RCDs). More
specifically, it relates to RCDs that have a test facility which, when actuated, causes
the device to trip.
RCDs are installed for protection against certain potentially dangerous
situations arising in electrical supply installations. As shown in Figure 1, an electrical
supply installation 10 has a number of conductors 11 (typically neutral and live
conductors for single phase A.C. supplies and three live conductors or three live and
one neutral conductor for three phase A.C. supplies). The conductors 11 connect to a
load circuit 12 (e.g. a domestic ring main to which appliances are connected). A
known RCD 13 operates by disconnecting the supply from the load circuit 12 when an
imbalance is detected in the current flowing in the conductors 11. This imbalance is
due to current flowing to earth indicating, for example, poor insulation or
electrocution of a person.
The RCD 13 has a current transformer 4 consisting of a toroidal magnetic core
surrounding the conductors 11. A sensor coil (not shown) is wound around the core
so that any imbalance in the current flowing in the conductors 11 causes a sensor
signal current 5 to be induced in the sensor coil, which current is proportional to the
current imbalance. An electronic signal processing circuit 6 analyses the sensor signal
current 5 to determine if the current imbalance is at or above a pre-settrip threshold
indicative of a potentially dangerous condition in the supply circuit. The device then
trips the circuit by providing power to an actuator 17 to actuate a switch 18 to isolate
the supply from the load circuit 12.
RCD devices are required to be fitted with a test button. Pressing the button
causes the device to trip, which allows a person to test satisfactory operation of the
device. Activation of the test button closes a contact causing a test circuit to introduce
a signal to simulate a residual current so that the whole signal path from the sensor to
the switch is included in the test. This may be achieved by the circuit shown in Figure
2. Some of the current in one of the conductors 21a of live and neutral supply
conductors 21a, 21b flows via a resistor 22 so as to bypass the current transformer 4
when a test button is pressed to close a contact 24. There are many disadvantages
with this approach. Firstly, connection of the test circuit to the mains conductors 21a,
21b is required, which can be mechanically awkward within RCD devices. Secondly,
the apparent residual current produced is voltage dependent and also dependent on the
tolerance and stability of the resistor 22. In practice, currents much greater than the
trip threshold are induced so as to ensure tripping (typicallytwo and a half times, and
in some cases as much as five times, the rated trip value). This tests that the device
will operate, but not that it will necessarily operate at the rated trip value. Thirdly, no
account is taken of any standing residual current already in the circuit. In the test, the
device simply adds the test residual current to any standing residual current already
present. Again this means that the test is not carried out at the rated trip value.
The following further problems may also arise. If the device fails to trip for
any reason when the button is pressed, and the button is held down, the resistor 22 can
quickly become very hot and burn. The device may be subjected to voltage variations
in the supply. As well as affecting the accuracy of the test, high voltage pulses that
may occur between the live and neutral conductors 21a, 21b can give rise to arcing at
the contact 24. RCDs are made with different trip threshold ratings and so the resistor
22 must be changed to suit the threshold, which is inconvenient for production.
Another known method of implementing the test function is shown in Figure
3. A magnetic field is introduced into a core 33 of the current transformer 4. A
second winding 31 is provided on the transformer core 33. The winding is placed in
series with a resistor 35 in a test circuit between the live conductor 21a and neutral
conductor 21b. When the test button 23 is pressed a contact 34 closes the circuit and .
a test signal current flows through the second winding 31. This will induce a current
in the sense coil 32. Typically the test signal current is much smaller than the sensor
signal current required to trip the device due to current gain in the transformer 4. A
100 turn winding means only 1/100* of the trip threshold current is required to
produce an apparent residual current sufficient to cause a trip. This method reduces
the problem of resistor heating, but does not overcome most of the disadvantages of
the previous method, such as supply voltage connection, inaccuracy due to standing
residual current, high voltage contact rating and resistor tolerance and stability.
Another problem associated with current transformers is that of remanence.
This is an effect where the magnetic material forming the core of the transformer
becomes magnetized. This effectively lowers its permeability and prevents it from
conveying further magnetic flux. The coupling effect of the transformer is then
effectively lost or reduced and the device becomes insensitive. Magnetisation can
occur when heavy fault currents flow and are switched off when at peak value by the
tripping mechanism leaving remanent magnetisation. When this has occurred and the
device is subsequently reset, insensitivity due to remanence means that the device
may be reset when a fault is still present in the supply circuit.
It is an aim of the present invention to provide an RCD which substantially
alleviates these problems.
According to a first aspect of the present invention there is provided a residual
current device (RCD) intended for tripping an electrical supply from a circuit to be
protected when a residual current imbalance in the circuit exceeds a predetermined
threshold rating, the RCD comprising:
sense means for generating an imbalance signal representative of residual
current imbalance in the circuit;
trip means intended for tripping the residual current device when the
imbalance signal exceeds the predetermined threshold rating so as to disconnect the
electrical supply from the circuit; and
test means for increasing the imbalance signal to a level which substantially
corresponds to the predetermined threshold rating whereby a trip at said rating
indicates a successful test.
It is an advantage that the device may be tested for whether or not the RCD
trips at or near the rated value. That is, a successful test indicates that the device is
operative to trip at the intended threshold rating. An unsuccessful test is one where
the device trips when the imbalance signal is below or above the threshold, this
condition indicating that the device is not operating at its rating. The test is therefore
more rigorous and accurate than the test provided in prior art devices.
The sense means may be operative for measuring an amount of any residual
current imbalance in the circuit.
The test means may be operative for calculating a difference value
corresponding to the difference between the measured residual current imbalance and
the predetermined threshold rating. The difference value may be applied such that the
increase in the imbalance signal is substantially instantaneous. Alternatively, the
testing means may be operative to ramp up or progressively increase the imbalance
signal from a low or zero value to the predetermined threshold value. This alternative
provides for determining the level of current imbalance at whichever level the device
trips. This advantageously provides for testing whether the device trips at a level
which is less than the predetermined threshold.
In embodiments of the invention, the test means effectively introduces a
simulation residual current imbalance into the device so that the sense means senses
the sum of any residual current imbalance in the circuit being protected and the
simulated residual current.
In a preferred embodiment, the sensor means comprises a current transformer
having a sense coil, the imbalance signal being an imbalance sense current induced in
the sense coil. The means for increasing the imbalance signal may include a test coil,
wherein a test current applied to the test coil is operable for introducing the simulation
current imbalance in the form of a magnetic field in the transformer, thereby inducing
the increase in the imbalance sense current in the sense coil.
The testing means may be coupled to a processor that monitors the imbalance
signal and determines the simulation current imbalance required to increase the
imbalance signal to a level that corresponds to the rated value. It is an advantage that,
if the processor detects a current imbalance below the rated trip value (a standing
current imbalance), then it determines how much to increase the imbalance signal to
reach the level that corresponds to the rated trip value, and thereby provides a more
accurate test than the prior art devices.
The processor may include an analogue to digital converter (ADC) for
converting the current imbalance signal to a digital form, a micro-controller unit
(MCU) for processing the digital signal and for providing a digital output signal, and a
digital to analogue converter (DAC) for converting the digital output signal to an
analogue test signal. The digital processing enables the generation of a test current
having a waveform and phase profile appropriate for providing the required sum.
An advantage of synthesising a waveform for the simulation current imbalance
directly from the processor is that it is independent of the electrical supply and any
variations therein. A further advantage is that the waveform can be synthesised by the
processor based on the standing residual current determined from the imbalance
signal. This means that whatever waveform, phase angle or frequency the standing
residual current has, the processor can synthesise a simulation current imbalance
waveform, which, when added to the standing residual current waveform, ensures that
die device is tested against the rated value.
Preferably, the processor is an integrated circuit in the RCD. An integrated
circuit is an effective, low cost, space-efficient processor, which is simple to assemble
into an RCD.
According to a second aspect of the present invention there is provided a
residual current device (RCD) comprising:
a current transformer for generating an imbalance sense current in a sense coil
in response to a current imbalance in an electrical supply; and
a degaussing coil for substantially removing remanence in the current
transformer by application of a degaussing signal to the degaussing coil.
The degaussing coil may be combined with a test coil forming part of a testing
means in a device according to the first aspect of the present invention as defined
above.
Degaussing is a method of removing a remanent magnetic field by driving the
transformer core with an alternating field which decreases in amplitude over several
cycles. Removing remanence means that a device, which has been desensitised due to
a remenant magnetic field in the transformer core, can be resensitised and thereby re--
establish the device's sensitivity'so that it will continue to function in the required
manner. By degaussing to remove remanence, the device can be re-set after a trip
while ensuring that the device will trip again within a very short time if the circuit still
has a fault.
The degaussing signal may be applied to the degaussing coil under the control
of a processor. The processor may be configured to apply the decaying alternating
field at a high frequency so that the degaussing signal is not detectable by the RCD' s
residual current detection system. This ensures that degaussing is achieved in a very
short time and that remanence is removed quickly when re-setting the RCD. The
RCD must be capable of tripping within a specified number of cycles of the A.C.
supply and so the high frequency degaussing signal ensures that remanence is
removed in fewer than the specified number of cycles. The high frequency
degaussing also enables the processor to be configured to control degaussing during
normal operation.
Embodiments of the invention will now be described with reference to the
accompanying drawings in which:
Figure 1 is a schematic circuit diagram of a known electrical installation
having a known RCD, as hereinbefore described;
Figure 2 is a schematic circuit diagram of a known test circuit for an RCD, as
hereinbefore described;
Figure 3 is a schematic circuit diagram of another known test circuit for an
RCD, as hereinbefore described;
Figure 4 is a schematic circuit diagram of a test and degaussing circuit for an
RCD in accordance with the invention;
Figures 5, 6 and 7 are graphs showing current waveforms as may be found in
an RCD according to the invention; and
Figure 8 is a graph showing a degaussing current waveform, for use in an
RCD according to the invention.
Referring to Figure 4 a power supply installation has a live conductor 40 and a
neutral conductor 42, for supplying current from a supply to a load circuit 44. An
RCD 46 includes a toroidal transformer 48 having a core 50 which surrounds the live
and neutral conductors 40, 42. A sense coil 52 and a test coil 54 are wound on the
core 50. A current induced in the sense coil 52 is supplied as an input to an electronic
processor 56. A switch mechanism 58, actuated by an actuator 60 under the control of
the processor 56, breaks the live and neutral conductors 40,42 when a predetermined
level of residual current is detected.
In the electronic processor 56 the input current from the sense coil 52 flows to
a transresistance amplifier 58, having a voltage output that is linearly related to the
input current. The output voltage of the transresistance amplifier 58 is then fed via a
lowpass filter 60 (to prevent aliasing) to an analogue-to-digital converter (ADC) 62
which outputs the voltage as a digital electronic signal. The digital signal is fed to a
micro-controller unit (MCU) 64 via a digital bus 66. The MCU 64 has an output 68
for controlling operation of the switch actuator 60.
The RCD 46 is provided with a test button 70 for closing a contact 72 to
initiate a test under the control of the MCU 64. A digital test signal provided by the
MCU 64 is fed via the bus 66 to a digital-to-analogue converter (D AC) 76, which
outputs an analogue test current to the test coil 54.
In use a current imbalance between the live and neutral conductors 40,42,
generates a magnetic field which induces a sense current in the sense coil 52. The
sense current is amplified by the transresistance amplifier 58 and converted into a
digital signal by the ADC 62 and read by the MCU 64. If the MCU 64 determines
that the current imbalance is above the predetermined rated trip value, then a trip
signal is applied to the MCU output 68 such that the switch acuator 60 actuates the
switch 58 to break the live and neutral conductors 40,42, and thereby interrupt
electrical supply to the load circuit 44.
The device may be tested while operational in an untripped condition.
Pressing the test button 70 closes the contact 72 and initiates the test. The MCU 64
determines the level of the current imbalance being sensed by the sense coil 52, and
calculates the amount by which the current from the sense coil 52 must be increased
for the RCD 46 to trip at its rated trip value. The calculated increase is provided by
means of the test coil 54. A test current is provided to the test coil, which generates a
magnetic field in the core 50 of the transformer 48. The magnetic field generated
induces an increase in the sense current in the sense coil 52. The MCU calculates the
test current required to test whether or not the RCD trips at the rated value.
The sense coil 52 is typically 1000 turns of wire and the test coil 54 is
typically 100 turns. The current in the sense coil 52 is linearly related to the residual
current by a factor determined by the turns ratio between the electrical circuit
conductors 40,41 (the primary coil of the transformer) and the sense coil 52.
Therefore, a 10mA RMS residual current induces a 10 micro-amp RMS current in the
sense coil 52 for the 1:1000 turns ratio. A working bandwidth from 20Hz to 2kHz is
readily achievable and adequate for RCD purposes. The transresistance amplifier 58
is characterized by having low (almost zero) input impedance which is necessary to
ensure the sense current is directly related to the residual current by a fixed 1:1000
ratio over the working bandwidth. The output of such an amplifier is a voltage
linearly related to the input current with a typical gain of 10000V/A.
The ADC 62 periodically samples the voltage and each time outputs a digital
electronic value of typically lObits. The ADC 62 can be time multiplexed so as to also
sample the line voltage of the supply via a potential divider network 74 allowing
mains frequency to be monitored. The processor 56 measures the frequency of the
residual current waveform and the sample frequency is adjusted such that a fixed
number of samples per cycle are taken. A rate of 64 samples per cycle of the residual
at 50Hz gives a sample rate of 3200Hz, whereas at 60Hz the sample rate is 3840Hz.
An algorithm executed on the MCU 64 determines the frequency of the residual
current, but in cases where it cannot be determined (e.g. the amplitude is zero, or the
signal is random, or the signal is outside the expected range of values) then the line
voltage frequency can be measured and used.
With the residual current waveform accurately represented by digital values, it
is possible to apply digital signal processing techniques to determine various
parameters of the signal and in particular to calculate its RMS value to cause a trip if
this exceeds the set threshold rating. The digital processing is performed by the MCU
64, which includes control circuitry, arithmetic circuitry, a read/write memory for
storage of variable values and a non-volatile read-only memory which stores an
executable software program for the whole MCU 64 to follow. Other peripheral
devices not shown are also present including power supplies, clock circuits and
power-on reset circuits.
The calculation of the residual current RMS is performed over a whole
number of cycles to ensure accuracy. Ten cycles of the residual waveform is a
sufficient period to perform the calculation and since the sample frequency is adjusted
to give a fixed number of samples per cycle (say 64) then the total calculation requires
640 samples. For a 50Hz residual current frequency this therefore takes 200mS to
process 640 samples and at 60Hz takes 167mS. In both cases tripping occurs within
the time set by published standards. The software is written into the MCU 64 at
manufacture using a non-volatile memory. The non-volatile memory also contains
associated configuration data, such as the tripping current threshold and calibration
data derived from measurements taken at manufacture.
The DAC 76 either directly outputs current or otherwise outputs voltage which
can be converted to current by a linear current-to-voltage amplifier (transconductance
amplifier) or more simply using a fixed resistor. The waveshape and amplitude of the
current signal produced by this system is controlled by the MCU 64 under software
control.
Most prior art devices drive a current of up to 2.5 times the tripping threshold
of the device using mains voltage to source a sinusoidal signal at 50 or 60Hz. This
ensures that whatever standing residual current may already be present, the test
current will swamp it and guarantee the device trips. This is effective in causing a trip
but does not really test the accuracy of the system. By driving a synthesized
waveform into the test coil 54, the test current is independent of supply voltage and
amplitude 3.0 and so its RMS value is 2.1. The equation actually only holds true if
the two signals are 90° out of phase as shown in Figure 6. It would be possible to
measure the phase of a standing residual current and add the test current at an
appropriate phase to generate the required resultant but this adds considerable
complexity and does not work with all wave shapes. It is therefore evident that the
RMS calculation has a dependency on the phase between the two signals being
summed and an accurate result is only obtained if the RMS is averaged over all
possible phase differences.
A simpler solution is to adhere to condition "a" and drive the test signal at a
different frequency to any standing residual current. As described above, the MCU 64
is capable of measuring the frequency, or in some circumstances it is assumed to be
the same as the measured supply frequency. The test coil 54 can then be driven at a
frequency 20 % higher or lower than the measured residual current frequency (e.g. 40
Hz if the measured frequency is 50Hz). The resultant is shown in Figure 8. The RMS
of the resultant is found to be correct as predicted by Equation 1 above, and will in
fact work for any wave shape of standing residual current. It is also true that any
wave shape for the test signal can also be used and the use of a square wave test signal
rather than a sinusoid can be simpler to synthesise. Another way of looking at this is
that the use of different frequencies means that the dependency, of the resultant RMS
on phase is lost because the two signals are added over time at all combinations of
phase.
Condition "b" above, requires measurement of the resultant of the standing
and induced test current signals to be performed over a great length of time to achieve
accuracy. The tripping time at the rated threshold for most RCDs is set at 300ms
maximum by the relevant standards. Therefore, when the test button 70 is pressed the
device has about 14 mains cycles (280mS) to initiate the trip. This number of cycles
does give reasonable accuracy but improved accuracy and tripping time can be
achieved with some care. With reference to figure 7 it is evident that a beat frequency
is present equal to the difference in the frequencies of standing residual current and
test current, in this case 10Hz for a 50Hz residual current. Over the ten-cycle period
shown (200mS at 50Hz) two beats are present and it is notable that the relative phases

of the three traces are the same at the start, and end of the period shown. The result is
accurate since all combinations of.possible phase between the two signals have been
used in the calculation exactly twice, meaning any initial phase is irrelevant and phase
dependency is lost. A measurement period which is not a multiple of the beat period
gives less accurate results since some phase combinations are seen more times than
others and so initial phase becomes a factor in the calculated RMS of the resultant.
The test signal is calculated as a fixed percentage of the standing residual current
frequency such that over the period where a fixed number of samples are used to
calculate the resultant RMS there will be an integer multiple of cycles of the beat
frequency produced between the standing residual current and test signal frequencies.
The test current calculation must take into account the turns ratio of the sense
and test coils so that the induced current ratio is correct, as well as the wave shape
used for the test signal. Also, initial tolerances in the-system can be accounted for
using calibration values stored in memory at manufacture to modify the test current
amplitude. Once the residual current frequency has been determined in the manner
described above, then on initiation of a test by operation of the test button 70 a test
signal of the calculated amplitude is driven into the test coil 54 at a frequency
different to that of the residual current 54. The measurement system will be operating
normally by continuously measuring the apparent RMS values detected in the sense
coil 52 over a fixed number of mains voltage cycles and causing a trip when
necessary.
Another feature of the device is the ability to effectively counter the problem
of remanence described above. To counter this problem the remanent magnetic field
in the transformer core 50 can be removed by driving the core 50 with an alternating
field which decreases in magnitude over several cycles. This technique is called
degaussing. Such a signal can be driven into the test coil 54 to permit degaussing
under software control. It is particularly useful to perform degaussing at startup of the
device as this is when the core 50 may have been left magnetized following a fault
which caused a trip. However, periodic degaussing can be implemented during
normal operation if desired, providing it can be done quickly without effecting normal
operation of the device. If the degaussing signal frequency is much higher than die
operating band to which the residual current sensing circuit is sensitive, then the high
frequency degaussing signal will not be seen directly by the measurement system. A
suitable type of waveform is shown in Figure 8. It consists of a decaying waveform
whose initial amplitude is sufficiently high to cause magnetic saturation of the core
(i.e. it cannot become more strongly magnetized). The wave form has a peak
amplitude of around 2A-turns of the test coil 54, so for a 100 turn test coil 54 this
means a current of 20mA peak is required. The subsequent decaying waveform
leaves the core less and less magnetized after each cycle. A high frequency waveform
of around lOKHz is suitable and a decay rate of 80% per millisecond over a two-
millisecond period achieves degaussing in a short time. However, the optimum
parameters of the waveform are greatly dependent upon the dimensions and material
of the toroidal core. There are no extra components required to perform degaussing as
the proposed components of the test circuit of Figure 4 are able to produce the
required signal. The synthesis of the waveform is undertaken by the MCU 64 under
software control. The waveshape used need not be sinusoidal as suggested, other
shapes such as rectangular waveforms are equally effective and are simpler to
synthesize.
We claim:
1. A residual current device (RCD) comprising:
a current transformer for generating an imbalance sense current in a sense coii
in response to a current imbalance in an electrical supply; and
a degaussing coil for substantially removing remanence in the current
transformer by application of a degaussing signal to the degaussing coil.
2. The device of claim 1, wherein the degaussing signal is applied to the
degaussing coil under the control of a processor.
3. The device of claim 2 wherein the processor is configured to apply the
degaussing signal so as to drive the transformer core with an alternating magnetic
field which decreases in amplitude over several cycles.
4. The device of claim 3, wherein the processor is configured to apply a decaying
alternating field at a high frequency so that the degaussing signal is not detectable by
the RCD's residual current detection system.
5. •¦ The device of claim 3 or claim 4, wherein the degaussing signal has a
sinusoidal or a rectangular waveform.
6. The device of claim 1. wherein the residual current device (RCD) is
configured for protecting a circuit by tripping in response to an imbalance signal
representative of residual current imbalance in the circuit, the RCD tripping the circuit
when the imbalance signal exceeds a predetermined threshold rating, and wherein the
degaussing coil is combined with a test coil forming part of a test means for
increasing the imbalance signal so as to test operation of the RCD against the
predetermined threshold rating.
7. The device of claim 6, wherein the test means calculates a difference value
corresponding to the difference between the measured residual current imbalance and
the predetermined threshold rating.
8. The device of claim 7, wherein the difference value is applied such that the
increase in the imbalance signal is substantially instantaneous.
9. The device of claim 7, wherein the test means ramps up or progressively
increases the imbalance signal from a low or zero value to the predetermined
threshold value.
10. The device of any one of claims 6 to 9, wherein the test means introduces a
simulation residual current imbalance into the device by applying a test current to the
test coil to induce a magnetic field in the transformer, thereby inducing the increase in
the imbalance sense current in the sense coil so that the sense means senses the sum of
any residual current imbalance in the circuit being protected and the simulation
residual current.
11. The device of claim 10, wherein the test means is coupled to a processor that
monitors the imbalance signal and determines the simulation current imbalance
required to increase the imbalance signal to a level that corresponds to the
predetermined threshold rating.
12. The device of claim 11, wherein the processor includes an analogue to digital
converter (ADC) for converting the current imbalance signal to a digital form, a
micro-controller unit (MCU) for processing the digital signal and for providing a
digital output signal, and a digital to analogue converter (DAC) for converting the
digital output signal to an analogue test signal.
13. The device of claim 11, wherein the processor generates a test current having a
waveform and phase profile appropriate for providing the required sum.
14. The device of claim 11, wherein the processor is an integrated circuit in the
RCD.

A residual current device (RCD) comprises a current transformer (48) for generating
an imbalance sense current in a sense coil (54) in response to a current imbalance in
an electrical supply and a degaussing coil (54) for substantially removing remanence
in the current transformer by application of a degaussing signal to the degaussing coil.