FIELD OF THE INVENTION
The present invention relates to an earth resistance monitoring apparatus and more particularly, a system for indicating the endurance and integrity of earth ground strips connected between main substation transformer earth ground and equipment to be protected.
BACKGROUND OF THE INVENTION
It is desirable to operate all electrical equipment with protective earth ground. The important methods of protective earth ground in industries include flat or strip earthing wherein a galvanized iron strip of various cross sectional sizes such as 50mm by 6mm, 25mm by 3mm etc. are used. The satisfactory operation of the protective earth ground depends on satisfactory connection between earth ground of main substation transformer and the earth ground fiat or strip. In case of any discontinuity of earth flat or strip due to mechanical damages caused by corrosion, erosion, ageing etc. such connection, between earth ground of main substation transformer and the earth ground flat or strip, may not be achieved. The earth flat or strip is then ineffective in discharging earth ground current to the earth ground of main substation transformer and there exists a danger of electrical spark or risk of explosion.
In the prior art, to prevent hazards due to danger of electrical spark or risk of explosion because of unhealthy earth flat or strip, it is required to monitor the healthiness of the earth flat or strip at known interval of time.
In European Patent No. EP 0057498, Stanley Howard discloses a ground

isolation monitoring apparatus for use between an AC power source and an electronic device with a protective circuit which automatically interrupts power to the device in the event that exposed conductive portions of the device become energized with hazardous potentials. The monitoring apparatus of U.S. Patent No. 3895264 utilizes a star arrangement of impedances connected at the power source and at the mobile machine to provide a monitoring voltage which circulates current through the ground wire and a monitoring impedance. In the system of U.S. Patent No. 3729653, a DC signal is impressed on the ground wire by a transmitter and when the current in the ground wire falls below a predetermined level, a relay is de-energized disconnecting the power source. However, in all the above inventions, there is no indication of fault and instead only isolation of power source alone is predominant. Similarly in the U.S. Patent No. 4389694, Cornwell Jr. discloses a cable continuity monitoring system for ensuring continuity and integrity of power distribution network, where a transmitter receiver arrangement is used for monitoring and the fault is not known locally. In U.S. Patent No. US 2003/0164770 Al, a device for indicating whether a conductive element has an electrical resistance equal to or less than a predetermined value or whether the conductive element is reliably connected to earth reference potential by a resistance equal to or less than a predetermined value. In this invention, only the grounding clamp healthiness is monitored and it uses a current to voltage converter, which involves plurality of solid state devices prone for frequent failures and thus of less reliability.

OBJECTS OF THE INVENTION
An object of the invention is to propose an earth strip continuity monitoring system with simple and reliable circuit.
Yet another object of the invention is to display the earth ground fault on the spot continuously through display means consisting of light strips.
SUMMARY OF THE INVENTION
According to an aspect of the invention, two indication signals are provided for indicating whether the earth flat or strip connected between main substation transformer earth ground and equipment to be protected is having electrical continuity to ensure the integrity of the power distribution and to avoid hazards due to danger of electrical spark or risk of explosion.
230 volt AC power supply is applied to a step down transformer for reducing the AC voltage level for applying across a four diode full wave bridge rectifier. Thus obtained unidirectional voltage at the output terminals of the four diode full wave bridge rectifier is filtered for unwanted ripple voltages using a filter capacitor and regulated to deliver a constant 12 volt DC voltage. The positive of regulated 12 volt DC voltage is applied to the positive terminal of an electromechanical relay and the negative of regulated 12 volt DC voltage is applied to the main substation transformer earth strip at the point entering into underground cable trench. The negative terminal of the electromechanical relay is connected to one end of the earth strip whose continuity is to be

monitored. The other end of the earth strip is connected to main substation transformer earth strip at the point entering into underground cable trench.
Under normal operating conditions, the plunger means of the electromechanical relay establishes connection between the positive of regulated 12 volt DC voltage and the anode of a green coloured light emitting diode. Under fault conditions, i.e., when there is no continuity in the earth strip, the plunger means of the electromechanical relay establishes connection between the negative terminal of regulated 12 volt DC voltage and the anode of a red coloured light emitting diode. The cathode of both the green coloured light emitting diode and red coloured light emitting diode are connected to the negative terminal of regulated 12 volt DC voltage.
Thus when the earth strip is healthy, green coloured light emitting diode is enabled to indicate the healthiness of the earth strip and when there is any discontinuity in the earth strip, the red coloured light emitting diode is enabled to indicate the failure of the earth strip,
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows the electrical schematic of the online earth continuity monitoring apparatus in accordance to the invention.
Figure 2 shows the outlook of the apparatus with LED strips in accordance to the invention.

DETAILED DESCRIPTlON OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1 shows the schematic of the earth continuity monitoring apparatus for earth strips embedded within the underground electrical cable trenches. The apparatus comprises of an electrical two pin plug means for connection to the alternating current (AC) source [1]. A step down transformer [2] is connected to the AC supply through a fuse wire [3] and an electrical power switch contact [4]. When the contact is closed AC electrical power is applied to the primary coil [5] of step down transformer [2]. The fuse wire is provided to protect the apparatus by opening the electrical contact in the event when electrical current through the fuse wire exceeds the current rating of the fuse wire. The contact terminals [6] and [7] of the secondary coil [8] of the step down transformer are connected to a four diode full wave bridge rectifier [9]. The step down transformer [2] reduces the applied 230V AC voltage from the AC source by stepping down to a 12V AC voltage level that can be applied to the bridge rectifier [9]. The bridge rectifier [9] is used for converting the stepped down AC voltage into a direct current (DC). Thus produced DC voltage is filtered through a filter capacitor [10] and applied across a constant voltage regulator [11]. The constant voltage regulator circuit [11] is used to provide a constant DC voltage to the earth continuity monitoring circuit.
An indication lamp [12] is connected across the constant voltage regulator [11] through an electrical resistor [13]. This electrical resistor wiU limit the current flow to the indication lamp [12] and avoid any damage to the indication lamp due to high electrical current flow. This indication lamp [121 will ensure normal working condition of the aforesaid AC to DC converting system. Under operating conditions the polarity available at the nodes [14] and [15] are

positive and negative respectively. About 12V constant DC voltage will be available at the terminals [14] and [15]. The negative terminal [15] is connected the earth ground [16]. This earth ground [16] is having continuity to the main electrical substation and hence termed as mains earth.
A 12 volt electromagnetic relay means [17] is connected to the positive terminal [14] and earth strip [18] running through the underground cable trench. The relay [17] consists of two pair of contacts [19] and [20]. Under normal conditions, when the apparatus is energized the relay closes the contact terminals [20]. When there is no electrical continuity in the earth strip [18] due to corrosion or due to any mechanical damages the relay [17] open the terminals [20] and close the terminals [19].
There are two indicating signs which is generally light emitting diodes. One indicating sign consists of green coloured light emitting diodes (LEDs) [21] and the other consists of red coloured light emitting diodes [22].
The anode of green LED [21] is connected to positive of 12 volt DC source available at the terminal [14] and the cathode is connected to the negative of 12 volt DC source available at the terminal [15] through the relay contact terminals [20]. The anode of red LED [22] is connected to positive of 12 volt DC source available at the terminal [14] and the cathode is connected to the negative of 12 volt DC source available at the terminal [15] through the relay contact terminals [19]. The green LEDs are enabled when the relay [17] operation closes the terminals [201- This condition occurs when there is electrical continuity between mains earth [16] and the earth strip [18] and thus when these green LED lights are enabled, it indicates that the earth strips are healthy.

Similarly, the red LEDs are lit when the relay [17] operation closes the terminals [19). This condition occurs when there is no electrical continuity between mains earth [16] and the earth strip [18] and thus when these red LED lights are enabled, it indicates that the earth strips are not healthy and requires attention.
Figure 2 shows the outlook of the apparatus with LED strips. In the exemplary embodiment the two LED indicating signs are embedded on top of a metallic housing [23]. AC cable [24] connecting the power supply circuit of the apparatus and the electrical plug [2] is taken through a hole using a cable gland [3] from the metallic housing [23]. The cable gland prevents damage to the AC cable [24] due any abrasion with metal. The AC cable [24] consists of two conductors, one connected to the phase of the AC source and the other to the neutral of the AC source.
The green LEDs are embedded on top of the apparatus in the shape of a ̒tick mark̕ [25] implying healthiness of the earth strip. Similarly, the red LEDs are embedded on top of the apparatus in the shape of a ̒X mark' [26] implying unhealthiness of the earth strip.
A thick conductor [27] insulated using poly vinyl chloride (PVC) compound is used to make electrical contact between negative polarity terminal [ 15] and the mains earth [16]. Yet another thick conductor [28] insulated using PVC is used to make electrical contact between negative polarity of the relay [17] and the earth strip [18]. The ends of the conductors [27] and [28] are terminated using eye hole cable lugs [29] and [30] respectively for fastening the conductor to mains earth and earth strips firmly.
A hooking arrangement [31] is made in the back end of the metal box [23] for suitably hanging the apparatus to walls.

We claim:-
1. An earth strip (18) continuous monitoring apparatus comprising: a AC power source, a switch contact (4), a step down transformer for stepping down the voltage level, a four diode full wave bridge rectifier (9), a voltage regulator, an indication lamp (12), an electromechanical relay (17) with plunger arrangement, a closed contact terminal (19) and open contact terminal (20), wherein a positive terminal (19) and negative terminal (19) of the electromechanical relay (17) are connected to positive DC supply and the said earth strip (18); wherein the earth strip's (18) health is indicated by display means.
2. The continuous monitoring of earth strip, as claimed in claim 1 wherein the display means are a first LED and second LED, each to indicate healthy and faulty condition respectively.
3. The continuous monitoring of earth strip, as claimed in claim 1 wherein the indication lamp (12) ensures normal working condition of the faulty wave bridge rectifier.
4. The continuous monitoring of earth strip, as claimed in claim 1 wherein the
LED (21, 22) is coupled to the DC source through the electromagnetic relay.

5 The continuous monitoring of earth strip, as claimed in claim 1 wherein the first LED indicates normal working and the second LED indicates faulty condition.
6. The continuous monitoring of earth strip, as claimed in claim 1 wherein substantially as herein described and illustrated.