A simple earth leakage indicator circuit discussed here can be used for getting some very useful results regarding current leakages from an appliance body into the earth pin. The idea was requested by Mr. SS kopparthy.
The circuit of the proposed earth leakage indicator is shown in the figure below.
Each such unit may be used for individual appliances having earth pins, or a single circuit may be placed near the MCB for detecting a possible common leakage from all the appliances.The circuit may be understood with the points explained below:
Circuit Operation
R2 is positioned as a current sensing resistor which should have a relatively low value so that the actual earthing feature doesn't get obstructed due to its resistance.
T1 here forms a current sensing and a voltage amplifier stage. The detected small voltage across R2 is quickly amplified by T1 and fed to the LED inside an opto coupler.
As long as the leakage is not relatively significant (below 20mA) the LED inside the opto does not respond, however the moment this value exceeds the set limit, the LED inside the opto illuminates switching ON the corresponding built-in transistor, which in turn actuates the red LED connected across its collector and positive lead indicating a possible earth leakage.
The supply for the entire operation is derived through a small trasformerless power supply using C1, D1, C2 as its main components.
The red LED may be replaced with a 12V piezo buzzer for getting an audio indication, or both may be used in parallel for facilitating a dual mode indication.
The value of R2 may be calculated using the following formula:
R = 0.2/I. where I is allowable current leakage through the earthing cable, assuming this to be 20mA we can calculate it as:
R = 0.2/.02 = 10 ohms
Since the collector resistance if T1 is quite high, T1 could get triggered with as low as 0.2 across its base/emitter, that's the reason why 0.2 is selected in the above formula.
The T2 stage is introduced for monitoring the "health" of the earthing connection, as long as it's at par with the neutral, T2 stays switched off since its base remains grounded via the good earthing, however the moment a weak earthing is formed, T2 base gets enough voltage through R5 to trigger itself and the opto which in turn triggers the connected alarm.
The situation of a weak or open earthing is indicated by the red and yellow LEDs together, while the red LED alone indicates an earth leakage.
CAUTION: THE CIRCUIT IS NOT ISOLATED FROM MAINS, ALL PARTS COULD CARRY LETHAL ELECTRIC CURRENT, EXERCISE UTMOST CAUTION WHILE HANDLING UNCOVERED.
Parts List
R1 = 1K ohms
R2 = see text
R3, R4 = 22k
R5 = 56K
R6 = 1M
D1 = 15V 1watt zener diode
C2 = 100uF/25V
T1, T2 = BC547
C1 = 0.47uF/400V
opto = any standard 4-pin type
The above circuit could be improved by adding a few more components to it, as shown below:
In this circuit we have added a rectifier diode D1 (1N4007) for an improved rectification.
T1 has been enhanced with another BC547 transistor T2 wired as Darlington in order to make the earth leakage detection even more sensitive and allow the use of smaller in-line resistance R2 for better "earthing" experience for the appliances.
C2 (0.22uF) ensures that T1/T2 does not get rattled with unwanted electrical disturbances.
Parts List
R1 = 1K
R2 = see text
R3, R4 = 22k
R5 = 56K
R6 = 1M
Z1 = 15V 1watt zener diode
D1, D2 = 1N4007
C0 = 0.47uF/400V
C1 = 100uF/25V
C2 = 0.22uF
T1, T2, T3 = BC547
C1 = 0.47uF/400V
opto = any standard 4-pin type
Test Set-Up for the above circuits:
Earth Leakage Indicator
The above diagram shows the test set-up for the proposed earth leakage indicator circuit.
It's conducted in the following manner:
The circuit powered up using an external 12V AC/dC adapter output, remember do not plug-in the circuit to mains while doing this procedure
In the set-up test 12V AC supply is connected across the earth/appliance points via a 12V bulb.
R5 link is kept disconnected for the time being.
The above implementation should instantly switch ON the red LED indicating a current leakage through R2.
Disconnecting the 12V bulb, the red led must also switch OFF, indicating the stoppage of the leakage condition.
Now reduce the 12V bulb load to some lower value, could be done by including another 12V bulb in series to it.
Even with such lower loads, the red LED should be able to indicate the leakages across R2 confirming the proper working of the circuit.
Now removing the above load should instantly switch off the red LED, assuring a correct working of the circuit.
Restore the circuit to its original condition and now it's ready for the actual installation near your MCB.
The functioning of the yellow LEd can be witnessed after the actual installation and connections are done.
If it starts glowing immediately after the installation would indicate a bad or incorrectly wired earthing line.
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