Monday, October 26, 2009

ELCB circuit

The diagram below shows how a single phase ELCB circuit looks like. However, before I get down to explain more, please take note that I am using the name ELCB as a generic name, just like a circuit breaker.

1. ELCB is a short for Earth leakage Circuit Breaker. When it first came to be used, the devise is actually a voltage-operated device that was designed to detect a current leaking through the earth path of electrical equipment and appliances. I used to see the actual circuit during my college days, but I have to dig it up and redraw it for uploading. You will see it in one of my future posts.
2. The circuit that you see here is actually an RCD (residual current devise), or RCCB (residual current circuit breaker). It is based on the principle of magnetic core balance and the trigger core is current operated instead of voltage operated as in the earth leakage circuit breaker.
3. In terms of performance, the residual current circuit breaker is definitely better. The performance of the old ELCB did not have any problem, but the residual current device can do the same and more.
4. I cannot remember when the residual current circuit breaker actually took over the whole wiring scene, but I cannot find the earth leakage circuit breaker in actual installation anymore. Everywhere that I know is unsung the RCD, but in the field the terminology used have not changed at all. Everywhere and in all design drawings the name ELCB has not been changed. Only some textbook and in the academic classes the RCD or RCCB terms has been used. Therefore, the convention has not been changed. That why throughout this blog I will keep on using the ELCB for this earth leakage device.
5. Originally ELCB was designed to detect earth leakage current and to disconnect the circuit immediately when a certain limit was crossed. Now RCD does the same. Therefore, the general practice to still call the residual devise an earth leakage circuit breaker is still correct. It is still an ELCB by purpose and function even though the internal circuit design has actually changed.
6. However, in certain scenarios both ELCB and RCD names need to be used for clarity. The discussion on internal circuit design of the device like now is one of the scenarios. That is why I will use the two names interchangeable in the following part of this post.
The picture below shows a typical single-phase double pole ELCB circuit. I think it is better to first explain the components of the circuit one by one and explain the how the circuit works afterward.

Diagram 1 – Typical ELCB circuit

a. The supply coil, the neutral coil and the search coil all wound on a common transformer core. On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.
b. In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the ELCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. This unbalanced flux is what we call a residual flux.
c. The residual flux will be detected by the will cross the winding of the search coil and produce a voltage that drives a current inside the wiring of the trip circuit. It is this current that operates the trip coil of the circuit breaker. Since the current has been driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current devise. With a circuit breaker incorporated as part of full circuit, it is called residual current circuit breaker (RCCB) or residual current devise (RCD).
d. The incoming current will come through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated. The circuit breaker can also be used to manually ON or OFF the circuit.
e. The load circuit is not part of the ELCB circuit. However, notice the earthing symbol at the load circuit. That is the earthing connection from the exposed metal parts of the electrical equipment or appliance to the electrical earth. This earthing connection will allow the ELCB fulfill its purpose of being in the electrical circuit. You can have a good operational ELCB unit properly installed and wire at the electrical panel, but if the earthing connection is broken or missing, the ELCB will not trip during an actual earth leakage situation.
f. The test pushbutton and the test resistor are arranged to provide a test function for the ELCB circuit. This part of the circuit bleeds away a fraction of the running current from the phase coil. So the neutral coil current will be higher the phase coil current. Therefore, a residual current will be generated in the trip circuit and trips the circuit breaker.

The above trip simulation tries to check the health of the ELCB circuit. In it works. However, it does not test the complete operation of the protection system that the ELCB is supposed to serve.

Read how you can get electric shocks here. Other related articles: a) Home electrical wiring, symbols and checking;

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prolix said...

Great explanation, thanks for the posting!
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