ELCB 3

[sarves]

There are two ELCBs(4 pole)instaled in a distribution board.
One is on the main supply (60 A,200ma) and other one for a perticular device (30 A,300ma).The tripping time of both is same 0.1s.But for a lighting time secondary was tripped but not main.After that I manually shorted the neutral and earth of the device on that time both ELCB tripped simultaniously.So I am unable to find the reson for this problem.Can anyone help me?

 

[RajT]

The main incomer ie 60A 200ma should have a higher setting then any of its downstream feeder which does not appear to be the case.
I would suggest that the main incomer be changed to 60A 200/300ma ,100msec. The extra delay should allow any down stream faults to be cleared in time.

 

[akhaider]

Your case theoratically appears normal, because in case of earth fault, the outgoing ELCB should trip first and in case of its failure the main should trip. It appears that the thermal protection of the outgoing ELCB had actuated, instead of the Earth Leakage protection (30 A). Otherwise main ELCB should have tripped because its current sensitivity is less as compared to outgoing ELCB.

Tip : The main ELCB shuold always have current sensitivity higher than those of outgiong ELCBs, because the phase to ground leakages which no matter how small, are always there and their cumulative effect causes the main ELCB to trip on earth faults at outgoing circuits. Therefore the current sensitivity of main ELCBs is always kept higher as compared to outgoing ones.

 

[jbartos]

Suggestion: Essentially, I agree with RajT, the upstream setting could be 300ma and downstream setting could be 200ma, or in similar proportions, if possible to ensure a selectivity in the protection and tripping.

 

[sarves]

Thank you for your support.

 

[Ozeng]

sarves, I'm not quite sure from yr post whether the problem is that only the secondary ELCB tripped for the "lighting time" (do you mean a lightning strike?) or that they both tripped when you shorted the neutral to earth. I suspect that in the first case the fault was of a very short duration possibly with a high dc component which caused only one ELCB to pick up and latch before the fault disappeared. In the second case, if the fault was higher than the setting of both ELCBs then it was inevitable that both would see the fault and trip as they both have the same time delay.

All the previous respondents have fallen into the common trap of assuming that if the upstream ELCB has a higher current setting (300mA v 200mA) it will provide discrimination, but as noted above if the fault current is higher than those settings, as most faults are of course, both ELCBs will see the fault and trip - the fault current is not limited by the relay setting but by the impedance of the circuit. The ONLY way to provide discrimination with sensitive earth-fault relays (RCDs ELCBs call them what you will!) is to use time discrimination. About 300msec should be enough with most modern devices. So don't change the ELCBs just adjust the time delays.

 

[sarves]

Mr Ozeng,
In the first case Secondary ELCB tripped due the lightning strike.When manually shorted bothe tripped.In the second case even though both tripped simutaneously is there any possibility to one tripped earlier.

 

[RRaghunath]

To my knowledge, there is no time grading facility with ELCBs (Ozeng may please correct me).

The ELCBs/RCDs are meant for protection against electrocution or fire due to leakage of current (caused by poor insulation etc.) and this explains why the operation of these devices shall be fast.

The ELCBs with current sensitivity of 30mA provide best protection against electrocution and are specified in individual outgoing circuits with limited amount of wiring downstream of ELCB to prevent spurious operation). Whereas, the ELCBs with sensitivity of 100, 300 or 500mA can be employed, as appropriate, in the incomer circuits and can provide effective protection against fire.

The current sensitivity to be selected is primarily decided by the amount of wiring downstream of the ELCB and the need to prevent spurious operation due to normal capacitive leakage currents to earth through the insulation.