RCCB Trips While ATS Source Changes from Normal to Generator

Perhaps an SST....In any case you need an EE to make a determination as to the solution...The 300mA choice might be acceptable in this setting...

https://onlinelibrary.wiley.com/doi/full/10.1002/2050-7038.12996

hope so sir, also need to know the cause of tripping. it doesn't happen during normal source restoration. if sst it might happen that time also. during normal source restoration same switching is happening in the ATS. ie transfer from generator source to normal source. the ATS manufacturer Zenith/GE. 4pole. neutral type- last break-first connect.

When's the last time you megged out your gen windings? What are the ages? Might need a redip...

Its new installation, there is reverese power relay is provided for each genset, so no chance of reverse feed. everything done pre-commissioning. commissioning also done 6month back. while now only started monthly testing with on load. initially there was not much load i think thts y there was no tripping that time.

The Commissioning Engineer's name will be on the commissioning documentation. Please describe the outcome of discussing this equipment's new behavio(u)r with this individual?

"ATS manufacturer Zenith/GE. 4pole. neutral type- last break-first connect.." - this suggests that the generator is connected first before normal incomer is opened. To me, it seems the neutrals of both transformer and the generator are paralleled briefly at the time of changeover. Generator produces third harmonics required for its magnetisation. When the neutrals are paralleled, third harmonics being similar like zero sequence currents circulate through earth and cause RCCB maloperation.

Please check and ensure that the 'break' is complete before 'make' happens.

Generally speaking, 30mA is too sensitive for a DB incomer. 30mA is suggested for an individual circuit. 100mA or 300mA (depending on number of circuits connected) is more appropriate for the incomer. I mentioned this because the healthy leakage just before changeover could have been close to 30mA and the disturbance during changeover could be causing RCCB maloperation.

ATS is open transition only. Neutral only provided the last break and first make, which ensure the neutral connection to the system, hence the earth and zero potential before the phase connects to the system. it is confirmed that there is no parallel operation in the ATS which is why 4 pole is used.

<...further clarifications are needed....>

Well, some idea of the meaning of those TLAs and FLAs would be helpful for those looking to learn from this thread.

Please let the forum know the outcome of talking to these people.

Without the power system grounding detail, it will be difficult to nail down the possible causes.

I agree with raghun that 30mA RCCB as a main for a DB distribution board we think maybe, is a poor choice. The RCCBs if applied at all should be on your selected load breakers. The 3rd harmonic present when paralleling rotating machinery grounded in multiple places can be enough to cause a ground resistor to sizzle. I’m impressed you could keep the power system running at all.

The distributed cable capacitance is a fine source of apparent ground fault current, that is called also charging current, and there are references published which allow you to estimate the magnitude. A typical industrial system at 480 volts 1500 to 2000 kVA might be less than 10 amperes, I have measured 2-3 Amperes, milliamperes, Ha!

It is unlikely you can achieve any sort of system selectivity with cascaded RCCBs, and since ground faults are the most common fault, you are sentencing your power system reliability and availability to a very low score.

And some extra typing explaining all of your TLAs may actually get some good answers from people who are tired of having to look up every TLA and FLA they run across, and then decide TL;DR.