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Anonymous Poster #1

RCCB Tripping After Change of Supply

05/11/2017 1:54 AM

I am resident of a building and am facing the problem mentioned below. All my knowledge regarding electrical system is from what I have read from the internet. Thus I might not be able to meticulously explain the problem/system. In my apartment I have a manual change-over switch to change the supply to DG power when the mains power is not available. When the mains supply is restored and the DG turns off I have to manually switch the supply to mains. As soon as this is done the RCCB 40A (connected just after electrical meter and before the Flat DB ) trips. The supply to the flat is as below. Mains: Transformer--MCCB--Bus Bar-- individual flat meter (3 Phase)-- *RCCB*--(1 phase to manual changeover and then to DB & 2 phase directly to DB in flat) DG: DG--MCCB--32A TPN MCB -- individual flat MCB-- manual change over in flat. It's noteworthy to mention that only one phase circuit works on DG. What could be possible cause for the tripping?

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#1

Re: RCCB tripping after change of supply

05/11/2017 5:03 AM

The only reason for tripping is that the vector sum of the currents passing through the device exceeds its trip setting.

The way the installation is wired cannot be seen from here. That's the trouble with <...from the internet...>. So, engage a qualified local Electrician to sort it out, perhaps?

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#2

Re: RCCB Tripping After Change of Supply

05/11/2017 7:03 AM

It would help if we knew where you are, only then can we decide if your installation falls under IEC (European) or NEC (American) regulations /codes.

Your reference to RCCD would indicate IEC.

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Anonymous Poster #2
#3
In reply to #2

Re: RCCB Tripping After Change of Supply

05/11/2017 8:00 AM

We? You can do that on your own, mate.

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Guru

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#4

Re: RCCB Tripping After Change of Supply

05/11/2017 11:02 PM

Yes .This will trip.RCCB works on the principle of Ia+Ib+Ic+In=0.

During this period it is not 0, therefore RCCB will trip.

Solution would be install 4 pole changeover switch instead of 2 pole change over switch.

This means during power failure , no power will be available from mains and only DG supply will be available in phase and neutral and no voltage will be available in other two phases.

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#5

Re: RCCB Tripping After Change of Supply

05/11/2017 11:38 PM

I'm going to take a wild guess, but I might not even be in the right ball-park on this.

Assumptions: 1.You're British and by "flat", you mean apartment/home. 2. "only one phase circuit works on DG" means that only 1-out-of-three works, meaning that the diesel is also 3-phase, but is using only one. 3. That your flat actually uses 3-phase power, but when it goes to diesel power, it goes to single-phase power/loads. 4. When the main power returns, there are no 3-phase loads restarting, and the diesel still powers the single phase loads as when the main was off. And it has to be manually turned off, and is not auto-off. That means that if there was no intervention, the diesel would still be carrying it's load until it either runs out of fuel or someone turns it off. I make this assumption based on the fact that if there were any 3-phase loads restarting and one of the phases is now on the diesel, then you would have an out-of-phase 3rd-phase (get it?). The only other way to avoid that would be to disconnect the diesel before the main was restored, and you never mentioned anything to support THAT assumption. 5. The RCCB 40A is now sending 2 phases to the flat and the 3rd phase to that xfer box, which means that you now have only single phase loads running on two different phased circuits (since the diesel is now shut down). What the diesel was powering, is now dead, and anything that is running are different loads on different phases.

From these assumptions, I'm guessing that at least one phase of that RCCB 40A breaker is carrying a significant load. And when you re-energize that 3rd phase, it puts an additional "starting current" on one of those other lines sending it over the trip threshold. Now when you go back to reset the breaker, you are now energizing all 3 phases at the same time, and the starting current stays below the threshold.

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#9
In reply to #5

Re: RCCB Tripping After Change of Supply

05/12/2017 2:55 PM

My guess is India or nearby!!

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#6

Re: RCCB Tripping After Change of Supply

05/12/2017 4:00 AM

I'm thinking the RCCB in question, is a 3Φ unit. And effectively you have two supplies in your DB. 1. Being essential supply (i.e. lighting and low power appliances), that is feed either from 1Φ of the 3Φ RCCB or the DG, via the manual change over switch (in the flat/apartment). & 2. The non essential supply for the higher power appliances (i.e. Air-Con), that is feed from the other 2Φ of the RCCB, but doesn't go via the manual change over switch.

If this is the case, when the mains supply is restored, the non essential supply is available in DB. So 2Φ of the RCCB are energized and feed current to the connected appliances via the DB. Now when you switch the manual change over switch from DG to mains, you are allowing current to flow in the third phase(Φ) of the RCCB. Now my feeling is that this supply uses the neutral (where as the other supply does not) and, the sudden surge could very well cause a momentary imbalance on the neutral (30mA probably) to cause it to trip. The RCCB probably only monitors 1Φ & Neutral, that is your essential supply. Can you identify an appliance(s) that is(are) connect to this supply, which is(are) not suitable for connection to a 30mA RCCB protected feed (i.e. something with high earth leakage currents)

This installation is not ideal, as the RCCB/RCD should be in your DB. In this case you could discriminate the supplies to the appliances with high earth leakage currents.

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Guru
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#7
In reply to #6

Re: RCCB Tripping After Change of Supply

05/12/2017 5:38 AM

<...identify an appliance(s) that is(are) connect to this supply, which is(are) not suitable for connection to a 30mA RCCB protected feed (i.e. something with high earth leakage currents)...>

Were one such appliance to be connected, the RCCB would trip on whatever supply it was taking the power from, and not just at the instant of changeover. Anyway, such an appliance is known as "faulty" and "dangerous", and is an ideal candidate for condemnation, removal and recycling under the WEEE Directive.

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#8

Re: RCCB Tripping After Change of Supply

05/12/2017 2:53 PM

Your load is a transformer, which is inductive. You might try isolating all loads from the transformer before switch over....

I am assuming that you are not switching over "live", which will definitely cause a trip.....as the DG and the mains are most unlikely to ever be "in phase" with one another....

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#10
In reply to #8

Re: RCCB Tripping After Change of Supply

05/12/2017 3:40 PM

There is that pesky "phase" bug again, I will be glad when we are all powered by light. At least that has only about 4 kinds of polarization, that I can remember.

User could also try and keep his same equipment, but make sure the DG is shut down, and all load from circuits disconnected before starting over on the mains. There appears to be a ground fault, since ΣI =0 is not a wish or a prerogative. It is a mandate for that RCCB system.

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#11

Re: RCCB Tripping After Change of Supply

05/14/2017 3:25 PM

Please clarify....

  1. Is the manual changeover switch to DG, [downstream of "tripping RCCB"] switching just live (single pole) or live and neutral (2 pole)?
  2. Is the DG connected to another flat or only yours?
  3. When you write "DG turns off", do you mean it is "switched off" and engine stationary?
  4. When DG is "switched off", is its (internal, automatic?) switch disconnecting live and neutral or only live?
  5. The RCCB trips after mains restore and DG off - does it reset OK (with mains OK) when you go and try a reset?
  6. Have you tried turning off MCBs of "DG phase" on Flat DB before switching back to mains? - restoring your load in steps will reduce size of unbalanced surge in RCCB currents.
  7. Is your RCCB "30 mA" sensitivity [Rated Tripping Current, which should be clearly written on its face]or higher?

You have not written if your flat is ground or upper floor, which will affect mainly the length of cable to your flat DB.

An RCCB trips because some of the current in one of the wires going through it is not returning through the rest of the wires through it (usually because current is flowing to earth wire, which does not pass through the RCCB, due to a fault).

In this case, I guess it is the capacitance of the generator and/or its neutral cable to earth which is causing the unbalance compared to a normal installation. RCCBs will usually trip in 1/50 second at little more than their rated current, they can be tripped by the charging current of abnormally high capacitance to earth (such as circuits having many computers or other equipment with radio interference filters).

Since you can probably do little about this [except, maybe, on points 1 & 4 above], I suggested in 6. above, trying a switch-back without the extra complication of maximum surge currents in live & neutral.

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