Complex motor start cuasing RCBO to trip problem

[pmgd]

HI,

I have a difficult to diagnose problem. I've had two qualified electricians try and sort it, but no solution yet.
It is tricky (interesting?) and would love to hear comments/suggestions.

I have a long barn conversion.

Supply enters one end into a CU. There is a 50A 30mA RCBO feeding a 16mm T&E which goes about 50yds to an annex where it terminates in a standard split load CU. (Two 30mA RCDs with assorted MCBs each side).

Because the earth conductor in 16mmT&E is of insufficient size, an earth spike is additionally connected at the annex end.

One MCB supplies a 'pump room' which supplies a bore hole pump and - recently - a variable speed water pressure pump.

Every time the variable speed pump starts it blows the RCBO. (Never the RCD).

But, it is not faulty. If plugged into a regular socket on my main house ring main it works fine.

I have a socket near the main CU also connected to the RCBO - for test purposes.
If the pump is plugged in there it trips the RCBO ONLY IF the annex DB main switch is on, and at least some MCBs are enabled.

The situation is fully reproducible - although putting MCBs on in turn gives inconsistent results.

My personal view is that this is related to the earth spike having a substantially lower inductance to earth for the startup motor current and 30mA is current flowing through some domestic appliance in the annex to the spike. However, this is difficult to argue as the problem occurs when the pump is pugged in at either end of the 50yds 16mm cable.

Supply appears to be TN-C-S (although there are 3 cables coming from the transformer outside).

Any clever ideas?

Peter

PS All Zs etc within spec. 1V shown on test equip between N and E in whole installation!

 

[stef]

Hi Pete,
there is a possibility that the pump starter contains a mains filter, causing the RCBO
to trip. I assume the motor itself is ok and does not cause an earth leakake.
Check the drive first. Let us know how it goes.

 

[stef]

Another thing is, when you say it trips only if the annex db is on, are there any loads
connected which may cause an earth leakage, such as computers, anything with a filter in it?

 

[Knobhead]

VSD’s have an inherent leakage. That will trip the RCBO.

You need to rethink the supply arrangement for the annex.

 

[monkeyelectric]

You got 30mA rcd's in series with each other so that needs sorting to start with! Have you ramped that rcbo?

 

[pmgd]

Wow - 4 replies within 2 hrs - thanks very much!!

I'm afraid I forgot a little bit of information:

When I plug the pump into a regular ring main in my house (connected to an RCD) - nothing happens. It just works.

From this I conclude that the pump itself is not actually at fault - it just shows up somthing.

Also, when I dont use the variable speed pump, the bore hole pump infrequently trips it as well.

In answer to the points raised:

Stef and Tony: I have measured the earth leakage during operation of the pump and it is effectively zero. This was measured by putting an AC ammeter in series with the earth lead. It clearly doesn't measure any start-up issues though.

Stef: Yes. Certainly there is other stuff in the annex turned on. It has a combined leakage of about 6mA. But then again there is other stuff in the house on the ring main as well - where it works fine.

Tony & Monkeyelectric: Yes - I know the two RCDs in series is stupid. The builder's electrician got it wrong. Unfortunately the fix is prohibitively expensive and would entail drilling holes in the walls of a listed building and digging a 50m trench by hand. I really don't want to go that route if I can help it.

Tony: Main RCBO ramped shows it tripping at about 25mA.

 

[jay b]

hi there what type breaker is it on?

 

[pmgd]

Wylex RCBO B50 30mA NSBS50-B/1

 

[telectrix]

could it be start up current tripping the overload side of the RCBO? try fitting a C type.

 

[pmgd]

No - it cant be overcurrent because of the experiment I did where:

I have a 13A socket fitted near the RCBO and connect to it. If I pull the DP isolator on the annex CU it works here fine. Same RCBO, just with annex disconnected.

Purely from experimental observation, it is something to do with the interaction of the pump and the annex.

Anyone like my inductance idea?

 

[snowhead]

Anyone like my inductance idea?

No, Cumulative earth leakage is the answer.

 

[monkeyelectric]

Cumulative earth leakage looks the favourite for me too, have u a leakage meter?

 

[pmgd]

Snowhead: I agree that would seem logical. And I thought that for a while as well, however I cant square it with my observations. The total leakage of all annex connected equipment (excluding pump) is about 7mA. The leakage of the pump is zero (unless it is more at startup). The leakage of appliances on my ring main (where it works fine) I assume to be rather more than 7mA. *However - I will do this check to confirm*.

 

[pmgd]

monkeyelectric: I have an AC ammeter which I can insert into the earth lead of an individual applicance. I also have a device which does a similar job to a ramp tester. (It is a decade resitance box. Just keep adding in 1mA until it trips).

 

[lewlec]

Hi
Have you done any insulation resistance tests on the annex? Possibly N-E fault on a circuit, had this happen before and I know the these vsd's and rcds can be trouble some. If using a vsd with an rcd it should be of a b type really. Hope this is of use.
Regards
Gary

 

[monkeyelectric]

Looking at what you have done its either cumulative leakage or the startup current of the pump is overloading the rcbo when everything else in the annex is on aswell, you need a clamp meter really such as this TM-EL9809 High Resolution AC Earth Leakage Clamp Meter to test for both

 

[pmgd]

Thanks everybody for their help. I will go and do some more tests and repost the results in a few days. Hopefully they will show a smoking gun ... though I suspect not......

 

[Knobhead]

Given some of you’re test methods I think you would be better off getting an electrician in.

 

[pmgd]

Tony: You may be right - but I've done that twice .. and paid for 2 x 1 days rate with no appreciable advantage. If possible I want to be able to present any electrician with the 'smoking gun' so that he can take remedial action.

 

[monkeyelectric]

Tony: You may be right - but I've done that twice .. and paid for 2 x 1 days rate with no appreciable advantage. If possible I want to be able to present any electrician with the 'smoking gun' so that he can take remedial action.

To be honest if they were decent electricians it should be them finding the problem not you! What exactly did they do to try and resolve your problem in that day they spent looking?