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!

 

[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?

 

[pmgd]

They did ramp tests of everything. They did various Zs tests. They discovered I have 1V between earth and neutral and pondered that for a while. They tried disconnecting things. They changed out the RCBO and then back again. The last one was just about to do insulation resistance tests - but ran out of time. If I can't find anything, then that will be the next thing - but I'll get him back to do that.

Have ordered the meter and will do measurements. However - have done the following experiment FYI:

in the property which does NOT fail. Completely empty ring main. 30mA RCD (blows around 23mA). Runs pump just fine.
I added my resistance box and loaded up 21mA of additional earth leakage before the RCD blew.

Seems to me to confirm that the pump itself doesn't have more than a few mA of leakage.

Also talked to the pump manufacturer. Apparently it is a soft start inverter. It has a start-up current of about 1.5x for a few hundred mS. It is rated at 0.9kW.

 

[Darkwood]

You supplied a little piece of info no-one seems to have picked up on ..... where exactly is the mains transformer is it nearer the barn by any chance than the house???

 

[pmgd]

Interesting question: It is about 60yds from the annex but about 40 from the house. However electrically it may seem rather nearer. Let me explain.

Transformer 3 wires (can clearly see L, PE, N) going 30yds to a post where the armored cable attaches. Then about 40-50yds of cable to the main supply fuses.

The direct line of sight from the earth spike in the annex to the transformer earth is definitely shorter

 

[Darkwood]

This could be down to a loose connection to the annex but only shows under heavier loading or inductive loading hence your motor, get the electrician to check all consumer side connections are sound, try plugging an heavy load into the offending circuit and see if this too causes similar tripping to re-enforce the theory.... all bus-bar links etc need checking also DNO connections could be and issue too, making a temp set up of the tails to the annex using the rcbo that trips this will eliminate any issues within your fuse board but obviously a competent electrician needs to do this... there is another possible cause but il reserve bringing this up so not to over complicate things at the moment.

 

[pmgd]

Darkwood: I will try the heavy load (eg 3kw heater). I will put the checking all connections on the electricians list (as well as an insulation test). Please can you explain the temp set of tails test? Just so as to utilise his time well.

Meanwhile - you really shouldn't keep us in suspense! What is your other suggested reason?
I will have to pay for most of the day with the electrician anyhow, if there is anything else to try I'd like to get it on the list before he arrives.

 

[Darkwood]

All i meant was fitting a temp supply and tapping direct of the tails if install permits then using this to feed the suspect rcbo with a socket attached... this will eliminate all the installation.... not all faults will show in tests... An insulation test has no real load but the fault could be only apparent under load . Again a competent electrician to do this but little surprised it hasn't been tried already..... if this still trips then the problem may be DNO side or even natural transients from the close nature of the transformer to the property hence i asked, this can cause nuisance tripping with loading but will require discussions with DNO.

Lastly if the motor has a drive then it will generate natural earth leakage which if small will probably be well below the trip threshold of the rcbo ... the issue is that inverter earth leakage isn't like natural earth leakage in that its variable in its waveform and frequency.... different rcd/rcbo's may react differently to this type of leakage if you can ask to have a rcbo of a differing brand fitted produced from a different manufacturer , if this clears it its a conflict of the noise dump to earth from the inverter. This can be overcome by wiring a circuit dedicated an instead of using a standard rcd/rcbo you use a SI unit which recognising the waveforms and frequencies generated by the inverter and ignores them and only looks at true leakage... having an inverter running through a RCD/RCBO should be avoided as it can cause an effect known as blinding where the device can actually fail to operate correctly in a true leakage situation while the drive is running... its unlikely the Electricians you employ will be clued up on this as it rarely an issue in domestic but you will be teaching them something i suspect ...... im trying to avoid over complicating this issue but these are real known causes but uncommon in domestic but as you said you have had Electricians scratching their head and if we are to assume their competence then the unlikely starts to become likely ..... i assume you have vetted the electricians before choosing them.

 

[lurch]

had a similar issue in reataurant. CU had a RCD and MCBs. Any nuisance tripping was knocking the RCD and whole restaurant out. I removed the RCD and replaced the MCBS for RCBO's. Every time the pump for the bar kicked in the rcbo tripped. I was luvky enough to run an swa in and put on MCB.

I know this doesnt help much but do feel motor circuits on RCBo's can be an issue sometimes.

 

[Darkwood]

At the end of the day its a pump and prone to water ingress eventually .. in the OPs case maybe borrowing a pump or hiring one and seeing if its has similar issues will be a cheap option to narrow the cause down .... but I see this dragging out mainly because im not faulting it ..... smug look lol

 

[peterb]

Does the pump have a capacitor start? I've seen faulty caps causing excessive earth leakage on start up which goes as the cap switches out as the motor comes up to speed.

 

[pmgd]

It is an interverter driven motor. Motor is three phase. Phases developed from an inverter. Speed is controlled from adjusting the frequency. HOWEVER, I am luck that this shows the fault every time. Before I had this puimp I STILL HAD THE TRIPS - just completely randomly. I assume this was due to a bore hole pump motor. That motor does in deed have a capacitor start. However, it has been replaced recently and the fault pre - and - post dates the change out.

Hence my belief it is somthing inductive.