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?

 

[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.

 

[pmgd]

Def not the pumps fault as if plugged into a totolly seperate building - or even the main CU of my own - it all works fine. I cannot get over thinking it is somthing to do with the earthing of the annex. However - I will read your essay - slowly and soberly tomorrow darkwood. Thank you VERY much for taking the time ....

 

[Knobhead]

Can't you get it in you’re head:
Without the VSD the pump runs OK. Using the VSD the supply trips!
VSD’s have an inherent leakage, if you look through the literature for it you will find it!

 

[Guest55]

i like how tony answered the query perfectly well in post #4 yet has been completely ignored by everyone else since lol

Here T , have a facepalm on me........

 

[Knobhead]

 

[pmgd]

Tony, biff55: I'm afraid that I disagree.
Yes VSD do present a challenge. I have researched them on the net quite a lot. I talked to the manufacturer yesterday.
The challenge they present appears to be due to the particular waveform they generate and perhaps common mode noise. NOT simple leakage.

Also - the pump works fine in a different socket. Doesn't that rather suggest that it is the socket at fault not the pump.

Also - I've had nuisance tripping in the annex for a year - apparently related to nothing we could identify. This was when the VSD did not exist! -- but a bore hole pump (regular capacitor start) did. Clearly something is wrong with the supply, we are just using the VSD to locate it.

So - NO I am not just going to swap out the VSD. I want to fix the supply.

Yes - I suspect giving the pump room its own (non RCD) supply will fix it. However - as I already said that is extremely difficult. Although I may be forced to spend the money eventually - I'd prefer to try without.

 

[snowhead]

What is in the pump room that must be on an RCD?

 

[pmgd]

17th edition regs unfortunately. It is fed with T&E which is behind plasterboard and within 3cms (?) of the surface and is not otherwise mechanically protected. Unfortunately!!!

 

[Guest55]

17th edition regs unfortunately. It is fed with T&E which is behind plasterboard and within 3cms (?) of the surface and is not otherwise mechanically protected. Unfortunately!!!

aah , bad design then.
you should design out the need for rcd's for equipment that have high leakage current at the installation stage.
your problems have then been further compounded by the selection of inappropriate protection devices.
back to the drawing board me thinks.

 

[Darkwood]

Tony, biff55: I'm afraid that I disagree.
Yes VSD do present a challenge. I have researched them on the net quite a lot. I talked to the manufacturer yesterday.
The challenge they present appears to be due to the particular waveform they generate and perhaps common mode noise. NOT simple leakage.

Also - the pump works fine in a different socket. Doesn't that rather suggest that it is the socket at fault not the pump.

Also - I've had nuisance tripping in the annex for a year - apparently related to nothing we could identify. This was when the VSD did not exist! -- but a bore hole pump (regular capacitor start) did. Clearly something is wrong with the supply, we are just using the VSD to locate it.

So - NO I am not just going to swap out the VSD. I want to fix the supply.

Yes - I suspect giving the pump room its own (non RCD) supply will fix it. However - as I already said that is extremely difficult. Although I may be forced to spend the money eventually - I'd prefer to try without.

As i pointed out with a solution that keeps rcd cover ...

 

[pmgd]

Darkwood: Many thanks for the long explanation yesterday. I've now read it in detail. A couple of points:

1. FYI an adjacent property had a problem whereby use of a large flymo in house A cause RCD to trip in house B. (Mine is house C). A little like the scenario you mention.

2. You say (imply?) that there are clever RCBOs available which 'recognise' the inverter maveform and discount it. If so - please can you point me at a spec/partnumber etc?

3. Electrician coming next week - will do all the tests. Meanwhile I have just received a clamp meter and will do some more accurate measurements myself.

4. "Vetted". Well Chartered Engineer & elecSa approved is all I know.

 

[Darkwood]

Darkwood: Many thanks for the long explanation yesterday. I've now read it in detail. A couple of points:

1. FYI an adjacent property had a problem whereby use of a large flymo in house A cause RCD to trip in house B. (Mine is house C). A little like the scenario you mention.

2. You say (imply?) that there are clever RCBOs available which 'recognise' the inverter maveform and discount it. If so - please can you point me at a spec/partnumber etc?

3. Electrician coming next week - will do all the tests. Meanwhile I have just received a clamp meter and will do some more accurate measurements myself.

4. "Vetted". Well Chartered Engineer & elecSa approved is all I know.

Regarding your point 1 that's what I briefly suggested earlier regarding proximity of transformer but with your added info regarding neighbours then this is more than likely the cause... ive heard of this problem before with neighbouring properties creating nuisance tripping in a different building, its possibly caused by issues between the properties using a common transformer that's in close proximity the exact cause was never identified but because it may be a neighbouring installation or the DNO's issue it gets very hard to resolve.

Point 2- Schneider do an RCCB in their SI range that's designed to recognise the generated earth leakage from power drives, computers, electronic lighting etc ---product code 25524 - this is a 40amp double pole RCCB unit so it would be fitted in a separate enclosure and doesn't provide overload protection so you feed it from a standard mcb then connect load to outgoing side.

Point 4.... let him peruse this thread, its not common knowledge this issue your having so may head him in the right direction. Note this hopefully will eliminate the inverter causing tripping but may not stop the other nuisance tripping issues you had although it does filter stray frequencies and may stop the nuisance tripping caused due to neighbouring installations and proximity of transformer but I cant say if it will or not.

 

[lewlec]

Hi
Just wondering did you get any IR tests done?

 

[pmgd]

Hi All,

well I did have an IR test scheduled but I have just cancelled it.

What I didn't mention was that the annex in question is rented out as a holiday home - and I really would prefer to not disturb them. I'll wait until november when we may have a spare week.

Also, I followed advice here and purchased some "proper" test equipment. Specifically a min/max clamp meter.

I discovered that the Variable speed pump has a max of 35mA at startup, and 20 thereafter. Not sure why my ammeter in the earth line didn't notice this, but there you go.

I have two solutions at the moment - both rubbish, but they work:

1. Connect the whole of the pump room to the other property - and show the problem doesnt recur .. thus blaming the pumps.

2. Replace the RCBO with an RCD and MCB. Logically equivalent, but I have experimental observations which says one blows and the other doesn't. [ I do notice the RCBO has an earth lead which according to literature enables it to 'blow faster'. ]

I still have a very funny earthing system.

There is one more thing I'd like advice with however:

With the new clamp meter I measured the current in the earth spike lead. Small 3mA. OK. But then I realised that my property and the property next door share a gas supply. The supply comes in copper into each property. In accordance with best practice installers have bonded these incoming copper pipes to the local earth.

To my mind this then creates more opportunity for ground loops etc etc. I measured the current in the gas bonding line. It was non-trivial. Varied but perhaps 10-20mA.

Any comments?