Complex motor start cuasing RCBO to trip problem

[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.