RCD Tripping on AMF switch-over to generator supply

[qwest]

Hi, I'm looking for some help and hoping I might find it here.

I am a house owner with a 10Kva stand-by generator (40Amp / single phase). I had the generator professionally installed by my local electrician but I supplied the Automatic Mains Failure switch and generator.

The set-up works perfectly with one issue - there RCDs trip. Given that the backup is supposed to be fail-safe this is far from ideal. The electricians that carried out the install have never worked with this type of equipment before so are not able to support the problem.

The generator is spiked to earth. The utility supply is PME. The PME earth is permanently connected to the generator earth spike so I believe the earth should be common to both generator supply and utility. There is no switching of earths.

The generator supplies the AMF panel through a 100mA RCD located at the generator.

The house is protected by a 100mA RCD at supply to the Consumer Unit (why it isn't 30mA I don't know). The consumer unit in this case is old (and may not actually be a consumer unit - I am not sure of the correct terminology) it doesn't contain RCDs - they look like RCDs but just break on current rather than earth fault.

The fault occurs on main failure (which my AMF panel can simulate). The generator starts up, stabilising for maybe 10 seconds and then the contactors switch over to generator supply. When this happens one of 4 situations occurs:

1. It all works as it should
2. RCD at gen trips
3. RCD at CU trips
4. Both CU & gen RCD trip

Some things I have noticed when trying to trouble shoot:

1. It isn't load related. There must be some load (it I turn all the breakers to off the fault never occurs), if I leave just one or two low current circuits operational then the frequency of RCD tripping remains the same as if full load.
2. I don't think there is an earth leak as once the RCDs trip and I flick them back on there is no recurrence of the problem.

I am now lost as to how I can trouble shoot further. I am not qualified as an electrician but have a reasonable understanding of electronic engineering. I don't particularly want to do this myself but I have struggled to find an electrician that can help.

I've seen a very similar thread here but couldn't find it a second time round for some reason - similar issue to what I have was discussed, alas with no actual diagnosis of problem.

So if I can get any indication of what to try next here that would be a huge help! Thanks.

 

[Darkwood]

Contact the suppliers of the gen' they will be aware of all issues customers will have had and possibly simple solutions to rectify such problems.....

Im a little saddened to hear your electrician has left this issue to you... has he tested the installation, monitored for earth leakage existing etc...there are many steps he can take to rule out several causes....but as an electrician doing the same job for you even without experience of the gen' system id personally put it on myself to ring tech' up and discuss possible issues they may know that cause such problems.... have you tried the mcb's one at a time to see if any particular circuit is the cause or whether this still occurs regardless????

 

[qwest]

Hi,

Electrician had a limited role. I did a bit of the cabling and donkey work. Just wanting to keep costs low.

The gen is about 6 years old and is being re-assigned this task. The background to this is that it isn't necessary - I had the gen spare and it was nice to have the back-up power as we have frequent power cuts 4 to 6 a year affecting things like heating, electric gates to the house and the fridge etc.

It isn't worth spending the £15k to £20k to do it with new kit specialist supplied and installed. But given I could get an AMF for £1,000 and the only other thing needed was some chunky copper I thought it would be simple - largely it was but this is a bit of a niggle I'd like to resolve.

I don't know what kind of earth I have on the gen - I think the neutral is tied to earth but not sure how to check - or even if that is accessible to check. The gen was originally in a truck and earthed to the truck chassis. That was professionally installed and it was used to power a range of things from lighting to computers for a mobile sales office.

I don't think I have MCBs - not the type that detect an earth fault. Just one RCD on the utility / gen supply to the bank of breakers for the individual household circuits and one RCD at the gen.

Switching any combo of breakers off in the "consumer unit" makes no difference. They all need to be off and then there is no issue - so I am confident there isn't an earth leak on the household circuits or appliances. I am also confident that this isn't an influx current from every circuit being switched on at the same time. Any load on any circuit could result in the RCD tripping on switch over.

The suggestions are appreciated though.

 

[Darkwood]

With so little info we can use here its going to be hard to suggest anything ....only an onsite visit by an Electrician experienced with Gen' systems is your best bet ..ive done a few myself but couldn't even contemplate throwing wild guesses at you ... Safety devices are tripping here and their may be a genuine reason maybe linked to the set-up...... without investigating first hand and testing the install i can't comment.

So my advice holds you will need to get in a competent Electrician or Engineer in to check out the install.... if its set-up incorrectly you or your family may be at risk.....

Your cutting down on costs here and doing a lot of the install yourself yet 'no offence meant' you seem unclear, vague to the gen set-up which in my experience is asking for trouble .. a little knowledge is a dangerous thing regarding electrics... and as you freely admit the electrician wasn't experienced with the generator he was in no position to be doing the job or advising you whether it was all done correctly.

I would be totally unprofessional if i wasn't to say disconnect it and get a experienced pro' in before it is used ....its your family at the end of the day and an area not worth the risk of taking shortcuts.

Try not to see it as a dig at your work or anything but your general vague responses means its crucial the work is checked over when using generators its crucial they are set-up correctly in line with the existing installation.

 

[Marvo]

Is the DNO neutral being isolated as well as the live before the contactors connect the generator supply to the CU on power fail? From you explanation I'm immediately worried about how this generator may have been connected and whether both poles of the DNO supply are being isolated.

Can you upload some photos to a photobucket account (or similar)? Photos of the AMF switch wiring and photos of the DNO head with the main earth terminal also photos of the CU with the cover on and the cover off.

I'd also emphasise Darkwood's view that there's lots that can be wrong with a back-up generator install, incorrectly installed generators can be lethal to the homeowner and also to DNO linesman so a professional electrician would be the way to go.

 

[qwest]

Not sure what a DNO is.

But from your question I guess you are asking what is switched within the AMF.

Within the AMF There is a relay that can put a PD across the energising circuit of one of two contactors. Never both. One contactor carries the utility L and the utility N to the CU via the household RCD. The other contactor carries the Gen L and Gen N to the CU via the same RCD. Each contactor is 100 Amp (I assume this is a continuous rating). The main RCD is 80amp and the gen is 40amp. So the contactors are more than adequately robust - and sound like a couple of bricks being clapped together when energised. They are both normally open.

The earth in both cases are common and not switched.

To this extent this install is competent. - No danger to linesmen since it is not possible to energise both contactors simultaneously.

There can be no risk to the household - that is double protected by RCD and the gen produces the correct voltage. At no time is anything unearthed and the house now has its own earth spike.

The system is all low voltage - there is no PD above 315 anywhere.

The only element I took care of was wiring the 12V system for auto start sequence on the gen and a couple of wires that determine PD on utility and gen circuits to allow the computer to determine the sate of supply and whether to start or stop the gen and what to switch.

Everything else was electrician installed but since I supplied the AMF and the issues could be with that then they won't support. I can accept that.

My suspicion is that on switching there is a momentary imbalance causing the RCD to trip or that the additional earthing is causing an issue - but in my view it is necessary as with PME it may be possible for a total loss of earth if there is a fault on N upstream somewhere. This would then leave my gen supply with no earth. So for that reason I took the decision to earth spike the gen.

I have measured the PD on L to E and P to E and L to N on the gen and there is nothing to be suspicious of. So I can only think that it is something momentary or some issue with the Earth.

 

[qwest]

^ DNO.... Distributor Network Operator.... just looked it up!#

Yes, any time I am making power they are isolated. Not interested in lighting up the rest of the street!

 

[Marvo]

Where is the power coming from that operates the contactors in the AMF panel? Reason I ask is maybe the initial inrush current of the contactor coil would trip the RCD if it's wired to a non-RCD neutral but once operated the lower holding current through the coil is below the RCD tripping threshold.

 

[qwest]

Utility powers contactor for mains.

Gen powers contactor for gen.

So this theory could be right. There is a chunky contactor operating with inrush current. 100mA is already below spec for a domestic supply. Is there some other type of RCD that may be less prone to this that I could try?

What can I do about it if I am stuck with current RCDs? That is in the design of the AMF panel so a little more tricky to resolve.

I think many AMF panels are a similar design. I thought this was an industry standard so I can't be the only one if this is the issue.

This could also explain why I see the same issue (RCD tripping) on return to utility supply though so the theory sounds good.

 

[Marvo]

Don't replace the RCD to work around the problem, that's treating the symptoms, rather find the cause. I wasn't questioning the general design of the AMF panel I was wondering out loud if the controls may be crosswired some way between an RCD live and a non-RCD neutral for example. To be honest you're unlikely to get your answer from a forum, it's the kind of problem you need an experienced electrician hands for an hour on to get it localised.

 

[qwest]

I suspect you are right. I will try to find someone with the right experience.

I like a challenge and would like to think I could figure this out but reached the end of my patience.

The actuation of the contactors can't be cross wired otherwise under power cut there would be nothing to actuate the gen contactor and the gen would have to be running to get utility supply.

 

[Marvo]

You obviously have a good understanding of the system you have and I'm sure you can work safely but if you're banging your head against a fault and you're at the end of your capabilities there's no shame in getting assistance and saving yourself a lot of gray hairs. The cost won't be prohibitive for an hour or two of time, your biggest problem is finding an electrician who's suitably experienced. If nobody offers their services in this thread I'd web-search for a commercial electrical company that specifically offers back-up power/UPS/generator services.

 

[Engineer54]

Oh Dear, .....this sounds like a real old mess you have here!! First and foremost, does this second hand generator have an AVR (automatic voltage regulation) included, as most genest's of this rating do not!! If it doesn't then i'm afraid your're generator is unsuitable for domestic appliance use, where those appliances include the use of electronics!!....

The earthing arrangement is also a very important aspect to any generator installation, has the alternator neutral been suitably grounded to the frame via an external earth rod/electrode arrangement etc. Somewhere along the line, it seems by the sound of it you have mixed the house and generator earthing systems up. You really do need to get an electrician in to check over this installation, and one that actually knows what he's doing!!

 

[qwest]

Gen has AVR - earth isn't mixed up - I don't think.

we have a PME here so N tied to E at various points upstream.

gen has N tied to E and its own Earth so I have added a further earth - I could switch the earth so the gen and house earth are independent but I can see no reason why this should be necessary. Can a PME also be staked to earth? The electricians did this and I can't come up with a theory of why this would cause an issue.

gen frame, engine, chassis etc all on a common earth too.

Despite the problem the project was fairly well researched.

all sensitive electronics in the house are behind a UPC so AVR.

the AVR produces a more stable supply than the utility from my very brief sample. Any AVR isn't going to be able to fix the momentary drop off when switching to load - they'll all drop a few volts. In any case the issue isn't stability related so far as I can tell.

 

[Engineer54]

You should not be mixing the PME earth with the generators earth rod earthing system.
It's fine to add a rod to the PME earth (and is standard practice in many European countries) at the house MET, but not to then join that earth to the generator earthing system. The generators earthing should be for the generator alone. A separation distance between any PME rod and generator rod should be at least 3 metres distant (more if a deep driven rod) from the generator earthing rod(s) House earthing to be left in place from the change over switch back to the house CU.


Don't forget here that on PME systems (in effect an artificial earthing system) there is a Neutral earth connection at the incoming cut-out....

 

[Darkwood]

Ill reiterate what i said before fully disconnect and get a knowledged competent person in ...Electrician means nothing nowadays with all the short courses and fast track qualifications so ensure who-ever you get is fully experienced with gen' set-ups...You could always fly E5 over and ensure a quality experienced knowledgeable sparks or taxi Marvo over as very good advice has been given ... anyone else you will need to vet them and ensure they aint just your routine sparkies!

As i said before you clearly have a bit of grasp of electrics but i fear not enough for this situation and as you have realised you maybe at risk here ...... a little knowledge is very dangerous sometimes!

 

[Engineer54]

gen has N tied to E and its own Earth so I have added a further earth- I could switch the earth so the gen and house earth are independent but I can see no reason why this should be necessary.

So what do you call this, if not mixing the earthing systems then??

 

[Darkwood]

So what do you call this, if not mixing the earthing systems then??

Suicidal :shame:

 

[Knobhead]

With PME you need automatic earth transfer to safely use a generator. This gets messy and very complicated, if you try to do this on your own.
You can’t use the PME earth for your generator.

You say you now know what DNO means, I think they would love to see this set up. It is downright bloody dangerous and they will remove you’re supply and PME earth immediately.

So you have two options, scrap the generator or employ an engineer that knows what they are doing.

No I’m not offering my services:
YOU CAN’T AFFORD ME!

Personally:
I think this is a very well put together Troll thread. A knowable Troll I’ll grant. There are far too many inconsistencies.

Biff!
It’s time to doff you’re cap!

 

[qwest]

I had guessed the earth may not be set up correctly.

The gen spike tied to the N at the gen is on good copper and is as close to the gen as possible - about 1m away. It consists of a spike about 1.5m long.

What I don't quite understand is what additional risks or benefits are involved in switching the earth system.

Currently because the Utility N is switched then so too is the utility E,

So on gen supply the earth is entirely to the gen spike. There is no utility earth.

On utility feed there is both the gen spike and utility N tied.

Now I could switch out the generator earth but I can't see that being the issue.

Incidentally the earthing system to the gen is on really heavy gauge copper - looks like three core car battery cable - the gen earth was tested by my electrician and they seemed to think it well within spec

I don't have test kit myself beyond a little meter but I have checked if there is any pd between the utility E and the gen E (when the two aren't connected obviously) and there is no pd.

What is interesting is that there is a regulation or best practice in having utility spike and gen spike a set distance apart.

My guess is that this is to prevent a potential backfeed to the grid from one earth to the other - so if a gen fault creates a current to earth it doesn't then find its way back into a nearby earth spike along the utility to a lower resistance E.

The is no chance of that happening since the only earth spike in on the gen.

So what would I gain by switching earth systems? I would gain a momentary period where the entire house has no earth at all.

Am I at risk from a utility fault? Potentially I could now be earthing the grid if their E was not as good as mine?

The AMF I have is 3 phase capable but I'm on a single phase supply so I can easily switch the earth using spares on the contactors but really not sure what I'd be gaining.

Either that or add a furthe spike and earth the PME again?