UK Generator connection to mains supply questions

[Chris Wilson]

I want to use a manual change over switch to safely connect a 21kVA single phase tractor PTO powered generator to my mains supply in case of power outages. The cable run from the generator to the outside mains incoming box will be about 40 meters. I intend to use 16mm SWA for that run. The generator control box has metering, fusing and an RCD. It's modern and made entirely by Mecc Alte. Not some Ebay Chinesium device.

Once the SWA is at the outside wall box where the underground feed from the supply company exits and the meter and master switch, 100 Amp utility company fuse is located, can I delete the two pole fused master isolation switch and use the two pole 125 Amp break before make change over switch and separate fuse as both an isolation switch to the house distribution panel and as a change over switch? I see a similar question was recently covered, but it was unclear as to whether doing this was advised against or regulated against.

Space inside the wall mounted outside box is limited. The alternative is to bring tails outside the box, to another water proof cabinet with the change over switch in, and return the cables back to the utility company box. Messy...

The property, a domestic bungalow, and has a PME earth system, how should the cabling from the generator be wired with regard to earthing at either end?

Thanks.

 

[pc1966]

What would cause a presumed earth loop that would trip the consumer panel RCD please?

If the change over switch changes the N, as it should, then you should see no difference and no danger of circulating N-E currents.

 

[pc1966]

I can't think why it would cause a Live / Neutral imbalance which is what should trip an RCD, surely?

Have you verified the polarity is correct when running off the generator?

If the incoming L & N are correctly swapped between sources there is no adequate explanation for this problem. If they are actually reversing L & N polarity (even if down to the cable feeding the 125A plug) then you might have strange things happen, and it is VERY DANGEROUS.

A socket check tester in one socket that is working on both mains and generator ought to reveal if this is a problem or not.

And yes, the electrician who installed it should absolutely have checked this, but the problem you are reporting ought to have been caught as well.

 

[Chris Wilson]

I have such a socket tester somewhere, I'll find it and check tomorrow, thanks very much!

 

[pc1966]

It is also just possible that you have a N-E fault that is not apparent on the mains TN-C-S supply due to the very short neutral to the common point, but it becomes enough of a problem on the generator TN-S with a long neutral path back to the common N-E point.

Again, this ought to have been tested, a global IR measurement would show that up. Though if anything has been changed/disturbed after the changeover switch, etc, was added then it might not have been present.

 

[Chris Wilson]

It is also just possible that you have a N-E fault that is not apparent on the mains TN-C-S supply due to the very short neutral to the common point, but it becomes enough of a problem on the generator TN-S with a long neutral path back to the common N-E point.

Again, this ought to have been tested, a global IR measurement would show that up. Though if anything has been changed/disturbed after the changeover switch, etc, was added then it might not have been present.

Thanks again for the reply pc1966

My best pondering is done with a few pints in front of me and last night's session sat with dogs outside in the lovely evening sun gave me some good pondering time

What I believe is happening is there's a neutral to earth leakage in the garage after its consumer unit. The reason it is tripping a 100mA RCD in the house consumer unit only when the supply is from the generator and not from the mains is our mains voltage is rather low, under 220V a lot of the time.

The generator automatic voltage regulator is set at 245 to 250V (I need to check with a calibrated digital meter as the panel analogue gauge is probably none too accurate). I'm thinking Ohms Law is causing a higher current leakage at the higher generator voltage and it's then enough to trip the house consumer unit RCD.

What I really need is an AC low current clamp meter, but I don't have one and decent ones are expensive, so I'll have do it peasant's way and start disconnecting neutrals one by one in the garage consumer unit until it goes away. I'm suspecting an outside alarm PIR that played up then failed a couple of years ago. It may have water ingress. But we'll find out! What fun.....

I suppose I could lower the AVR voltage on the generator to say 200V and see if it stops tripping, but an excessive neutral to earth leakage needs fixing anyway. I'll try and do it properly.

I can see me spending hours in the sweltering and cramped loft space above the garage again, and I'm not as nimble as I was!

EDIT:

It's a fluorescent light circuit in a workshop fed off the garage consumer unit. I used a Megger to test every separated neutral off the busbar until I found one with 400 or so Ohms to ground. Of course it's the lights in my workshop with a car lift in it, so they are hard to get to "up in the Gods", but at least I now only have eight fluorescent fittings to check. Running all on gennie power as I type

Thanks for the help. Anything I may have missed or misunderstood please let me know but we've been on generator power, house and workshops, running all and anything for two hours with no issues.

 

[pc1966]

Hopefully the N-E fault you have found is enough to explain things, though to trip a 100mA RCD you would need > 50mA and at 400 ohms that is 20V N-E which is really quite high for any sane cable voltage drop that you would plan (e.g. 5% is 11.5V and if L and N cables identical, then under 6V drop expected along N to appear N-E).

But it needs fixing anyway, so one done check and see if it has solved it.