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.

 

[davesparks]

Can you post a picture of the meter box so we can be clear on what switchgear is currently installed in there?

 

[Chris Wilson]

Can you post a picture of the meter box so we can be clear on what switchgear is currently installed in there?

I should be able to do that tomorrow, but basically incoming SWA service cable from underground to 100 Amp service fuse, then meter, then to a two pole fused switch and onwards to the consumer unit within the house. There are a couple of junction boxes in there, too, where outputs FROM the CU in the house splice into SWA cables to outside lights and to the garage. Thanks for your reply.

 

[Chris Wilson]

Here's a photo of the inside of the supplier's wall cabinet, the fused switch to the consumer unit inside the house has an 80 Amp fuse in it. The other fused switch has a 63 Amp fuse in it and serves a dedicated panel in the garage for a 63A socket for my TIG welder.

 

[UNG]

Looking at the wiring in that meter box I would suggest you employ an electrician as I doubt that the additions in that meter box have been ever been tested and parts do not appear to be compliant with the regulations

With a 2 pole changeover switch I would have concerns about the earthing still being connected to a PME system in the event of a power cut as it could be unreliable as an earth point and in the event of a fault could introduce fault currents into other properties located nearby

 

[pc1966]

That looks rough, and there are boxes feeding various bits that kind of look like no over current protection, etc. As @UNG says you really should be getting that lot tested and documented before considering anything new.

As a first point of principle you should get an isolator switch that isolates everything, so all of the various things are down-stream of that including any transfer switch. It is the only safe and above-board means of working on the supply side of any CU to fit or fix the transfer switch, etc.

You must have an independent means of earthing any generator as you cannot assume the supply earth connection would be intact in the event of a fault that made you need to use the generator. A generator of that size would still be a TN supply (earth and neutral common at the generator) but that needs to be referenced to the true Earth by means of a rod or similar.

 

[snowhead]

Althought the wall cabinet is owned and maintained by the property owner, the space inside is (supposed to be) reserved for the DNO and Energy supplier, apparently it's a condition of supply and is mentioned in their terms and conditions.

So if you are proposing altering any of what's there (excluding meter and fuse cutout) the simplest thing to do would be to move all the switch fuses to a new adjacent box and install the changeover switch in there.

If the meter needed to be changed to a "Smart Meter", either by request or the existing one going past it's use-by date or it failing, then I suspect the supplier would refuse to fit one as there would not be enough space.

 

[loz2754]

there are boxes feeding various bits that kind of look like no over current protection,

OP says they are fed from the consumer unit in the house.
But I agree, it looks very amateurish.

 

[Chris Wilson]

OK, thanks for then replies, I imagine various things have been added on over the years and it has become somwhat of a pig's ear in there. What sort of secondary box would be required, are the regulations about that? I would like to have one mounted and ready for an electrician to populate, with any cable gland holes drilled to suit. Do cables between two boxes need to be SWA, or can T&E or tails in conduit be used? The SWA cable feed to the garage would really need to remain as it goes straight down into concrete and I will not be wanting to start digging up the concrete...

 

[UNG]

You could fit a similar box to what you have now

You are being redirected...

They have a few different sizes available so you should be able to get one to fit all the equipment you need

 

[pc1966]

Cables in the boxes must have secondary insulation, so tails (double sheathed single conductor) or T&E are fine as it is not exposed to UV light or likely to be damaged by rodents, etc. You should take care that any holes have protection to avoid damaging cable going between them so grommet, gland, conduit, etc.

The SWA coming up to that adaptable box could be tidied up with a new box and cable exiting to the new adjacent meter-style box (assuming it can go to the right of current one). I would re-do the cable gland so it has the cover for the exposed armour in place, though maybe it is tight to assemble it given the stupidly short bit left out of the concrete!

 

[Chris Wilson]

Cables in the boxes must have secondary insulation, so tails (double sheathed single conductor) or T&E are fine as it is not exposed to UV light or likely to be damaged by rodents, etc. You should take care that any holes have protection to avoid damaging cable going between them so grommet, gland, conduit, etc.

The SWA coming up to that adaptable box could be tidied up with a new box and cable exiting to the new adjacent meter-style box (assuming it can go to the right of current one). I would re-do the cable gland so it has the cover for the exposed armour in place, though maybe it is tight to assemble it given the stupidly short bit left out of the concrete!

Thanks, that's very useful, will order a new secondary cabinet and call the sparky.

 

[Chris Wilson]

Resurrecting this as I have been away. The electrician rewired the outside supply box and added a second box with the two pole transfer switch in it. He wired a 125 Amp socket in the workshop tp plug the generator into. What I find is the generator makes power and although the house lights work, the consumer panel RCD trips. I think only the sockets are on the RCD as all the house LIGHTING circuits work OK on the generator.

The generator has neutral and earth bonded n the alternator box and it has the chassis grounded to a six foot earth spike near the machine. The outside electricity board box states the supply is a multiple earth system. What would cause a presumed earth loop that would trip the consumer panel RCD please? The electrician is away abroad on holiday. .... Thanks.

 

[davesparks]

He wired a 125 Amp socket in the workshop tp plug the generator into.

That should be a plug (aka inlet connector) and not a socket which is installed for the generator to connect to. The socket must be on the generator side of the connection.

What I find is the generator makes power and although the house lights work, the consumer panel RCD trips. I think only the sockets are on the RCD as all the house LIGHTING circuits work OK on the generator.

Does the changeover switch isolate the whole installation or does any part of the installation remain connected to the grid supply when the generator is in use?

 

[Chris Wilson]

That should be a plug (aka inlet connector) and not a socket which is installed for the generator to connect to. The socket must be on the generator side of the connection.


Does the changeover switch isolate the whole installation or does any part of the installation remain connected to the grid supply when the generator is in use?

The changeover switch is a 2 pole break before make affair, So live and neutral are switched. Gennie ground is permanently wired to mains ground in he service provider box. Thanks for your interest.

BUT, I have now found this out:

OK, nailed it I think to the consumer unit in the garage. House consumer unit has a 32 Amp MCB that I was TOLD was for the garage, and is marked as "Garage". With that and ALL other MCB's off on the house CU as soon as i turned the main house consumer unit isolation switch on the main RCD tripped. If I turn the master switch on the garage consumer unit OFF all is OK. If the garage CU is turned on, even with all the MCB's OFF, the house CU RCD trips and it's a 100mA one.

I just need to get my head around how the housebuilder wired this stuff up, and why, with all MCB's OFF on both the house and garage CU's but the main CU switches ON, and the garage CU master switch on, it trips....

On normal mains these all work as expected.

Anyway the gennie runs EVERYTHING in the house and workshop, including a rotary three phase converter and all the workshop stuff without even getting to half its load capacity, which was the main purpose of the installation test


One MCB in the garage consumer unit didn't like the inrush to an air bottle compressor and has had repeated resets over the years, maybe that MCB is knackered but I can't think why it would cause a Live / Neutral imbalance which is what should trip an RCD, surely?

Things may be clearer tomorrow whilst I get to grips of it in my mind. <LOL>

 

[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.