Coupled with the time delay discussed, breaking the neutral on the downstream final circuits will be important to achieve desired selectivity on N/E faults. 

Thanks Andy, Yes we have installed a changeover switch and a large separate enclosure with Din Rail terminals large enough to take up to 185mm conductors. When there is a power outage a generator will be delivered by the hire company & connected up to these terminals by their operatives. 

The building itself is a  butchery (the whole reason for the temporary generator is to keep the meat within the fridges / freezers supported during any extended power outages) 

We have a mixture of 3 phase DBs with each have existing final circuits protected via a combination of MCBs (With no RCD protection) & 30mA RCBOs on some circuits. 

So providing we get a resistance of lower than 20 ohms - Than I'll probably do as that & inform both the client and generator hire company of the resistance and leave it up to them. 

Many thanks for everyone's insights - very helpful as usual!

Thanks,

Tim

(the whole reason for the temporary generator is to keep the meat within the fridges / freezers supported during any extended power outages) 

I think that would make me keener to ensure some level of discrimination on faults - it might defeat the purpose somewhat if a minor fault in a light fitting or some such was only disconnected by the generator's earth fault system - so taking the whole installation out. Maybe a few extra RCDs might be in order.

   - Andy.

Best of luck with 20 ohms! 

No problem with 20 ohms unless you have stone, chalk or gravel in the ground.   Use a 900 x 900 earth mat 600mm down and a bare 70mm copper earthing conductor. 

JP

Or to turn that round, OK with wet clay, peat or salt marsh ;-)

Best I've ever seen in UK personally was Essex clay, and about 40 ohms on a skinny 2ft earth rod. Round here (hants, glacial outwash with lots of gravel) a single 4 ft rod is more like 120-150 ish, so you need a  few.

The RCD protection can be designed to cope with ADS with generator supply but what about your point Andy on how to deal with L-N fault where genset can't provide enough energy to clear submains fault in less than 5 seconds does this not create fire risk issue? Then only way to counter this is to oversize gen set?

what about your point Andy on how to deal with L-N fault where genset can't provide enough energy to clear submains fault in less than 5 seconds does this not create fire risk issue? Then only way to counter this is to oversize gen set?

Simple answer to that one is to ensure that In ≤ Iz throughout - the conductors should then be thermally protected no matter how long it takes to disconnect (5s is a nice to have and makes adiabatic calculations easier but really isn't a safety issue for faults between live conductors).

   - Andy.

Right chaps/ladies- this seems to be an appropriate place and thread to get educated better in my understanding. 

Please may someone help me and my understanding of generator earthing requirements. 

I don't do Generators all day every day, but periodically do for construction site set ups and temporary supplies for construction site compounds. This is not my day in and day out job, but not uncommon either. 

So most Generators larger than 10KVa arrive as standard, here in the UK as a TNS system. I believe that is standard operating procedure around here.

So far all mine have. Hired from Large national hire companies.

So the generator runs and operates as a TNS System. Earth fault path is a TNS system. 

However, I understand if the Generator is NOT earth staked, the neutral and earth systems, because they are common at the Genset - can float up and down with reference to terra firma - the ground next to the Generator - and RCDs don't know when a fault is occurring as this floating value confuses the RCDs......, yes?

All of my generators have come with a earth fault relay RCDs that I'd normally set up, (I commonly call these "viggi units") as described in the above sort of way, with a 300mA 0.25 second time delay and cascading RCD settings out from there.

Clearly, this needs to have an earth reference point to tell the RCD what value earth is. Of course it does - no dispute there. 

So - I think I'm all on track so far......

Now, to the earth reference point; the earth stake, matt, or commonly for me, on railway sidings or construction sites out in the country side where a metal fence has been installed, a metal fence that is "owned by" and managed by us, and I have permission to use - then I'd use the metal fence, I like a metal fence................... and its required value in ohms.

So, hypothetically,  lets pretend that the generator was laid on a glass slab, or floating in the air - no contact with the ground at all, and the earth reference point (The earth stake hammered into the ground and connected to the Generator earth point for example) is effectively an infinite value as an earth fault path back to the Generator, but it does provide the earth part of the generator with a reference point to what the earth value should be. I understand that the earth fault relay RCD "viggi" unit would function perfectly well.

So to my question - why should we try to achieve a 20 ohm value for this earth reference point - its just an earth reference and not an earth fault path - surely? 

(Obviously I'm not talking about a TT system here - I understand the need for that earth stake to be less than 200 ohms and all its difficulties, I think)

If the generator earth reference point was 200 ohms or 2000 ohms, would it make a difference? I don't think so.....

I feel an ohms Law coming on as the explanation here.........(V=IR maybe?)

But it's not a fault path, just a reference point........Education required please.

If i stuck a good old metal shaft screw driver in the ground it'd (The RCD part of the Genny) probably function perfectly well surely?

an additional read for interest   electrical.theiet.org/.../