Earthing, with a generator...

[Sparks]

Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a TN-S installation (Earth supplied via the armor on the mains supply) then if you install a generator, you cant rely on this earth. - seems sensible, as the fault that has taken you off supply could be because your cable has been cut.

So you have to install an earth rod if you have a generator connected to the installation with a transfer switch (Rather than if you just run things from the generator from an extension lead).

Now where I am unclear is, I understand you have to switch the earth connection when you changeover to the generator, you can't have both the TN-S earth and the rod connected to your earthing system at the same time. Why is this?

Surely the bonded water main would act like an earth rod in a TN-S supply anyway?

Or is it just you must disconnect the suppliers TN-S earth when running the generator - and if so, why?

Or am I barking up the wrong tree here?

Ta!

[Tim..]

I can't fully explain why, but if you are using a generator through a change over switch then you must convert to a TT system with a local earth rod, and have the generator floating N strapped to E.

Tim..

[Sparks]

That's fine, but does this mean the local earth rod MUST be disconnected during "normal" supply and the electricity companies earth MUST be disconnected during generator supply - or can they both be left connected at all times?

:-)

[Martin Bonner]

No, not at all. The local earth rod is connected at all times.

Yes. Otherwise you could end up feeding voltage through to the linesman trying to repair the cable.

My understanding is that "converting to a TT system" means that you disconnect the nice safe earth the supply company provide, and use an earch rod /instead/. That way you just have to switch over the neutral and live when you switch to generator mode.

With luck, one of the professionals will be along shortly to tell us all the actual requirements

[Andy Wade]

Agreed.

Well, no, actually. Absolutely no. There is *no* requirement to disconnect from the supplier's earth. In fact the regs generally forbid inserting switching devices in protective (earth) conductors, for fairly obvious reasons. Only hard links needing a tool to disconnect them are allowed for testing purposes.

Nothing needs to be converted to TT; you run the generator supply as a TN-S system with a 3-core cable (or 2+armour) between genny and house switchgear (5-core if it's 3-ph). One side of the generator winding (or the star-point for 3-ph) is designated to be the neutral and is earthed at the generator end, together with the all the generator's exposed metalwork. (In other words the earth and neutral conductors of the cable are connected together (and to earth) at the generator end.)

The need for an RCD comes about because the Ze value (effectively the supply's line-earth impedance seen at the origin of the house installation) may be a fair bit higher from the genny than from the much stiffer public mains, so the standard assumptions that ensure that fuses blow or MCBs trip sufficiently quickly don't necessarily hold true when on the genny. A common RCD here ensures that quick disconnection will always occur (possibly at the expense of discrimination, but that's usually acceptable on a standby system that's only used occasionally).

[Sparks]

Thanks for that Andy!

A few more question for you, if you don't mind!...

With an 8KVA generator, what size earth would you require between.... The generator & the earth rod? The Generator and the consumer unit?

Ta!

[Andy Wade]

Treat as for an earthing conductor. If you're using ordinary G/Y insulated earth wire and it's buried for part of its length it needs to be 16 mm^2. For bare copper in the ground you'd need 25 mm^2. If it's not buried it could be 4 mm^2, or even 2.5 if protected in conduit.

Just use 16 insulated and you can't go wrong. Remember that the connection to the rod needs to be accessible unless welded, soldered or brazed etc., and labelled "safety electrical connection do not disconnect".

Electrically than can be quite small, because RCD protection is in place. What size and type of cable connects the generator to the transfer switch? The armour of 2-core SWA will be quite OK, assuming it's 6 mm^2 or larger. For T&E the included CPC will be OK. If you're using singles or single-sheathed wires then I'd be inclined to make it one size smaller than the line & neutral. The minimum size for any separate protective conductor that is not in conduit or trunking is 4 mm^2.

[Tim..]

That is exactly what is done! MY neighbour has just had it done, and I have quizzed the Spark this morning. He had a pole mounted earth provided by the electricity company (which was actually found to be disconnected, but thats another story!) but now has a local earth spike at the property linked by

10mm cable (only has to be 4mm according to the regs now- madness!)

Tim..

[Dave Liquorice]

And presumably a 100mA time delayed "whole house" RCD? Under TT the earthing doesn't have to be quite so chunky as it is assumed to have too high an impedance to allow really hefty fault currents flow to blow fuses or trip MCBs in the required time. Hence the 100mA time delayed wholes house RCD.

[Sparks]

I have 10mm between the generator box and the spike, but it is not protected, but the run is only about 30 cms, along a fence. Is this OK, or should I put it in some conduit?

Do you need to protect the connection form the rain, in somthing like this?

I got one of these, it would mean the conduit can juct push into it.

I have a 100mA RCD in the transfer box (Inside) is this OK? (There is a thermal type fuse on the generator that pops out if there is a fault)

Okay, currently it is 6mm T&E between the CU and a IP66 box on the house (Run of about 10m) It then changes to 4mm SWA from this plastic box to one of these

run is about 4m This connects to one of these on the box the generator lives in. From the back of this, it terminates in another 32A plug, whch is connected to a flying length of SWA I have installed on the generator panel (It only comes with 2 16A sockets as standard for some reason)

I did want 6mm SWA, but the dozey tit who laid the new patio got 4mm, I only realised after he had finished. I do have a spare conduit layed in, so I can upgrade this is needed - do I need to? (The generator is acually only 7.5KVA) Basically the older red one of these!

--

Thanks Andy!

[Martin Bonner]

and lo, > > > >> That's fine, but does this mean the local earth rod MUST be disconnected

OK. That's why I wanted a professional.

Oh sure. I wasn't saying anything about a switch! I just thought that they company earth was permanently disconnected. Presumably, it would be /allowed/ to disconnect the company earth, but a silly thing to do (why disconnect a perfectly good earth).

[Andy Wade]

It should really be 16, or in conduit.

That's one option. The conduit adaptor solvent welds on to the end of the conduit. Those little boxes don't leave much working space inside though - I prefer to use a plastic earth pit, such as

you can finish neatly at ground level.

Well, having the RCD at the far end of the feeder means that you're relying on the generator's cut-out for protection in the event of a cable fault. With that 4 mm^2 you only have a 1.5 mm^2 earth! You'd have to ask the genny manufacturer about its fault characteristics, and you might find it hard to get an answer. Much simpler to have the RCD at the genny end - alternatively upgrade to 10 mm^2 SWA, or run a separate 6 or 10 mm^2 earth alongside the T&E.

gennies like that are earthed to a centre-tap (115-0-115 V out), which obviously is not what you need. Make sure you disconnect any centre-tap connection before running it up with the neutral end earthed...

[Andy Wade]

You didn't get one though...

Having checked, there is an exception allowed in the new edition for this and earth switching is allowed where essential, subject to various conditions [see 537.1.5 for details].

There's no compulsion to use a supplier's earth facility, either TN-S or TN-C-S/PME. The TT option is always open, but of course conversion would usually involve fitting at least one additional RCD. I don't see any need or point in doing it though, if there's a good TN earth available, unless the supplier or distributor imposes it as a condition of having a generator.

[Sparks]

which you can finish neatly at ground level.

It's 6mm T&E and three core 4mm SWA

Sounds like the best idea, Will install a 100mA RCD in the generator box.

From the earth pin on the generator, it seems to be putting out 115 between the L and E and the same with N and E

If I strap one of these to Earth, I get...

230v between L and N 230v between L and E 0v between N and E

It didn't go bang or blow it's fuse!

Do I still need to disconnect somthing?

Thanks again!

[Roger Mills]

No, you'll be fine! Your figures suggest that the earth was floating, and sat in the middle when left to its own devices, but was quite happy to be strapped to one side.

That's what I've done with my Honda generator. I was pretty sure the earth was floating but initally connected N to E using a 60w light bulb rather than a hard link on the basis that if it *wasn't* floating, the bulb would glow and I would be able to measure a finite voltage across it. Result: no glow and no voltage, so I replaced the bulb with a hard link.

[Dave Liquorice]

One also assumes that you have stuck a rudy great label on the set or made the link *very* visible so that some one coming along expecting the two phases to be flaoting (as is normal for a portable generator) is aware of the link. Trouble is they might not be aware of the safety implications such a link has...

Best to leave the Gen set unmodified and put the link in the connector you use to connect to the "normal" wiring scheme. With a warning notice and instruction not to operate the set with that connector unless the earth spike is also connected to the sets frame.

[Roger Mills]

The link is in a well-labelled 13A plug plugged into one of the two outlets, with the actual load being plugged into the other one.

I don't anticipate that anyone other than me will use it, anyway!