Connecting a generator to a caravan and tools???

[legrandfromage]

A friend of mine plans to connect his brand new 4kVA generator to his mobile home while he works on his (rather spectacular) renovation. He also intends to use the generator for tools.

I advised him that he should link the neutral and earth wires in in his mobile home, before the consumer unit, and fit an earth spike. I also suggested he labels the connection plug that it must not be connected to the mains. I also advised him that he should disconnect the generator from his mobile home when it is being used for power on site, as the measures to make it safe to power his home make it unsafe to simultaneously use for tools.

[legrandfromage]

[OOPS - hit the wrong key!!]

He thinks I am talking nonsense (which I may be), so I'm going to see him at the weekend and either apologise or give him a strong talking to regarding electrical safety.

Is my advice correct? Also, might it be advisable to connect the generator to his mobile home (it will be 10-15m away) with armoured cable?

salut

legrandfromage

[NT]

Its common with portable gens to just not bother with earthing.

NT

[Andy Wade]

See

you're going to do that.

Otherwise I'd say the OP has it about right, although the neutral is probably better earthed at the generator end rather than in the caravan. Ensure the caravan CU includes a 30 mA RCD. Best to advise using 110 V for the site tools, either via an isolating transformer or straight from the genny if it's dual-voltage. A safe way to run the caravan and site tools together might be to use a 230/55-0-55 V isolating transformer with the secondary CT earthed to a separate electrode placed some distance away from the one on the 230 V side.

Note the point in the IET article about the need for frequent inspection of cables, plugs, etc.

[TMC]

As the caravan will have an electrical input (16 amp blue socket) what would be wrong with connecting the generator to that using the same cable as he would use to connect the caravan to an external mains supply when on a caravan site? Why mess with any wiring in a caravan that is designed to accept an external supply?

Or am I missing something here?

regards

[legrandfromage]

I think the issue is that the power from the generator, being "floating" will not trip an RCD in the mobile home in the event of a fault. I believe that the blue plug is a common place to make the earth-neutral connection.

I also think that making the neutral/earth link turns the 115-0-115V generator into a 230-115-0V power source, so it will probably be unsafe to use as power source on site - hence the need to be unplugged from the mobile home.

legrandfromage

[Dave Liquorice]

RCD's do not need an earth to operate, any imbalance in the L/N currents will trip 'em. Of course without either pole of the genset connected to "earth" the alternative return path might have to high an impedance for enough fault current to flow to trip the RCD...

If anything the generator chassis needs to be connected to an earth spike at the generator and the designated N pole connected to that, but if you do that you need to ensure that the caravan chassis (not just the "earth" of the wiring) is also bonded to an earth spike. The wiring "earth" should be connected back to the generator chassis/earth spike through the supply cable.

How? Assuming your 0V is the earth spike if you connect anything to that it also becomes 0V (high currents and/or cable resistance excepted...). The floating supply which could be anything from

230-0-0 to 0-0-230 (L E N) is made into 230-0-0.

[legrandfromage]

You are of course correct on how RCDs work, but given that there is no return path to earth for an isolated generator, there will be no imbalance to detect. Unless I'm missing something...

You are probably correct, but I prefer not to alter the generator. I figure it is designed to power tools etc. as it is, and I don't want to compromise whatever safety measure it has.

I've not seen the generator, but I assume it provides 230V and two

115V outputs at least. If you earth one of the ends of the voltage range, it causes the centre to become 115v and the other extreme to become 230V relative to earth. Someone using 115V might not expect to be using equipment that is in fact at a potential of 230V above their surroundings.

What am I missing?

Salut

legrandfromage

[Tim Watts]

On Thu, 15 Apr 2010 13:05:34 -0700, legrandfromage wibbled:

Anyone using 110V "site" tools will expect a 55-0-55V supply with the centre tapped 0 being spiked to the ground so it's probably a moot point.

115V (relative to ground) is considered to be in the same danger bracket as 230V.

I would put the earth link and connection to a ground rod on the generator side. That way, when (if) the supply to the mobile home is replaced with a mains feed, no one can forget anything and get an unexpected surprise (eg a hard E-N link on a TNS supply[1] could be a fire risk in its own right).

[1] I realise you *may* not be able to link a TNS supply to a metal mobile home - it's not an area of the IEE regs I've ever needed to read.

Cheers

Tim

[Dave Liquorice]

A "return path" with an impedance less than about 7k ohms will be enough to trip a 30mA RCD...

The basic problem is trying to mix two ways or working. The unbalanced, needs return path of some sort, RCD method and the balanced 55-0-55 no return path with nominally "safe" voltages to earth.

It may vary from generator to generator but I'm fairly sure that most alternators have two 115v windings and you then series/parallel connect those with a switch to give you 115v or 230v. You can't have both outputs live at the same time.

One way to avoid moding the genset and caravan wiring would be to check that the earth pin of the 230v output is bonded to the set chassis (also connected to a spike) and put the N-E link in the plug, suitably labeled, that is used to connect to the caravan (chassis also bonded to an earth spike).