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.

On Apr 14, 6:34*pm, legrandfromage wrote:
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.

[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

On Apr 14, 6:39*pm, legrandfromage wrote:
On Apr 14, 6:34*pm, legrandfromage wrote:

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.

[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

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


NT

NT wrote:

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

See
http://www.theiet.org/publishing/wir...s.cfm?type=pdf
if 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.

--
Andy

"legrandfromage" wrote in message
...
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.


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

On 15 Apr, 16:31, "TMC" wrote:
"legrandfromage" wrote in message

...

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.

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

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

On Thu, 15 Apr 2010 09:08:21 -0700 (PDT), legrandfromage wrote:

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.

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.

I also think that making the neutral/earth link turns the 115-0-115V
generator into a 230-115-0V power source,

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.

--
Cheers
Dave.

On 15 Apr, 19:43, "Dave Liquorice"
wrote:
On Thu, 15 Apr 2010 09:08:21 -0700 (PDT), legrandfromage wrote:
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.

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...


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...

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.


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 also think that making the neutral/earth link turns the 115-0-115V
generator into a 230-115-0V power source,

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.


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

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

On 15 Apr, 19:43, "Dave Liquorice"
wrote:
On Thu, 15 Apr 2010 09:08:21 -0700 (PDT), legrandfromage wrote:
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.

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...


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...

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.


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 also think that making the neutral/earth link turns the 115-0-115V
generator into a 230-115-0V power source,

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.


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?

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

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

On Thu, 15 Apr 2010 13:05:34 -0700 (PDT), legrandfromage wrote:

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...

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

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.

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.

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.

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).

--
Cheers
Dave.