Re: Portable generator - Grounding

On 11 Feb 2006 02:32:18 -0800, "John" <johnmanders@xxxxxxxxxxxxxxxxx>
wrote:

AFAIK there are 2 ways of running a generator.
In order to get a shock, there has to be a complete electrical circuit
via you. Since you are normally connected to the ground, that is the
most likely current return path. A generator produces a voltage across
two terminals. If neither of these are connected to ground, you cannot
complete the circuit that way. This is earth isolation and is favoured
for portable generators as it saves having to find a good earth. If the
system is run like this, the only way to get a shock is to contact both
of the conductors. The down side is that, as there is no voltage
reference to earth, the power side can escalate to high voltage. Again,
this is normally only a problem if the insulation brakes down providing
an earth connection. Since there is normally no earth by design, it's
best not to make such a connection.
The other way of running a genny is to fully earth one of the power
terminals. This mimics domestic wiring. Metal appliances are now
earthed and protected but touching the live lead will give a shock.
By definition, portable equipment is used in unusual environments where
cables are likely to get trodden on etc. Here the earth isolation
method is best as both cables have to be touched to make danger.
I would always advise a RCD for safety but be warned that some domestic
plug in types also look for earth connection. If they are used in an
earth isolated system, they trip as soon as power is applied. Not a lot
of use.


Thanks John, Dave and Martin for your replies. My vintage generator
has one 240V AC o/p connected directly to the frame, the earth pin on
the o/p socket is also bonded to frame. (This is an ex recreational
vehicle / ice cream van Onan genny). The wiring of the new
lightweight Chinese genny is to be investigated. I will look for RCDs
for both.

That leaves me with another question. In the event of a mains power
failure I would be persuaded to use one or the other generator to
provide emergency lighting and cooking facilities and run the CH pump
and boiler. For every device I would use an extension lead from the
generator and plug in the lights (table top lamp variety), an old
table top standalone oven and cooker ring and the CH boiler, and I
have no doubt the idiot's lantern. The boiler would be unplugged from
the mains and plugged into the extension lead. The TV aerial sockets
outer are unearthed.

This seems to me to be running in the earth isolation method except
that the CH boiler is earthed because the incoming gas feed is
connected to building earth and also by means of the copper pipes to
the boiler and hence all the CH radiators.

In the above scenario, with the two choices of generator, one with
o/p terminal bonded to frame, the other with both o/p terminals
isolated from the frame (and ground), but with the connection to
building ground through the CH gas pipe being fixed, what is
recommended for grounding of either generator?

I hope this doesn't degenerate into a Part P discussion!

--
cerberus
.

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