Generators and RCD's

[twinaxel]

Hi I help to run a small caravan club in the summer we use a marquee
power is supplied by a 2.5 kv generator which is not earthed.

In the past they have used metal clad sockets with no protection at all!

I have purchased two IP44 RCD triple socket supply units on the
understanding that no earth was required.

Is this information correct.

Thanks

 

[Spark123]

An RCD is almost useless when being used with an electrically separate source, theres a bit of info here: http://www.blue-room.org.uk/index.php?showtopic=12655
(I hope you meant KVA )

 

[twinaxel]

Thanks for reply yes it is 2.5KVA . Have looked at the thread you
kindly provided. Hmmm it seems to be a grey area not sure what to
do next. We are considering buying a new generator have phoned
the supplier for his advice. Reply was just stick an earth rod next
to the genny. Thats everybody else doe's.

 

[Spark123]

My advice is speak to the manufacturers technical dept, sticking a rod next to it because everyone else does doesn't sound to me to be the best advice.
What I was told to do by the manufacturer of my generator was to install an earth electrode for my genny, (this rod needs to be tested to confirm contact with the ground by using a special tester, you may need an electrician to do this for you). I was then told to make up a lead with the neutral and earth jointed together in the generator plug and labelled as such that it is only to be used with the generator and only when the generator is earthed. The lead I made also incorporates an RCD. This is the advice given to me by Generac for my 2.2KVA genny, you should clarify this with your generator manufacturer to make sure this arrangement is acceptable.

 

[plugwash]

thats pretty much the standard arrangement for using a portable generator on an installation (as apposed to just running a tool or two)

the generator as supplied is an IT system which is idea for running a single tool or so but not so good as the installation gets bigger. What you describe will convert it to a TN-S system.

 

[Adam_151]

So you have got an RCD to clear phase earth faults, because presumembly the supply impedance of the gennerater won't be low enough to be able to drive enough current into a fault to open an overcurrent device?

So if this is the case, what happens with a phase-> neutral fault? does the generator just shut itself down/stall ?

 

[plugwash]

well assuming no breaker either in your wiring or in the generator itself tripps (and it may be the case that they do trip just not in fractions of a second) i imagine the generator would stall, even on maximum throttle (i persume generators have automatic throttle designed to achive a constant running speed) the petrol engine simply won't have the power to drive the generator against a dead short (dead shorted motors/generators are VERY difficult to turn).

 

[ban-all-sheds]

An RCD is almost useless when being used with an electrically separate source

Why?

 

[Adam_151]

Because the only complete circuit is from live to neutral, theres no earth because it just isn't needed, and you can't have a complete circuit to terra firma if the supply has no relation to it.

Its a nice system, but only suitable for small things, the reason we don't use it for the mains supply, is because with a system of any reasonable size that is electrially separated from ground is that it has a tendancy to float upto some high dc voltages with respect to ground, so we have
to 'tie' it down to prevent that happening

EDIT: I'm sure my typing is getting worse

 

[Pensdown]

Because the only complete circuit is from live to neutral, theres no earch because it just isn't needed, and you can't have a complete circuit to terra firma if the supply has to relation to it.

So are you saying you can't get a shock to earth from a portable generator?

 

[ban-all-sheds]

Because the only complete circuit is from live to neutral, theres no earch because it just isn't needed, and you can't have a complete circuit to terra firma if the supply has to relation to it.

If, for whatever reason, there is a PD between the supply and earth, or any other object a person might touch, sufficient to drive a dangerous current through them then there'd be enough to trip an RCD

 

[Adam_151]

FFS Adam, Stop pressing quote when you want edit

 

[Adam_151]

So are you saying you can't get a shock to earth from a portable generator?

If neither side of the generator is connected to earth in anyway, then I can't see how you are going to get a shock from any part of the system to the ground on which you stand because there just ins't a circuit there.

Unless the whole system decides to float at a dc voltage relative to the earth because of atmospheric conditions (thunderstorm style), I'm guessing this is what ban might be getting at? (but I'm not really sure that an RCD would be much of a help here)

If I'm missing something obvious then put me out my missery, but I can't think of anything I might have missed

 

[plugwash]

yeah the electrical seperation approach works brilliantly for small systems, especially if the kit is double insulated.

unfortunately if there is a fault from one side of the output to "earth" (the earth core of the wiring which may or may not be connected to real earth but is connected to exposed metalwork) while a person touches the other side of the output then they get a shock and a RCD sitting right up next to the generator isn't going to make any difference (since the only ways out of the generator and back into it are through the RCD).

 

[JohnD]

thinking