Earthing a generator

Birka

I have a 3Kw petrol generator that I'm planning to use with an emergency circuit that's completely independent from the house wiring. The plan is to run a connection from outside to feed several double sockets to run the likes of freezer, oil fired central heating etc. There will be no connection to the house consumer unit and the new sockets will only be live when a generator is connected to the external connection.

My electrician has tested the generator and found that its output is reading 80V between neutral and earth and 140V between live and earth. He can't get the generator's RCD to trip when he tries. A generator supplier suggested an earth rod. That was fitted at the location where the generator will run and connected to it but made no difference. I went back to the supplier who said that as well as the earth rod, the new emergency circuit needs the neutral and earth connected (as would normally be done at the consumer unit) and that this would sort things.

Does this make sense or is there an issue with my generator?

How are generators used in the wild, say on building sites or campsites? Do they always need an earth rod and a joined neutral and earth to work safely?

zg3409

Building sites are supposed to use 110 volts centre tapped earth, so 60 volts from neutral to earth and 60 from earth to live.

On most generators it says to connect chassis to earth rod. On most generators neutral is connected to earth. Beware measuring voltages with a meter, as they load so little they can give false readings. Ideally you need a load such as a 60 watt light bulb across the meter to see real voltage. There is often electronics and filtering that may give false readings. You also need a meter with "true RMS" to give correct voltages.

Many generators are cheap junk. What make model and can you link to manual?

Birka

https://www.boards.ie/discussion/comment/118118168#Comment_118118168

Thanks for the info. My electrician was using a Megger to read voltages and to try and trip the RCD.

The generator is a Hyundai HY3100L and wasn't very expensive when I bought it maybe ten years ago. It hasn't had much use and is in good condition. I've attached the manual

hy3000i.pdf

2011

"My electrician has tested the generator and found that its output is reading 80V between neutral and earth and 140V between live and earth."

I would say that your generator has no reference between neutral and earth or live and earth until the earth rod was installed. Even when it was installed this is quite a high impedance to connection to earth so 80V and 140V readings would not suprise me.

"He can't get the generator's RCD to trip when he tries."

For the RCD to trip the current imbalance must be >30mA, I would assume that this value is unachievable via a fault path that includes earth due to the high impedance as the system is not neutralized.

"A generator supplier suggested an earth rod. "

As above a single earth rod may not be sufficient or it may be connected to the wrong point. Where does it connect?

"I went back to the supplier who said that as well as the earth rod, the new emergency circuit needs the neutral and earth connected (as would normally be done at the consumer unit) and that this would sort things."

As above, this is called "neutralizing" and it is normally done in the ESB meter cabinet.

"Does this make sense or is there an issue with my generator?"

No.

Ask youself this: What do you think would happen if your generator system had no earth?

Birka

@2011 thanks for your comments and information.

In answer to your first question above, the earth rod is connected to an earth terminal on the control panel of the generator.

Anything other than wiring a plug is above my head so I know that I have a very incomplete understanding of this. I've always thought of smaller generators as being a portable device that you'd run behind a market stall or in a campsite, most times without an earth rod. As far as I can see, if I plug something to the generator with or without an earth rod, the generators RCD won't trip as the circuit isn't neutralised. Is that correct and if so, is that not dangerous?

Risteard81

You just have two phases out of your generator (and no neutral). Without an Earth reference neither conductor is neutral. You should Earth the chassis, but this is just an Earth reference. You should also Earth one pole of the supply, which will then be neutral. Then obviously have an RCD to provide fault protection (as well as additional protection).