To earth or not to earth

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Rikkitic

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#213867 16-Apr-2017 12:46

I was looking through the manual for our generator the other day, and there was a section about earthing it if connecting to earthed devices. I don't really understand this. I would think that the generator's 220 volt output would act the same as an isolation transformer, precisely because there is not an earth connection. In a case like this, wouldn't earthing it actually make it less safe? Can someone explain this to me please?

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gregmcc

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#1765007 16-Apr-2017 13:18

Your right about the generator effectively been similar to an isolating transformer. If it's a small generator, typically they are centre tapped 220V generators with the centre tap connected to the earth giving 110V on either pin to the the earth pin, Effectively this makes it a SELV system (segregated extra low voltage) not a low voltage system as it should be. Technically it's illegal to fit a standard NZ 3 pin socket to a generator like this, but it doesn't stop the company who sold it to you from doing that.

k1w1k1d

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#1765060 16-Apr-2017 15:57

A bit of light reading on this subject;

https://www.electricalforum.co.nz/index.php?action=more_details&id=1367459376

Aredwood

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#1765061 16-Apr-2017 15:57

If it is a center tapped generator. Never plug it into a caravan or the generator input of a building. As often the neutral and earth are linked together. So you will probably short out one of the generator windings. Or the generator frame will be live with 110V to ground. As a lot of these generators only have overload protection fitted to only 1 of the 110V windings.

They are originally designed for the US market. And have been kludged to provide NZ 230V. Far better will be if their output was completely isolated.

Rikkitic

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#1765066 16-Apr-2017 16:22

The generator is a Yamaha, so not cheap Chinese. Output is full sine wave 220 vac. There is a warning in the manual that it should be earthed if used with earthed equipment. I don't understand the point of that advice. If it is not earthed, it acts like an isolation transformer and the only way to get electrocuted is to put yourself between phase and neutral. I would think this would be safer than re-introducing a return path via earth. That is why I asked my question, which still isn't answered. Why do it this way at all? Isn't no earth better in this specific situation?

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Rikkitic

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#1765078 16-Apr-2017 16:30

k1w1k1d: A bit of light reading on this subject; https://www.electricalforum.co.nz/index.php?action=more_details&id=1367459376

Thanks for the link. It seems to confirm what I already thought. My little 1.2 kVA petrol generator (which does not have RCD protection) should not be earthed and the advice in the manual is wrong.

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richms

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#1765085 16-Apr-2017 16:53

I got a 3000va one from bunnings to run stuff when vector were replacing a powerpole and I dont recall seeing anything in its manual about earthing anything.

Richard rich.ms

Rikkitic

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#1765087 16-Apr-2017 17:01

It is definitely there. Very clear. Maybe I will dig it out and copy it.

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Aredwood

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#1765142 16-Apr-2017 21:12

Another possibility - does the generator have a 12V output? If so that output might not be isolated from the mains. The generator might have the earth, neutral, 12V neg, and metal frame all electrically linked. If the output is isolated, you want it to be 100% isolated. Not maybe or might be isolated. There is a similar recommendation in the manual that came with my approx 15 year old inverter. It says that if I want to run something that is not inside my car, then I should install an earth stake to the mass of earth.

The only reason I can think of as to why. Is if you get 1 earth fault somewhere, and the frame is isolated from ground. Someone touching the generator frame might then get electrocuted. But you could still manage this hazard by using an RCD on the generator output. The generator manufacturer is probably trying to strike a balance between safety and number of callbacks. Especially as they have no idea and cannot control what their generators will be used for. Someone might even make up a suicide lead (cord with a male plug on both ends) so they can plug 1 end into the generator, and the other end into a power socket in their house. As a very dangerous way of connecting power from the generator to the whole house.

Rikkitic

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#1765251 17-Apr-2017 10:26

I like the idea of the suicide lead. I actually thought about doing that (just thought about it, no intention to carry through) but decided it would be too risky. The way to do it would be to first switch the main switch off to break the connection to the power lines. We have old house wiring, no RCD, neutral common to earth, wire fuses. On a system like that, I don't see a problem in principle but I still wouldn't do it. With a small generator like we have it just makes more sense to directly connect any devices we want to run.

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gregmcc

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#1765254 17-Apr-2017 10:41

Rikkitic:

I like the idea of the suicide lead. I actually thought about doing that (just thought about it, no intention to carry through) but decided it would be too risky. The way to do it would be to first switch the main switch off to break the connection to the power lines. We have old house wiring, no RCD, neutral common to earth, wire fuses. On a system like that, I don't see a problem in principle but I still wouldn't do it. With a small generator like we have it just makes more sense to directly connect any devices we want to run.

Don't even say stuff like that, no matter how much you say it's a bad idea there is always someone out there who will give it ago......