Small generator earthing.

[JohnW2]

Thanks, John, I will check tomorrow if there is a N-E link on the Genny and report back. Closing down now, Saturday night and things to do.

Thanks. If there is a N-E link, then I don't think there's any doubt that the genny supply does have to have an effective earth (and ideally an RCD), because all the 'normal' risks of electric shock then show their heads.

Kind Regards, John

 

[ban-all-sheds]

And if there isn't you can always add one.

 

[JohnW2]

And if there isn't you can always add one.

Well, yes, of course - but if there is not one (N-E link) then, as we've seen, the question about earthing becomes 'a matter for discussion'. However, as I said, with an N-E link I don't think there is any scope for discussion or debate.

Kind Regards, John

 

[ban-all-sheds]

Then add one, and end the debate

 

[JohnW2]

Then add one, and end the debate

It may end the debate, but (as we've seen) some would argue that it introduces an unnecessary hazard.

Kind Regards, John

 

[ban-all-sheds]

I guess it depends what you want to do with the supply. If you don't want to be constrained to at most 1, preferably no, Class I appliances, and would like RCD protection....

 

[JohnW2]

I guess it depends what you want to do with the supply. If you don't want to be constrained to at most 1, preferably no, Class I appliances, and would like RCD protection....

Quite so. The OP's application ("...for running my oil fired cooker boiler, and possibly a couple of lights..") probably qualifies as "at most one ... Class I appliances" and, as westie has said, with a floating supply, RCD protection is not really appropriate/relevant.

I personally probably would never do it (I certainly don't with my standby genny), but I can see the case in some situations.

Kind Regards, John

 

[SimonH2]

http://electrical.theiet.org/wiring-matters/39/portable-generators.cfm?type=pdf

Amusing that a leaflet about portable generators has a photo of something else on the front. Looks like a poker (vibrating compactor for concrete) drive to me.

 

[ericmark]

I have talked about generator earthing a lot and without knowing how the generator is internally wired near impossible to give advice.

I was rather surprised to find a Honda which I considered as a well know and respected make to have the earth when selected to 230 volt corrected to a tapping giving 57.5 - 0 - 172.5 volt however when you think about it since your not allowed to switch an earth connection it is impossible to have a generator which has 110 and 230 volt options to completely comply.

Either it switches the earth or earth is not centre tapped in 110 volt mode or not at neutral voltage in 230 volt mode so first thing has to be to work out what the generator wiring is before deciding what to bond.

Also one has to remember the definitions and a rod driven into the ground and bonded to neutral could be an earth electrode or an extraneous-conductive-part physical connections are the same but name changes depending on if classified as TN or TT or if local generator or DNO supply is used.

Currents under fault conditions can be very high if the earth bond is very good and the idea of the lower the resistance to earth the better is flawed and having a 60 ohm reading on the rod with RCD protection is likely better than 6 ohm when one considers the currents which will flow under fault conditions.

So I would use a single rod with a pit giving some distance before any live animal can contact the earth of likely 3 foot from where the rod is placed but this is to be based on a risk assessment of the installation rather than blindly following regulations.

I would attach to the installation certificate the risk assessment stating why I had selected what I had done so should it all go wrong in the future any court could see I had taken all reasonable steps to ensure safety. Even if I got it all wrong the fact I had documented why I had selected the method would likely mean I was not likely to be prosecuted should it go wrong.

It is a watch my back exercise and there is no cut and dried answer.

The question is to notify or not. Personally if there is any likely hood of it going wrong then notifying means you have passed the buck. If at your own home and you are reasonably certain nothing will go wrong then one may try to get away with minor works but if customer and you think they may add or change things after you leave than full installation and notify.

What one has to remember the certificates protect the electrician as well as customer and should things go wrong being able to produce paperwork to show this is what it was like when I left is likely to mean courts will believe what you say and lack of paper work will likely mean courts will consider you did something dodgy.

 

[maltaron]

Thanks to every one for a lively and informative discussion. I can now confirm that there is NO connection between neutral and earth on the genny. So should I (1) connect the genny earth to a spike or (2) leave the genny supply floating or (3) connect the genny to a spike, connect neutral to earth and use a RCD.

Perhaps I should point out that this is for a particular situation, It is likely that "Bob the Builder" on site will just plug in his drill or whatever and fire up the genny, with no concern for earths or RCD,s. (although on large sites it will be 110v)

 

[SimonH2]

... however when you think about it since your not allowed to switch an earth connection it is impossible to have a generator which has 110 and 230 volt options to completely comply.

Either it switches the earth or earth is not centre tapped in 110 volt mode or not at neutral voltage in 230 volt mode ...

Or it's a lot more expensive and has separate windings for 110 and 240, or has a transformer. Ever seen one ? No, neither have I.

Or, thinking a bit more, with some much more complicated switching, it could be done - with 3 or 4 separate windings.
But I can't see any manufacturer seeing it as being worth while - since few (if any) of their customers are likely to complain.

 

[JohnW2]

Thanks to every one for a lively and informative discussion. I can now confirm that there is NO connection between neutral and earth on the genny. So should I (1) connect the genny earth to a spike or (2) leave the genny supply floating or (3) connect the genny to a spike, connect neutral to earth and use a RCD.

As I said before, I honestly can't see what useful purpose would be served by (1) without (3). By (2), I presume that you mean leave the genny frame floating, since unless you do (3), the genny supply will still be floating, even if you do (1).

Opinions obviously vary. For the exact application you mention, it probably does not make that much difference (provided the boiler electronics are happy with a floating supply).

As has been alluded to, the main theoretical hazard of a floating supply arises if two Class I appliances with 'exposed conductive parts' are plugged into it. In an (incredibly unlikely) scenario, an L-'E' fault in one of them and an N-'E' fault in the other could result in full supply voltage appearing between the casings of the two appliances (hence a problem for anyone who touches both!), with any fuses, MCBs or RCDs being oblivious to the existence of any problem. One could, however, argue that one might be more likely to be hit by lightning than to encounter that pair of simultaneous faults!

FWIW, as I've said, for my standby genny (essentially like yours - primarily for boiler and CH controls etc. plus a few lights, but also a few sockets around the house - which could theoretically be used for anything), I've done (3).

Kind Regards, John

 

[ericmark]

Or, thinking a bit more, with some much more complicated switching, it could be done - with 3 or 4 separate windings.
But I can't see any manufacturer seeing it as being worth while - since few (if any) of their customers are likely to complain.

I have seen generators with 12 winding which can be configured star, delta, or zig zag, with parallel or series giving 110, 230 or 400 volt single or three phase in my case a 12.5 kVA but it is hard wired not switched and that is the point.

Opinions obviously vary. For the exact application you mention, it probably does not make that much difference (provided the boiler electronics are happy with a floating supply).

As has been alluded to, the main theoretical hazard of a floating supply arises if two Class I appliances with 'exposed conductive parts' are plugged into it. In an (incredibly unlikely) scenario, an L-'E' fault in one of them and an N-'E' fault in the other could result in full supply voltage appearing between the casings of the two appliances (hence a problem for anyone who touches both!), with any fuses, MCBs or RCDs being oblivious to the existence of any problem. One could, however, argue that one might be more likely to be hit by lightning than to encounter that pair of simultaneous faults!

FWIW, as I've said, for my standby genny (essentially like yours - primarily for boiler and CH controls etc. plus a few lights, but also a few sockets around the house - which could theoretically be used for anything), I've done (3).

Kind Regards, John

I think John is spot on. It's down to a risk assessment and there is no one size fits all.

Bonding all earthed parts even without any connection to neutral would mean two appliances one with live 1 fault to earth the other with live 2 fault to earth would mean a direct short circuit and the automatic disconnection should result in a fail safe.

Connecting one of the lives permanently to earth giving a neutral inside the generator would mean only single pole switching and MCB's are required in fact you are no permitted to put a fuse in the neutral but you are allowed a fuse in live 2 so it requires a risk assessment rather than simple answer.

With a single voltage fixed generator at 230 volt I would want to within the generator earth one live and install a RCD plus provide a point for earth rod connection. This would also be the case with a hard wired generator with multi options. However with a switched output I would leave as supplied by manufacturer and just bond after the generator.

With a generator moved from site to site I would only earth as per manufacturers instructions within the generator but outside bond as normal.

Clearly I would consider the risk and adjust as I thought best fitted the situation and so those guide lines are not fixed in stone.

In theory you should not need to consider what the next guy will do. In practice you should and to alter any generator from manufacturers standard or to bond a line to earth to form a neutral outside the generator so in essence what I am saying is only bond a live to earth in a fixed generator.