Generator voltage

[rich79l]

Hello, I am an electrician and I have wired a dwelling which is to supplied by a mobile generator. I performed all of my dead tests and everthing ok.
I have wired it up as a TT system, spike ect.
My tester won't allow me to carry out earth loop and Rcd tests.
There is voltage on both live and neutral outputs from the generator, are about 170 volts on the live and about 60 volts on the neutral.
These are both to earth. There is 230volts between the live and neutral.

Is this correct?

Also the generator is about 5 years old and has had repairs on it, eg the rectifier and the stator have been worked on, previously. As I was told by the owner.

 

[Spark123]

A system supplied by a generator ought to be TN-S, not TT. In essence, if it was to be TT you'd need a rod for the generator and a separate rod for the dwelling.
For a TN-S system the generator should be earthed with a rod*, the neutral connected to this earth point and this used for the earthing in the dwelling. If the generator is putting out an odd supply i.e. 170-0-60 then you need to investigate why.

*This rod should be less than 21ohms to ground which needs to be tested with a 3 or 4 wire earth electrode tester.

 

[ericmark]

I will have a guess as had this before. I will expect to find the generator is duel voltage 110 or 230 volt so two windings with a centre tap on one winding connected to the earth tag so in 110 volt mode all is OK 55 - 0 - 55 but in the 230 volt mode that earth is around 60 - 0 - 180 giving a often unfused 60 volt output. Sounds if in your case the line and neutral have been swapped. The one I found was a Honda and the problem is you should not switch earth and without switching it is very hard to swap earth from centre tap to connected to neutral.

I looked at altering generator wiring but then when switched to 110 it would be 110 volts to earth so I asked which was used most and they said 110 so the change over switch and 230 volt outlet was removed and we got them a 110 to 230 volt step up transformer.

You can't change it outside the generator one has to do it inside the generator and in your case remove the 110 volt option or at least 110 volt centre tapped option. Even as 110 volt there is a problem with the earth loop impedance as only one winding is earthed. It really does need an earth leakage trip to protect the generator winding as an earth fault could over load one winding if nothing trips.

 

[endecotp]

Isn't it just floating?

 

[JohnW2]

There is voltage on both live and neutral outputs from the generator, are about 170 volts on the live and about 60 volts on the neutral. These are both to earth. There is 230volts between the live and neutral.

What on the genny have you earthed?

Kind Regards, John

 

[ericmark]

Careful if you bond earth and neutral and it is as I say you will have a direct short and likely no protection so most likely result is expensive blue smoke.

 

[rich79l]

Thanks for all your comments.
I thought there was not going to be a problem with the generator as they have been using it for some years.
The genny is spiked down at its location and I have it spiked at the dwelling.
When I finished on that day it was late, so didn't have much time to investigate. So I left it and am going back next Wednesday to try and sort it.

 

[Spark123]

What is the spike connected to? The chassis of the genny? The earth terminal of the genny? I do you have a conductor between the earth on the genny and the house?

 

[JohnW2]

What is the spike connected to? The chassis of the genny? The earth terminal of the genny? I do you have a conductor between the earth on the genny and the house?

Quite so - I haven't yet had a reply to ...

What on the genny have you earthed?

Kind Regards, John

 

[Spark123]

Yes, I was waiting for those answers too but it looks like he has gone away again.

 

[ericmark]

I want to see if my guess is right. I was surprised as I thought of Honda as a good make. I can see the problem however with a duel voltage generator either you switch the earth tapping and not permitted to switch the earth or you earth to the neutral and in 110 volt mode it is not centre tapped, or you have split phase as if not bonded to earth you can't call it neutral and have the 60 - 0 - 170 volt output. The last is not to earth at the generator and allow the user to select what he earths.

Being honest it is rare for generator users to actually connect an earth electrode, I suppose we should as the user select what we earth, however if you do and the generator is swapped again there are problems. Over 16kVA we are normally expected to hard wire, it is only those under 16kVA which likely can be simply plugged in. I know you can get 126A plugs but not very common after 63 normally not plugged in. However I can see the advantage in plugging in you can hire a replacement.

I have come across generators with 12 output windings which could be user configured to give single or three phase 230 or 110 volt using zig zag configuration for single phase. Again to be frank I was not impressed it was too easy for the user to make an error and get a winding wrong.

 

[Spark123]

It may be a Honda engine but not necessarily a Honda alternator stuck on it. Smaller units are usually left floating which is fine for small supplies i.e. supplying a single appliance, when we go into supplying multiple circuits, floating supplies (aka electrical separation) become more complicated and require all pole fusing as double fault scenarios need to be taken into account.

 

[endecotp]

Just knowing the make and model of the generator would be useful....

 

[rich79l]

Good morning, the genny is earthed to the spike from the earth terminal on the genarator.
Not sure what the make is and there arnt any markings on it eg. Kva etc.
It is duel voltage 110 and 230. The output is fused through a double pole 16a breaker.
I have the earth in the house on a spike and it is not electrically connected to the genny.
Hope that make sense!
Thanks

 

[endecotp]

Maybe ask the customer where it originally came from, and see if you can track down manufacturer's instructions that way? It would be easiest to get right if there is a nice wiring diagram somewhere!

Otherwise I'd try measuring resistances between the outputs, with the load disconnected; that should identify whether the earth is a centre-tap. It's not obvious to me what you should do in that case.