It's likely that the PE contacts of the sockets are just connected to the frame of the machine, and not directly to the windings - the 90V each way is probably just the result of capacitive coupling and the high impedance of modern test meters. Test with something that draws a significant current to verify (e.g. shunt the meter with a filament lamp).

As you say, floating systems are generally only recommended for small system - e.g. one appliance according to BS 7671. For more extensive systems, it's far more usual to link N & PE and earth to a local electrode - then ADS can be provided (if, due to the low fault current available, by RCD rather than overcurrent device).

   - Andy.

Nothing to do with earthing, but petrol seems to be an odd choice of fuel. It can be stored only in small quantities (legally) and for occasional use, it is likely to become stale - the modern stuff seems to have a very short shelf life.

If the earth pin of the sockets is only connected to the frame, and not the windings, then connecting frame to neutral won't cause any problems, and will effectively give you a TN-S supply.  You would need to make sure the frame was earthed, without relying on the supplier's earth.

To me, 90V suggests some capacitive coupling, while a centre tap would give 115V.

No Simon, that is not correct. One side of the output should be connected to the frame and an Earth rod to give a TN-S supply.

It is currently a floating supply that will not operate RCDs or MCBs with Earth faults. It is an IT supply without Earth fault monitoring.

I agree that petrol seems an unlikely and maybe unwise choice of fuel for an 8 kw generator. Only 30 litres of petrol may lawfully be stored, and 30 litres wont last very long in an 8 kw generator.

Returning to earthing, I would proceed as follows. Provide at the changeover switch a fixed 32 amp ceeform plug via which the generator may be connected. Bond together all the following, the blue and the green/yellow wires from the generator, the blue and the green/yellow wires into the changeover switch, a green/yellow wire to a local earth rod, and a green/yellow wire to the MET of the existing electrical installation.

Provide a standard 32 amp ceeform extension lead, with a plug for the generator end and a trailing socket for the connection to the changeover switch.

Presuming that the stated capacity of the generator is 8 kw short term or intermittent, with the long term capacity being 7 kw or less, then  32 amp connectors will be fine. If however 8 kw can be supplied CONTINUALLY then 63 amp connectors should be used.

This will ensure that neutral of the generator supply is connected to the general mass of earth and to the CPC. This then resemble a public supply with an earthed neutral.

I would advise against modifying the internal connections of the generator as that renders it non standard and will void the warranty.

It is an IT supply without Earth fault monitoring.

Small point, but I think BS 7671 would class the generator, in the manner it is usually used, as a separated system (section 413 or 418.3) rather than an IT system (section 411.6) - the difference is that exposed-condutive-parts (either left unconnected, or connected together) aren't deliberately connected to Earth, as they would need to be in an IT system.

   - Andy.

Normally you can earth the neutral of such a genset (and should if you want to use it to run an installation where the ADS relies on an NE bond) , but you should perhaps try to do so via a limiting resistance first to check that nothing untoward happens.

Do you have a web link to the genset datasheet?

Note that without an interconnection between windings and CPC, then RCDs will not work, and single pole MCBs and fuses will not be in the right pole, or more correctly without a clear neutral, there is no clear live either.

Be aware that you will need to provide an NE bond and an electrode, as during an external mains fault you cannot rely on the mains for earthing - the cable be be disconnected in the street for example, and then be aware that in areas cvered by the ecsqr, that NE bond has to be on the other side of a double pole switch to the one from the DNO.

Mike

Yep, it was not my choice of generator. The customer chose it it

broadgage said:

I agree that petrol seems an unlikely and maybe unwise choice of fuel for an 8 kw generator. Only 30 litres of petrol may lawfully be stored, and 30 litres wont last very long in an 8 kw generator.

Not my choice of generator, already purchased and currently connected via widow maker cable .

Returning to earthing, I would proceed as follows. Provide at the changeover switch a fixed 32 amp ceeform plug via which the generator may be connected. Bond together all the following, the blue and the green/yellow wires from the generator, the blue and the green/yellow wires into the changeover switch, a green/yellow wire to a local earth rod, and a green/yellow wire to the MET of the existing electrical installation.

Provide a standard 32 amp ceeform extension lead, with a plug for the generator end and a trailing socket for the connection to the changeover switch.

Presuming that the stated capacity of the generator is 8 kw short term or intermittent, with the long term capacity being 7 kw or less, then  32 amp connectors will be fine. If however 8 kw can be supplied CONTINUALLY then 63 amp connectors should be used.

This will ensure that neutral of the generator supply is connected to the general mass of earth and to the CPC. This then resemble a public supply with an earthed neutral.

I would advise against modifying the internal connections of the generator as that renders it non standard and will void the warranty.

Thank you, all of the above makes sense and is my design intent, I just don't want to bond N and PE if it will damage the genset

mapj1 said:

Normally you can earth the neutral of such a genset (and should if you want to use it to run an installation where the ADS relies on an NE bond) , but you should perhaps try to do so via a limiting resistance first to check that nothing untoward happens.

OK

Do you have a web link to the genset datasheet?

This is a link to the Hyundai HY10000LEK-2 and there are links to the datasheet and user manual on the page

Note that without an interconnection between windings and CPC, then RCDs will not work, and single pole MCBs and fuses will not be in the right pole, or more correctly without a clear neutral, there is no clear live either.

Agreed

Be aware that you will need to provide an NE bond and an electrode, as during an external mains fault you cannot rely on the mains for earthing - the cable be be disconnected in the street for example, and then be aware that in areas cvered by the ecsqr, that NE bond has to be on the other side of a double pole switch to the one from the DNO.

Agreed, the cables are overhead and it is a TNCS supply, if the cables are brought down by tree then no mains earthing. Thanks for the reminder about the position of the NE link