Earthing of standby generators

[Km1777]

Afternoon guys. Recently been asked to do a generator changeover switch for a friend it’s a 6.0kva standby generator . The earthing system is tnc-s . Just seeking a bit of advice with regards to earthing requirements as it’s a portable gen . Also it will be sitting on top of block paving . Can’t see an easy way to drive an eart rod in. The output wiring is neutral and earth are separate . I’m a fully qualified electrician have all up to date qualifications just haven’t had any dealings with standby generator equipment . Any advice would be appreciated.

 

[pc1966]

I would go with option A
consider the inlet on the house as the origin of supply and link N to E at that point.

Probably the least-worst option.

Don't forget to label the inlet as intended for floating-neutral generators and not RCD protected ones, could save some grief further down the line.

 

[ianscf]

Ok so I have a similar, but different issue. I got a 7KVA 55-0-55 generator, which i want to use as standby for my house.
I am using a 5KVA site transformer rewired as a step up to get my 240v.
My question is, should I keep the cte from the generator to the transformer, or loop n+e in the generator? There is almost no chance this generator will be used for any other purpose, but for the sake of safety, it is better to never say never.
If I keep the CTE, should I isolate the generator grounding from the house TNC-S and use an earth spike on the generator, or loop the earth through and stake the input socket on the house, or both? I am using a 3 pole changeover switch?
*I know I am kind of answering my own question, but I want to be sure.

 

[Lucien Nunes]

To make the 55-0-55 part of the system true reduced-low-voltage, the centre-tap of the generator output should be referenced to earth. There should be no connection to the centre-tap of the transformer otherwise circulating currents will occur unless the voltage symmetry is perfect. However, the interwinding screen, if there is one, should be earthed. This would mean separating any internal connection that might exist between the screen and the CT. The screen would otherwise be at an 'artificial' neutral held halfway between the two live conductors by the transformer itself. Not a big deal but not technically correct. I can't see any advantage to making this part of the system 110V TN.

The two sides of the transformer are electrically separate so it shouldn't matter whether the earthing systems are interconnected or not. The output of the transformer, with N-E linked, is regular TN-S so that just needs an earth reference as well.

Are you providing thermal protection to the transformer, and if so, how? Is the 5kVA rating intermittent or continuous? (just trying to visualise the tranny). How close has the output voltage worked out in practice? I'm expecting some contributors here to be thinking that this is a cheeky way to get 230V but TBH it's the sort of thing I would do so I'm not shooting it down!

 

[ianscf]

To make the 55-0-55 part of the system true reduced-low-voltage, the centre-tap of the generator output should be referenced to earth. There should be no connection to the centre-tap of the transformer otherwise circulating currents will occur unless the voltage symmetry is perfect. However, the interwinding screen, if there is one, should be earthed. This would mean separating any internal connection that might exist between the screen and the CT. The screen would otherwise be at an 'artificial' neutral held halfway between the two live conductors by the transformer itself. Not a big deal but not technically correct. I can't see any advantage to making this part of the system 110V TN.

The two sides of the transformer are electrically separate so it shouldn't matter whether the earthing systems are interconnected or not. The output of the transformer, with N-E linked, is regular TN-S so that just needs an earth reference as well.

Are you providing thermal protection to the transformer, and if so, how? Is the 5kVA rating intermittent or continuous? (just trying to visualise the tranny). How close has the output voltage worked out in practice? I'm expecting some contributors here to be thinking that this is a cheeky way to get 230V but TBH it's the sort of thing I would do so I'm not shooting it down!

5kva continuous so I have a 20A double pole mcb on the output side. I am looking for a cheap 10KVA on ebay so that I can use the full output of the generator.

To make the 55-0-55 part of the system true reduced-low-voltage, the centre-tap of the generator output should be referenced to earth. There should be no connection to the centre-tap of the transformer otherwise circulating currents will occur unless the voltage symmetry is perfect. However, the interwinding screen, if there is one, should be earthed. This would mean separating any internal connection that might exist between the screen and the CT. The screen would otherwise be at an 'artificial' neutral held halfway between the two live conductors by the transformer itself. Not a big deal but not technically correct. I can't see any advantage to making this part of the system 110V TN.

The two sides of the transformer are electrically separate so it shouldn't matter whether the earthing systems are interconnected or not. The output of the transformer, with N-E linked, is regular TN-S so that just needs an earth reference as well.

Are you providing thermal protection to the transformer, and if so, how? Is the 5kVA rating intermittent or continuous? (just trying to visualise the tranny). How close has the output voltage worked out in practice? I'm expecting some contributors here to be thinking that this is a cheeky way to get 230V but TBH it's the sort of thing I would do so I'm not shooting it down!

Thanks. what happened was I got a 7KVA road tow lighting tower really cheaply (£385) because it was advertised as 110V only. It has a 1500RPM Kubota diesel engine and runs great. I want to keep using the lights from time to time, and it also has a couple of 110V CTE outlets. I thought I could be clever and change the jumpers on the generator, but it is 55-0-55 only.
The idea was to use it as a backup for the house, as I said. The transormer is one of the big metal box ones so I can see all the connections. I don't think overheating will be a problem as it is rated at 5KVA continuous. I have the inlet on a DP MCB on the 110V side, and the 240V output the same at 20A.
My plan is to get a 10KVA transformer cheap on ebay to get the benefit of the full output capabilities of the genny.
I can remove the centre tap connection on the transformer, and leave the screen connected, that way I think everything should work fine? I can loop N&E on the 240v side inside the box. I then stake the generator so that has an earth path, and stake the house inlet so it has an earth on changeover.
What do you think?

 

[Simon47]

You can now add an earth electrode to a PME supply and at one point it was expected to find it's way into BS7671 as a requirement. Plus it's forbidden to have any switching in a CPC.
So just add your earth electrode to the installation MET and use that to earth the genny and the case of the transformer.

Then as you say, earth one side of the 240V to make it TN-S and feed that into your C/O switch.