Earthing of standby generators

[James]

Instructions say it is a floating N but nothing else.

 

[pc1966]

Looking at the diagrams it has a schematic at the end:

https://cdn.shopify.com/s/files/1/0624/4745/2318/files/CPG4000DHY-eu-technical-drawings.pdf

Looks like a 3-phase alternator and the "control unit" might be the inverter that converts rectified DC off that to good sine-like single phase AC. However, there are no RCD on the outputs and I'm guessing the floating supply approach is the means of reducing the risk of shock on contact with a live conductor instead of ADS from a RCD.

Without modifying the generator (not a trivial task) you can't really win:

• Earth the neutral some place and you have no additional protection on the generator's socket outlets, against the regs. Also if you ever use a properly engineered generaotr with RCD it will trip as a result of the link.
• Don't earth the neutral and the house has an IT supply without skilled supervision, also against regs

Also the instructions are clearly for the USA market, in spite of it being sold here with UK outlets:

Use a ground fault circuit interrupter (GFCI) in damp areas and areas containing conductive material such as metal decking.

Connection to your home’s electrical system requires a listed 30A transfer switch installed by a licensed electrician and approved by the local authority having jurisdiction. The connection must isolate the generator from the utility power and must comply with all applicable laws and electrical codes.

Do not use generator for medical and life support uses.

In case of emergency, call 911 immediately.

That also fits, as the USA typically has N-E links in the incoming panel/DB, so the issue of a floating supply is not happening. I don't know if they have any similar blanket RCD rules on socket outlets as we do.

 

[goasis]

Looking at the diagrams it has a schematic at the end:

https://cdn.shopify.com/s/files/1/0624/4745/2318/files/CPG4000DHY-eu-technical-drawings.pdf

Looks like a 3-phase alternator and the "control unit" might be the inverter that converts rectified DC off that to good sine-like single phase AC. However, there are no RCD on the outputs and I'm guessing the floating supply approach is the means of reducing the risk of shock on contact with a live conductor instead of ADS from a RCD.

Without modifying the generator (not a trivial task) you can't really win:

• Earth the neutral some place and you have no additional protection on the generator's socket outlets, against the regs. Also if you ever use a properly engineered generaotr with RCD it will trip as a result of the link.
• Don't earth the neutral and the house has an IT supply without skilled supervision, also against regs

Also the instructions are clearly for the USA market, in spite of it being sold here with UK outlets:

Use a ground fault circuit interrupter (GFCI) in damp areas and areas containing conductive material such as metal decking.

Connection to your home’s electrical system requires a listed 30A transfer switch installed by a licensed electrician and approved by the local authority having jurisdiction. The connection must isolate the generator from the utility power and must comply with all applicable laws and electrical codes.

Do not use generator for medical and life support uses.

In case of emergency, call 911 immediately.

That also fits, as the USA typically has N-E links in the incoming panel/DB, so the issue of a floating supply is not happening. I don't know if they have any similar blanket RCD rules on socket outlets as we do.

Thankyou for your detailed response.
I'm thinking I may have bitten of more than I can chew here.
Option A, create a neutral earth link in the plug inlet on the house. There's no additional protection on the outlets of the generator but that is the case anyway?
If the cable from the inlet to the changeover switch is installed in a way which doesn't require rcd protection would that then be OK?

 

[James]

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.

 

[UNG]

Champion 3600 Watt Frame Type Inverter

Champion 3600 Watt Digital Hybrid Petrol Generator. Portable and easy to move. Wireless key start. Perfect for backup power, home use and even camping in the UK.

www.championpowerequipment.co.uk

Shows the level of quality when the warranty is limited to 1000Hrs for domestic use and 500Hrs for industrial and commercial use

 

[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.