I suggest you grab the IET Code of Practice if you've not already got it, which specifically discusses this.

Why are you creating a TT?

If creating a TT, don't forget the inverter leakage current will be significant which will affect RCD trip current and device type.

Many thanks for your response and I have the COP so happy to adhere to those guidelines in respect to array bonding.

I was trying to alleviate leakage current back to the site providers installation hence the remote TT and I know the ramifications this has with the selection of earth leakage devices.

Appreciate your input, thanks Jam.

Bill Young said:

I was trying to alleviate leakage current back to the site providers installation hence the remote TT

The issue is, the 'leakage' will have to go somewhere. If you don't send it back through a low resistance path, it can corrode the frame stanchions ... and possibly  increase AC leakage currents and harmonics, especially with non-separated inverters. You will almost certainly see currents flowing through the soil trying to make their way back to the transformer neutral.

Secondly, there are also AC protective conductor currents to deal with. If they don't have  low impedance paths back to the transformer, they will possibly again appear as harmonics.

Most inverters perform best from a harmonic perspective (and leakage current perspective) if they are in a TN system (often TN-S to reduce circulating soil currents).

Bill Young said:

I was trying to alleviate leakage current back to the site providers installation hence the remote TT

The issue is, the 'leakage' will have to go somewhere. If you don't send it back through a low resistance path, it can corrode the frame stanchions ... and possibly  increase AC leakage currents and harmonics, especially with non-separated inverters. You will almost certainly see currents flowing through the soil trying to make their way back to the transformer neutral.

Secondly, there are also AC protective conductor currents to deal with. If they don't have  low impedance paths back to the transformer, they will possibly again appear as harmonics.

Most inverters perform best from a harmonic perspective (and leakage current perspective) if they are in a TN system (often TN-S to reduce circulating soil currents).

Hi Graham,

Many thanks for this in depth response and I will now change my system design approach based on your summary. This is why I became a member of the IET to allow me to tap into the years of experience of it's members, once again this has been very helpful.

Rgds, Bill

gkenyon said:

If you don't send it back through a low resistance path, it can corrode the frame stanchions

Side note: On the expecation that you're using glav steel framework, I would recommend bituminous paint on any exposed bonding connections and try to make them in a relatively dry part of the structure, otherwise bimetallic corrosion, accelerated by the leakage current, will make a mess over time.

Many thanks again, the frame work is going to be aluminium I believe but still to see the detail of that however I will make sure that all these bonds are well protected for sure, much appreciated.