TL inverter & array frame bonding..confusing at best!

[bumcrack]

Hello!

I am putting in a 4Kw ground mount scheme with a TL inverter and I can't make sense of the DTI earthing tree. Framework to be of unistrut construction, and effectivley insulated from tera firma by concrete.

The main incomer is PME, and if I follow the DTI tree, the end result is that I bond the array frame to a local earth spike. It's the footnote to this in the DTI tree " do not take PME out of equipotential zone " that concerns me.

My intention is to mount the inverter onto metalwork on the underside of the array. Cable wise, 3 core SWA to come from below ground to AC isolator, then from isolator to inverter with 3 core SY type flex. I will use core 3 of the SWA and SY as earth all the way to the inverter earth terminal.

The inverter hangs on a metal backplate, bolted to the array frame. In doing this, am I effectively taking the PME out of the equipotential zone?

Surely there will be continuity from the inverter earth terminal to the inverter chassis/metalwork,backplate, and this will extend the PME to the array frame?

My only solution would be to mount the inverter backplate on insulators, and then bond the framework down to an earth spike.



Does that make sense?

cheers...

 

[BruceB]

I would not class damp concrete as an insulator. I suspect if you measure resistance to earth it could well be extraneous.
Two choices really:
- extend EPZ and retain PME You need to bond the array framework. The easiest way will be to make sure the SWA you are using is at least 10mm2 so that you can use the earth wire also as a bonding conductor.
- TT the external elements to a local earth spike (inverter and array framework).

Actually a third coice - you could do both.

Regards
Bruce

 

[pvman]

I would put in an earth spike, not too much extra cost and your covered.

Tom

 

[bumcrack]

Cheers boys...

The Dti tree was pointing me to an earth spike anyway. I'm not fussed about cost. The cable out to the array is a 16mm 3 core SWA

So if I read this correctly, I can extent the EPZ of the building by using core 3 16mm as a main earth bone?


The only other swerving ball is that the 3 core is not going back to the main house CU. It goes to a board in the loft that is fed by what looks like a 10mm T&E. The earth in this will be what, 6mm?

Access to the main CU in the house is very tricky, and the metering is remote again from this. I'm not sure I can extend the EPZ in the true sense.

 

[GaryM]

It goes to a board in the loft that is fed by what looks like a 10mm T&E. The earth in this will be what, 6mm?

Nope, 4mm

 

[bumcrack]

Cheers.... I dont recall seeing any additional earthing up to the loft CU, clearly a 4mm is too light.

So I can't extend the EPZ out to the array, so its a rod job, this is no problem.

Back to the original question then, the inverter will need to be earthed, do I need to ensure this earthing which is PME, does not extend to the array metalwork?

thanks so far for your help, its much appreciated !

 

[moggy1968]

you cannot transport the earth, end of.
you need a spike to earth your frame and the earth from the inverter must also go to the spike (you could use an earth block). you cannot eart it back to the building.
you cannot take any of your earths back into the building because you are outside the equipotential zone. You should of course also earth the metal sheath of your SWA to the spike.

 

[yellowvanman]

you cannot transport the earth, end of.
you need a spike to earth your frame and the earth from the inverter must also go to the spike (you could use an earth block). you cannot eart it back to the building.
you cannot take any of your earths back into the building because you are outside the equipotential zone. You should of course also earth the metal sheath of your SWA to the spike.

SWA should be earthed at the source ie. in the house. I would suggest terminate the SWA in an insulated box and then earth everything after that from the stake.

 

[moggy1968]

yes, but surely the source in this instance is the panels. the major current is going from the array to the house so the SWA should be terminated at the array?

 

[yellowvanman]

yes, but surely the source in this instance is the panels. the major current is going from the array to the house so the SWA should be terminated at the array?

You're not wrong but a fault would get most of the current from the grid, and generally the overcurrent device is at the remote end of the cable from the inverter.

 

[moggy1968]

once upon a time it all seemed so simple didn't it!!
yes I see your point there, if you put your spade through the cable you want the earth and protective devices to be functioning towards the mains end. I give up, my head hurts!!

 

[BruceB]

you cannot transport the earth, end of.

and yet it get discussed on every electrical board every week of every month.....

 

[yellowvanman]

As it is you're still going to get a 5 second shock (as opposed to 0.4sec) from the inverter!

 

[powervan]

all these ideas all come to one thing!!!!!!!!!TT it !!!!!!!!!!!!!!!!! or sack the previous spark who waisted a 16mmswa by joining it to a 6mm t/e cable! shud of gone to scrappy

 

[bumcrack]

you cannot transport the earth, end of.

Cheers moggy. I dont disagree with your statement, but to me, having all the earthing arrangements at the array and inverter relying on spikes seems odd. If the link between the rods and array goes ( say through corrosion ), its game over! Maybe just my way of thinking?

What about having the inverter and SWA sheath bonded to to PME, but nothing else?

Or is your statement really as straight forward as you say, i.e, NO PME taken out of EPZ, regardless if its insulated from external metalwork?

cheers

 

[moggy1968]

it is an odd thing I would agree, we rrecently did a board change in a static caravan and the council asked us to TT it. previously it was using the Earth from the main supply to the park.
before inserting spike, Ze,about 0.2, after inserting spike, about 76, so by complying with the regs we made his earthning worse.
In my view the ideal would be PME with an additional spike outside the zone but you can't have 2 earths.

 

[bumcrack]

Moggy...

I think I have answered my own questions to you. If PME is at the inverter ( but isolated from ground ) and the array is at ground, there is the potential for a potential, between inverter metalwork and PV frame...!

cheers..

 

[moggy1968]

It is simple as, except it isn't!! for my first part P inspection I rewired my garage, which was detached and about 5 meteres from the property. Prior to the inspection I rang ELECSAs tech support and asked if I had to spike it and they said as long as it's not too far away it would be ok to transport the earth. I asked how far away not far away was and they wouldn't give me an answer. Me and the inspector had a lengthy chat about it afterwards but still non the wiser, oh, and I transported the earth and he was fine with it.

 

[GreggElectrical]

and yet it get discussed on every electrical board every week of every month.....

Any when you tell your suppliers you dont need three core to supply that garage e.t.c they look at you funny. Most round here don't even stock 2 core.