Earthing the Array - Practically

[edexlab]

We've done a lot of transformerless inverters and I tend to bond to across the rails ie if there is four rails for a portrait system these have all been TT ,and the array earthed to a rod
as i posted on another thread i have tested for voltage on the Tl jobs we've done and there is always a voltage of >50v (highest being 108v) on the frame turn the inverter off and it vanishes ,i tell our customers to turn the system off when cleaning panels or working near the array
I did have a pdf explaining this which was possibly from Sma so i'll have a look for it

 

[malcolmsanford]

Can I ask where this grid comes from? ............is it from the MCS.

The point about the exported earth of a PME system is that any extraneous conductive parts in an out building or now if you prove that a array frame is one, then you need to bond the ECP back to the MET.

In many outbuildings that are several metres from the building it can be cost prohibitive to run a 10mm bond many metres back to your MET, and so in these cases it is better to make the outbuilding or frame a seperate TT, but AFAIK there is no regulation prohibiting you exporting a PME earth and bonding the ECP to the MET.

It just rankles a little when I see something like that chart that prohibits something that is not prohibited.

 

[SolarCity]

It just rankles a little when I see something like that chart that prohibits something that is not prohibited.

I don't think the DNOs are too keen on you taking a PME earth out of a building either.

 

[Worcester]

@malcomsandford,

It's part of the regulations that MCS installers have to comply with it's the
DTi Photovoltaics in Buildings, Guide to the installation of PV systems 2nd Edition, Published in 2006
https://www.electriciansforums.net/...llation_of_pv_systems_2nd_edition-pdf.120211/

 

[edexlab]

the pdf i was talking about is titled capacitive discharge currents from Sma this explains how the voltage transfers to the array,hence the need to discharge to earth with Tl inverters,I'm sure i've got another regarding earthing the array,so i'll keep digging

 

[malcolmsanford]

the pdf i was talking about is titled capacitive discharge currents from Sma this explains how the voltage transfers to the array,hence the need to discharge to earth with Tl inverters,I'm sure i've got another regarding earthing the array,so i'll keep digging

I can understand when your not using a transformer invertor as your not using seperation via the transfromer, that there is a chance that a fault on the DC side can easily be transferred into the the AC system, if the array frame is not bonded.

 

[Worcester]

That'll be this then:

http://www.sma.de/en/service/downlo...pe=281&cHash=d3a10c9cf6160b1340d21fc2730cb5a9

It's on the 4000TL downloads page - choose English International.

 

[malcolmsanford]

Thanks for the post Worcester I have to cede that your knowledge of PV is far above mine. I just take exception to the fact that a body as categorically stated that a PME earth can not be exported, and that is just not true.

As I posted providing that certain requirements are met there is no reason why a PME earth can not be so.

I'm sure you saw that I was arguing here a week or so ago why the ECA thinks that a PV system that is installed in a domestic situation on a split RCD board contravenes regulation 314.2, as I can not see why that is correct.

I feel that there is a lot of uneccessary red tape concerning this part of our industry, and that certain organisations are trying to re-write the basic electrical fundimentals. It is a new branch of our industry but when section 710 for medical equipment comes into force I wonder also how many "bodies" will try and put their interpretations on the Regs.

 

[Worcester]

@malcolmsandford, no problem, appreciate the debate and being challenged once in a while

The problem we have is that the 'inspectors' are not technically minded, they are process driven. Some don't even like a variance in the placing of an isolator (even when the drawings show two variations!) ...

The PV industry and certainly the regulations / guidelines are really only now being tested (just talk to the DNO's, some are still strugling to understand FITS 15 months on)

 

[Markc]

Not needing to have bond any arrays (yet) we had discussed similar to Biggssolar only using the fixing bolt, roof anchor to rail rather than rail joiner but then we also discussed where rail slip joiners are used for expansion and considered these as not an effective means of continuity as only one end is fixed to the rail and the other floats inside the rest of the rail. So as and when we need to we will use a additional one turn bolt into the rail as a specific bonding location and again on a floating rail. We would also bond every rail. We use Schucco system by the way.

 

[Worcester]

Thanks to all that have contributed, seems to me that BiggsSolar and Markc are suggesting 'currently adopted best practice' until specific guidance comes out (if ever)

To summarise, currently adopted best practice is:

1) Bond each rail.
2) Bond across rail expansion joints.
3) Single earth cable (10mm2) to appropriate earthing point as per guide.

How to bond:
Connections: Use the fixing rail mountings, a dedicated bolt to the rail (stainless steel), or an external earth bonding strap.
Bonding method: 10mm2 Braid or equivalent

p.s we put warning stickers on EVERY panel saying 'LIVE DURING DAYLIGHT HOURS' - does anyone else?

Not sure as I'd want to put a sign up saying "Isolate before working on the roof" otherwise where would you put it, and how many places would you put it... putting by the isolators is a waste of time, and a big sign on the roof?

 

[SolarCity]

p.s we put warning stickers on EVERY panel saying 'LIVE DURING DAYLIGHT HOURS' - does anyone else?

Again, we had exactly the same conversation yesterday! My colleague thinks we should be putting these on each VISIBLE side of the panels and a "Do not disconnect under load" sticker. I disagreed as I don't think it is necessary. I think it is fair to say at the very least that anyone messing with the panels would switch off the DC isolator. This removes any load and therefore 99% of the minimal risk when unplugging the panels.

 

[BruceB]

Well I think I agree with you Worcester that what you describe would be best practice if you have to bond the array. For a number of reasons I think it is over the top and I also know the DTI guide makes some incorrect statements about PME which lead it down what I believe is an unnecessarily pedantic path. I have only been on this board a couple of months and have a faint recollection that someone here mentioned around the time I joined that the DTI guide was being updated. Anyone know?

One other point about best practice - if you are going to do all the bonds you mention above then they are de facto part of the main protective bonding regime. Therefore every joint must be labelled with a "Safety Electrical Connection - Do Not Remove" label in accordance with 514.13.1.

Regards
Bruce

 

[dansk]

"p.s we put warning stickers on EVERY panel saying 'LIVE DURING DAYLIGHT HOURS' - does anyone else?"

We dont do this - not to say its a bad idea - but anyone working on the PV system should be suitalby qualified and therefore know there "live during daylight", and anyone not; should not be touching with things they dont know about.

 

[Worcester]

@BruceB

if you have to bond the array

Agree entirely.

every joint must be labelled with a "Safety Electrical Connection - Do Not Remove"

good point

@dansk, The problem is you get roofers, builders, TV aerial fitters, Sky etc all crawling around there, who may know nothing at all about 'that PV stuff'.