Array frame earthing

[paulsamuel1984]

Having read the draft PV guide edtion 3 out for review, I see very little instances where you would earth or bond an array frame.

I appreciate there are instances where you should but these seem quite rare reading the new guidline, even with transformerless inverters.

is anyone still earthing their array frames when using transformerless inverters or already starting to move over to the new guidelines and keeping the majority of their frames floating??

 

[JulianC]

The new guidelines are still in draft form and subject to change. You shouldn't be working to it, no matter how convenient it might be. If you are fitting a transformerless inverter, bond the array frame every time.

 

[paulsamuel1984]

Thanks Julian,

Thats still my thought process at the moment. I am still earthing / bonding my array frame as per the original guidlelines. Thanks for the response.

 

[Coleman]

Thanks Julian,

Thats still my thought process at the moment. I am still earthing / bonding my array frame as per the original guidlelines. Thanks for the response.

I would'nt bond the array unless you can touch the panels from a velux window etc. because your potentially introducing a fault path this was confirmed by ECA

 

[powervan]

last week nic part p inspector said NO NEVER EARTH ARRAY!! i just dont use TL invertas now!!!!!!!!!!!!!

 

[moggy1968]

my concern with this has always been if there is a fault on the DC cabling causing the array frame to become live without bonding the first you'll know about it is when you fry yourself going up there to investigate. It does concern me having a large metal structure, potentially having potential, so to speak, and no earthing, even with a transformerless inverter. It's a concept I'm having trouble getting my head round.
Without bonding what would stop you getting fried?
Any explanations at a suitably simpleton level gratefully received, I am after all a bear of very little brain!!!

 

[edexlab]

My last Niceic inspection and Mcs inspection the assessors knew very little about Pv and most of the assessment seemed to be talking about the pv side of the install i was using.
I know that in Germany its common practice to earth all arrays TL or not and if you read the inverter manuals the manufacturer usually reccommends you do so ,even though the Regs/DTI guidelines etc say otherwise

 

[powervan]

bear of very iccle brain!! u be a senior member in no time

 

[powervan]

ooops u r ))))))))))))))))))))))))))))))))

 

[SolarCity]

We're still bonding frames.

In general, a fault from the panels to the frame shouldn't be that dangerous in itself as there shouldn't be any potential to earth anyway. That is unless I'm missing something.

 

[powervan]

i spoke to window cleaner yesterday who cleans them for 3 of his clients! i said did he shut it down first? no? i did point out if he had ladder on wet grass(silly but true) then he holds on at top to ladder! other hand sponge!!(yes i no they dont use sponges) but u get the idea!!! if fault (voltage) on array he will be a FLYING oops FRYING window cleaner!! but what do i no? nothing thats y i steeeeeer clear of TL invertas unless totaly stuffed!!

 

[moggy1968]

Big solar
as I understand it, and as I say I do have a conceptual problem with this, if there is a short between the DC and the frame, that frame will hold a potential. it will continue to hold that potential until it is given a path to earth, through a bonding cable if present, or the window cleaner if it isn't!!

 

[powervan]

by jove i think he got it!! perhaps we need someone to demostrate it? any takers!!!!!

 

[bumcrack]

All elasatictrickery wants to do is get back to it's point of origin.

If a DC pos is touching the array frame, it will do nothing untill it finds the DC neg of the source. It can't flow until that path is made.

This is why bonding DC down to ground is frowned upon, in doing that, you are creating a path.

 

[GreggElectrical]

All elasatictrickery wants to do is get back to it's point of origin.

If a DC pos is touching the array frame, it will do nothing untill it finds the DC neg of the source. It can't flow until that path is made.

This is why bonding DC down to ground is frowned upon, in doing that, you are creating a path.

That is my understanding too. The DC +ve is only interested in getting to DC-ve. This means even if the +ve is shorting to the frame, you shouldn't (in theory) get a shock as there is no potential to earth. Kinda like if you were to wire up 10 12V batteries (120v) in series an touch the live terminal. Am I on the right track?

 

[JulianC]

Have a read of the DTI guide:

"a) Isolating transformer: An isolating transformer is one in which the input
and output windings are electrically separated by double or reinforced insulation
(see BS 3535).
While the hazards presented by an array frame reaching the system d.c. potential
may be significant, the potential fault/shock current is typically much less than
that from a mains fault. Hence it is the electrical separation of the mains from the
d.c. using an isolating transformer that is the key determining factor when
assessing the requirement for array frame earthing."

Array frame bonding required when using a transformerless inverter due to increased liklihood of AC appearing on the frame in the event of a fault, due to lack of electrical separation.

 

[moggy1968]

I like the battery analagy, I can see that. I know what the guide says but, like I say, had a bit of a conceptual problem with it as touching the live on AC will produce a fairly unpleasant effect as you are connecting it to earth, but now I think I see.
I avoid transformerless inverters if I can 'cos of the aggro of running an earth

 

[bigsparks]

It is interesting to read all the different opinions on the array earthing. If you follow the DTI flow chart, they advise that you should earth the frame if it is a transformerless inverter, because there is no galvanic isolation between DC & AC, but the SMA TL's say that have a safety device built in that would ensure that AC does not reach the DC side & vice versa in the event of a fault. The regs say you should install a DC type B RCD which cost £300 each, but the SMA has a device that appears to negate the need for one.
Also, if you follow the 17th Edition (or just good practice) all dedicated PV circuits should be protected by an AC 30mA RCD which again would isolate the supply if the inverter developed a fault to earth over 30mA ie AC onto the array framing, so again the window cleaner should not get a fatal shock if the frame becomes live with AC (he may fall & break his back however) but good sense would tell him to turn the AC side off first or as a minimum hold a volt stick to the frame to see if its live with AC current.

Additionally, and probably most crucially, most of the pv panels on the market are double insulated which, even in the event of a fault, the current that could cause a shock should not be apparent on the frame if they are double insulated.

So taking all those factors into account, PV panels must be the safest pieces of equipment we install!! :lol:

 

[moggy1968]

you don't need to fit an RCD if you have simple seperation according to the big green book (assuming none of the other criteria for RCDs are there).

another reason for using transformer inverters.

Also some inverters (4000TL for example) don't like 30ma RCDs.

If your inverter is next to the board, say if your installing in the garage I really don't see any need for an RCD on a transformer inverter to protect less than a meter of visible AC cable.

If your using an RCD it must be a type B, the usual domestic type A may not operate at all or not operate within the required timeframe.
A common error is to confuse a type B MCB (very common) with a type B RCD (very uncommon!). next time you look at an RCBO see what I mean, it says type B MCB, type A RCD. As above type B RCDs are very expensive and not easy to get hold of.

I would suggest a very large percentage of installations out there don't comply with this.

 

[BruceB]

Also, if you follow the 17th Edition (or just good practice) all dedicated PV circuits should be protected by an AC 30mA RCD ...............

People get blinded by domestic 17th practice, but there is no general requirement for a 30mA rcd in 7671. There are reasons to have a rcd of some sort, for example:

- TT
- agricultural
- cables buried <50mm in walls
- etc

but it will often be better to design out the requirement.

Also the regs do not say "you should install a DC type B RCD which cost £300 each..." for TLs. They say if you have an rcd for a TL inverter then it should be type B, which is subtlely different.

Regards
Bruce