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

 

[SolarCity]

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!!

Surely there will still be zero potential to earth?

 

[SolarCity]

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.

It isn't frowned upon - it is a necessity on TL inverters.

Beyond taking an earth outside of the equipotential zone, I can't see why it should be a problem.

 

[SolarCity]

I avoid transformerless inverters if I can 'cos of the aggro of running an earth

That surprises me. We rarely use anything other than TL inverters if we can help it. The increase in efficiency is significant and it doesn't take long to bond the frame.

 

[paulsamuel1984]

Biggsolar - as well as bonding the array frames, if required, are you earthing the array frames? i.e down to separate earth spike for TN-C-S or back to main earth for other earthing systems?

 

[SolarCity]

As far as I can see, you are bonding the array and not earthing it. Earthing, unless I'm somehow mistaken, would have no effect whatsoever as the DC voltage will always be present.

 

[paulsamuel1984]

So you are bonding back to a separate earth spike or back to the MET (depending on existing earthing system)? I am sure it is me who is mistaken when referring to earthing!

 

[SolarCity]

back to spike or MET depending on earthing system (much to some electrician's dismay! - Malcomsanford).

The Dti guide mixes up earthing and bonding and to be fair I think most electricians mix the terms up. I'm sure I often say "earthing the array" when what I actually mean is "bonding the array".

 

[paulsamuel1984]

Thanks biggsolar. I will and have been continuing to bond the array frame back to spike or MET as we mainly use transformerless inverters. But as suggested on this post, my concern was that it could potentially (no pun intended!!) do more harm than good.

 

[and]

Reading this thread initially prompted me to unbond my array; read some more of this thread and then went and re-bonded it;now to be on the safe side I have half of it bonded and half un-bonded.

Damn, this thread is hard to follow!
I hope my system is hazard-free:
TL inverter
array bonded (earth wire connects each of the four rails)
earth wire connected all on its own down to a brand new earth spike.

 

[bigsparks]

The upshot of all this is there are pro's and cons to both sides of arguments reference earthing the array with tranformerless inverters. It comes down to everyones own judgement for each different site. some might say earthing array to electrode is good in event ac gets onto frames etc, others would say if u have an ac 30mA rcd protecting the ac pv circuit any earth fault current over 30mA will isolate the supply and protect the 'window cleaner', some would say that earthing the array just increases likelihood of lightning strikes due the earth grounding. The nic tech guys we spoke to said if u have minimised the risk by: installing an ac rcd to pv circuit, using a transformerless inverter which has an in built protection which would not allow ac into the frame in event of fault and the panels are double insulated are you really gaining anything by earthing the array other than creating a higher potential for lightning strikes etc. The chances of the frames becoming live with ac is very slim with all the above things in place. u just need to ensure u have civered yourself in the event of an incident and as said before there are pros and cons for earthing and not

 

[edexlab]

I would disagree with the statement ''The chances of the frames becoming live with AC is very slim"" and suggest you check the voltage from the array frame to earth rod/Met when using a TL inverter
I've checked this on most of the Tls I've installed which is a fair amount and found that on good days the voltage has been upto 126v down to 56v Ac (this does seem to vary with different makes of panels )being the lowest on an overcast day so in my opinion it's a good idea to follow the manufacturers guidance and bond Tl's ,and I also advise customers to turn the array off when cleaning just in case the bonding connection should ever be compromised as the Ac potential vanishes as soon as the system is turned off

Also most Tl's will normally have residual currents upto 50mA and with some over ,this is why you should'nt use 30mA Rcd's

 

[powervan]

u got more chance of killing urself keep messing with ur array!! best stop reading this and leave it to the EXPERTS!!!! let me no when u find one)))))))))))))))))) u makes ur choice takes ur chances!! or do what some senior members do in there spare time wrap urself in PVC (safe as houses) if not bit swetttty

 

[SibertSolar]

Interesting thread....

"Also most Tl's will normally have residual currents upto 50mA and with some over ,this is why you should'nt use 30mA Rcd's" - important observation. As is Edexlab's comments about PV arrays having a potential difference present when using TL inverters.

PV arrays will generate parasitic capacitive leakage currents (AC) naturally, especially when there is dew/moisture present in the mornings and things start to warm up. Combine this leakage current with anything inherently produced by the TL inverter and you can often "nuisance" trip a 30mA RCD. This is why SMA, Fronius, PowerOne etc recommend the use of a minimum 100mA trip limit RCD (300mA for PowerOne I think) when fitting their TL inverters. Whether this requirement conflicts with a pre-determined need to fit a 30mA RCD in compliance with wiring regs/BS standards (indirect contact fault protection, for example) is a debatable point.

Interestingly enough, I did hear that bonding your array to earth can exascerbate the generation of capacitive leakage currents by the array.

Btw, these slowly rising capacitive leakage currents can often not be detected by an internal RCMU (residual current monitoring unit) in the inverter as RCMU's typically only identify sudden rises in leakage current and will often not result in disconnection until max 300mA is reached. Relying on the inverter's built-in RCMU to act as a circuit protective (safety) device should not be recommended.

In addition to these naturally generated capacitive leakage currents, if a component of that leakage current is pure DC, with any greater than 6mA amplitude, a Type-A RCD manufacturer cannot guarantee that the RCD will continue to operate within it's required trip characteristics (required as a safety device to conform to its own standard/Reg 133.1.3). Some Type-A RCDs have been seen to have operated up to 30% over their intended trip limit when such DC leakage current components exceed 6mA pure DC. Type-AC RCDs will simply find themselves with a saturated trip coil and will NEVER trip, under fault conditions.

Anyway, I digress somewhat....my take on it is somewhat like this: If you have a TT supply, don't use a TL inverter - or if you can't mitigate against the BS7671 requirement for a 30mA RCD, don't use a TL inverter unless you can ensure that a 30mA Type-B RCD will function without nuisance tripping. If you can use a TL inverter, earth the array and fit a Type-B RCD for AC and DC circuit leakage fault protection. If you're concerned about the risk of lightning strikes then follow the risk assessment (as part of the recent BS7671 443 ammendment) and fit Type2 SPDs to the DC and AC circuits. If an LPS (lightning protection system) is already in place (or you have a TT supply) then fit Type1/Type2 SPDs accordingly depending on whether adequate separation distances are respected from the LPS and the array frame.

See here and here and here for some info that you may (or may not) find useful,

Andy

(don't shoot me if I've completely hijacked the thread, my apologies, it's been a long day and I'm a wee bit tired so apologies if the above makes no sense at all!)

 

[powervan]

WOW if ur tired hate to get lecture offfff u in morning

 

[moggy1968]

Aye, he's an intersting fella, knows his acorns! had a long chat with him the other day about RCDs and in particular about TT systems and RCDs IIf you remember andy we discussed wether a 100ma RCD was adequate on a TT supply if there was no other requirement for a 30ma RCD)

 

[SibertSolar]

Lol, indeed, I missed that point but was reminded by Bruce's comment on another thread, cheers

 

[Rw1001]

All mcs approved panels are class 2 double insulated. The dc cabling is also double insulated. For this reason I wouldn't earth the mounting system any more than I would earth metal guttering or tv aerial bracketry.​

 

[edexlab]

All mcs approved panels are class 2 double insulated. The dc cabling is also double insulated. For this reason I wouldn't earth the mounting system any more than I would earth metal guttering or tv aerial bracketry.​

I would suggest you read some of the threads regarding this subject if you have'nt already done so ,and the DTI guide and BS7671 is worth a look, and Siberts post above
and instead of assuming that it'll be ok you can test a TL array to earth and if you're happy not earthing it then good luck, and you'd better hope no one ever climbs a aluminium ladder to fix/replace the aerial (or clean the panels) and makes contact simultaneously with the array and ground