Array Frame Earthing

[poheZ]

Hi

I just wondered how other installers are earthing array frames. At the moment we are bolting onto the rail (and following the DTI earthing decision tree)

Does any use bracing bars between rails? Do any of you use a smaller csa (than 10mm) for TNS systems?

 

[Worcester]

You can't just earth one rail, you need to ensure that all are connected and not rely on the panels to do that.

So all the rails should be joined together - horizontally across mechanical and expansion joints as well as vertically.

 

[Tiberius]

Think you'll find the main answer is many are not doing it...

 

[SolarCity]

Think you'll find the main answer is many are not doing it...

I think you're right.

 

[Worcester]

We do as above in 10mm on every TL job ...

There again, if some aerial fitter falls off a ladder, I won't have to go back and retro-fit.

 

[edexlab]

We bond all the rails together again using 10mm although I have to say we don't do it across the mechanical joints ,I would like to think they are secure enough to provide a good connection/continuity between joints ,I have also looked into the feasibility of using a single piece of rail or aluminium flat bar or similar
(still have'nt found anything of the correct csa or larger which would be cheaper than 10mm cable)
which could be fixed by the roofers across the rails when they are putting them on which would allow them to finish the job without being held up by busy electricians , although we would then have to test the frames continuity is good enough .

 

[MorganPVI]

We bond every single panel.

 

[Richard]

I was on a course this week, and he said that there is NO NEED to earth the array, full stop. Is this correct? as it seems to condradict what you say??

Thanks

 

[DomB]

I talked to NAPIT tech support one time who said not to bond the array as it would do nothing more than attract lightning. I followed the reg's and did bond it but more for insurance reasons than anything else!

I do wonder what the point is tho - I guess it's just in case of AC leakage onto the DC side & then a fault on the DC cabling which allows the leaked AC onto the frame?

 

[JulianC]

Don't believe all they teach you on courses! I've yet to go on a course where the tutor has had a good grounding and knowledge of the subject they're teaching. Got sick of the number of times I had to correct the idiot taking my NICEIC PV course. Not as sick as he got of me though, I'll wager. Read and understand the DTI guide - it's all in there.

Used bonding clamps around each rail and 10mm back to the MET (TN-S) on the last TL job I did. I'm happy that this is a secure and effective means of bonding. Rail joints were bolted on each side. I didn't bond across these but my meter read 0 ohms across all rails so that was good enough for me.

 

[GaryM]

Used bonding clamps around each rail and 10mm back to the MET (TN-S) on the last TL job I did. I'm happy that this is a secure and effective means of bonding. Rail joints were bolted on each side. I didn't bond across these but my meter read 0 ohms across all rails so that was good enough for me.

When you say bonding clamps do you mean a specific type of clamp or just a BS951 clamp used for bonding of pipe work?

 

[JulianC]

Yup, basic BS951 clamps (the blue EC15 ones for use in damp/corrosive environments).

 

[GaryM]

I'm surprised there isn't a specific clamp available for purpose but your method is a good alternative. Also good to see people using the correct type of clamp for environment and not red fits all!

 

[JulianC]

We usually use the K2 light rail, which is pretty much a square section with rounded off edges, so bog standard clamps work really well. I'm sure it's only a matter of time before someone comes up with a specific PV rail clamp - at a specific PV price!

 

[Tiberius]

Another set of regs we all read but come to different conclusions. Iv read the DTI section and still no wiser but one thing i do know is that when i spoke to a panel maker rep at a trade show and asked about earthing the frame he said dont as it is double insulated equipment, doing so will also void any warranties, so just beware when literally attaching earths to panels direct.

 

[SolarCity]

Earthing the panels and earthing the mounting system is different though. Even though in practice they will obviously be at the same potential.

 

[krismo]

Pipe clamps? Well, I've just got the same idea.

BTW, we were just installing solar thermal next to the existing PV system with TL inverter. While working on the roof around the copper pipe, I felt a slight tickling in the back of my palm. It was a bit tight space around with the roof ladder squeezed between two systems and touching PV rails. I felt a tickling once again and realized it must be some sort of current and started wondering where it was coming from. Then I realized it must be some leak from PV. I was standing on the straight ladder just by the gutter and tested voltage from PV rails to my hand. It was 5V. Then I tested the voltage from PV rails to the copper solar thermal pipe. It was 38V (!). Couldn't believe it.

 

[BruceB]

In that circumstance where you have solar thermal equipment alongside I would argue that it is important that the solar pv and solar thermal are at the same potential. The EPZ has de facto been extended to the roof by the solar thermal being there so the solar PV array should be bonded back to the MET - even under TN-C-S conditions.

 

[moggy1968]

I looked into this recently when I found some SWA earthed by a bonding clamp round the armouring. The advice I was given is that bonding clamps are specifically for bonding to pipes and any other use is not authorised by the manufacturer. Therefore bonding using an pipe clamp is not acceptable.
I use a looped crimp and self tapping screw.

 

[SolarCity]

In that circumstance where you have solar thermal equipment alongside I would argue that it is important that the solar pv and solar thermal are at the same potential. The EPZ has de facto been extended to the roof by the solar thermal being there so the solar PV array should be bonded back to the MET - even under TN-C-S conditions.

Shouldn't the pipework from the loft space onto the roof space be non-conducting? I don't install solar thermal so I'm not sure if this is common practice.

Absolutely agree though. If the equipotential zone has been extended to the roof due to a solar thermal installation then the array should be bonded back to the MET.

 

[BruceB]

Shouldn't the pipework from the loft space onto the roof space be non-conducting? I don't install solar thermal so I'm not sure if this is common practice.
.

Perhaps that is one way, but it is non-trivial. The pipe has to be able to cope with perhaps 6 bar pressure and superheated steam at say 160°C because the water in the thermal panels can boil and turn into steam if it stops circulating during a power cut. So normal practice is to use continuous stainless steel flexible pipe with a high temperature insulation cover all the way from the solar thermal panel to the solar station, where pump etc is, with no joins en route.

So normally that pipe will be exporting the internal earth to the roof.

I am MCS solar thermal as well.

Regards
Bruce

 

[SolarCity]

Thanks, as always, for the helpful input.

 

[robassais]

Pipework from loft space to the roof should also be rodent proof so metal pipework is always used

 

[krismo]

Thanks Bruce.

One worry: when you think of bonding PV array back to MET (assuming TN-C-S and solar thermal scenario) then you still are facing the same hazard coming from the PEN failure, aren't you?

Would it not be a good idea to install supplementary equipotential bonding between both thermal and PV as an additional protection but still have the PV array grounded with a spike?

It seems like being on the roof you might be in two different potential zones depending whether being in contact with a ladder that stands on the ground or touching metallic parts of a solar thermal system.

Question two: would you still bond PV array to solar thermal or back to MET (bringing both systems to the same potential) when the inverter has a transformer?

K.

 

[BruceB]

The problem with earthing and bonding solutions is that you can hardly ever protect against every failure mode at the same time. Protect against the most likely and you often make yourself vulnerable to a less likely failure mode.

Supplementary bonding plus an earth spike would be good. There is a Paul Cook document on the IET site about earthing and bonding iirc that recommends an additional earth spike in domestic installations even with TN-C-S. It might help a little but does not eliminate the broken pen problem in your ladder scenario because most of the voltage drop would be across the earth spike.

If the inverter has a transformer then go back to basics and measure the resistance to earth of the array framework. If >23kΩ then it is not extraneous and does not need bonding.

My view is that there is often a bit of engineering judgement to be applied on-site to get a good answer on earthing and bonding issues. They are rarely black and white - witness the likely changes in the DTI guide.

Regards
Bruce