Array bonding needed?

[rich.250]

strange how when managing projects it's the time served apprenrice trained electricians who have tried to insist everything has to go on an RCD under 17ed, and that it's fine for the PV to go on a shared RCD, and aren't prepared to listen to anyone telling them different. At least the short course guys tend to be willing to listen and learn, unlike the arrogance of some time served sparks.

Well you always get some. Tend to be the ones who hate updates and testing there own work as they know it's good LOL

 

[moggy1968]

yep, I'm afraid it's not how you trained that matters, it's how good you are at your job

 

[moggy1968]

TBH the whole issue of having to go to a spike if it's TNCS makes no sense to me, it's totally inconsistent to my mind. If we say an attached garaged is in the equipotential zone then why is a structure attached to the roof not? It's even acceptable to bond back to the MET on a detached garage according to ELECSA as long as it's not too far away (this was the scenario on my first assessment, they declined to say what 'not too far way' actually meant).
I can't really fathom why there is such an inconsitencey.

 

[rich.250]

You can extend a equip zone anywhere in theory. Aslong as you bond any services or parallel paths in that part back to the MET at the origin with a unbroken cable. I.e normal bonding rules. The only thing is you need to calculate the size of conductor required to stay under your 0.05 ohms.

As for bonding the array. I don't see that it should be introduced into the zone. As it is not in contact with the ground. So no parallel path. At all.

This reminds me of an example. Would you bond your knife and fork, there metal. And you can touch them and a bonded pipe..... LOL

 

[rich.250]

Theres a sticky on here to look at on exporting tncs

 

[moggy1968]

I don't see why it should be either, but if it has to be I don't really see why it needs a spike on a TNCS when other things don't.

This reminds me of an example. Would you bond your knife and fork, there metal. And you can touch them and a bonded pipe..... LOL

I've seen door handles and bannister rails supplementary bonded!

 

[rich.250]

The stake on a tncs is an absolute no no. I'd never even contemplate it.
All you will do is cause a potential difference.

I seen this on a farm, well reversed.
Retarded spark put new cct on tncs. And the difference in potential between the farm (TT by requirement)
And the new cct. Killed cattle.

 

[malcolmsanford]

tbf to the dti guide, this was a problem with your tutor, not the guide.

The guide says bond to MET if it's in the equipotential zone to avoid this issue.

Are you a PV installer, becasue even though i'm not I still know that the current DTi guide tells you that if you have a TNC-S system that needs bonding it must be spiked ....................or am I wrong

 

[malcolmsanford]

yep, I'm afraid it's not how you trained that matters, it's how good you are at your job

Is that a serious remark .................... your telling me that it is irrelvant how you are trained

 

[Gavin A]

All you will do is cause a potential difference.

potential difference to what though?

under the DTI guide 2006 decision tree the frame should only be bonded to a seperate earth spike in situations where there is nothing else for there to be a potential difference to. If there is anything within reach of the frame that was bonded to the MET then the frame would have to also be bonded to the MET.

That's the argument in favour of this as far as it goes, along with not unnecessarily bringing the equipotential zone out on to the roof (via bonding to the MET), as anyone then accessing the roof / frame via a metal ladder, or scaffolding and touching the frame would then introduce the danger of a potential to earth via the ladder / scaf that was different enough to that via the MET bonding to cause problems if both were touched at once (pretty likely).

Personally though we've not been installing earth spikes, or earthing the frame at all unless we actually measure a potential to earth from the frame since last Autumn when I realised that the new draft guidance matched our real world experience and measurements that show bonding the frame to not be necessary unless there already was a potential to earth.

Comparing the risks from the few micro-amps worth of leakage current the panels / frames may end up carrying with a TL inverter vs the risk from either earthing or bonding the frame resulting in a potential to earth that didn't exist before, and IMO the safest option is not to bond or earth the frame.

If there is potential to earth via the frame already, then that changes things.

The risk of a slight tingle from the frame in damp conditions can be entirely removed by switching the DC isolator off anyway before accessing the roof, which is the instruction we give to our customers.

 

[Gavin A]

Are you a PV installer, becasue even though i'm not I still know that the current DTi guide tells you that if you have a TNC-S system that needs bonding it must be spiked ....................or am I wrong

you're wrong.

In the equipotential Zone? >> Yes >> bond to MET*


if it's not in the equipotential zone, then you would bond to an earth spike if you were following the 2006 DTI guide.






*actually it calls it the consumer earthing terminal, but I take it you'd accept that this is what it means.

 

[malcolmsanford]

Are you still not following the 2006 guide, as the new guide is still under consultation I believe ..............or have you started working to it?

How is something on a roof in the equipotential zone, unless you extend the zone out to it which the 2006 guide tells you can't ?

 

[rich.250]

Yes that makes sense. I can see that bonding it to the met in a TN system could introduce a potential to a metal ladder. Which as you said. Is exactly why it should not be bonded. At all unless you do somehow measure continuity from the frame to the met.
I think more people need to just stop for a second and think before just following these guides like yourself. It will make perfect sense to them if they think about it. And after all they are just guides.

 

[Gavin A]

Are you still not following the 2006 guide, as the new guide is still under consultation I believe ..............or have you started working to it?

mostly we are, but in this case no we're not because we believe it to be unsafe, and the draft guidance gave me sufficient support for what I was already thinking to take the decision to stop doing something that was unnecessary and potentially dangerous just because some 6 year old guidance written when TL inverters had only just hit the market said we should.

I'd be surprised if the authors of the 2006 DTI guide had installed anywhere near as many TL systems as we have, and our real world experience indicates that the issue they were trying to guard against is simply not a problem in reality.

How is something on a roof in the equipotential zone, unless you extend the zone out to it which the 2006 guide tells you can't ?

it's in the equipotential zone if some other bonded metal system is already bringing the equipotential zone on to the roof within reach of the frame (eg gas flue), or it's below a velux and within touching distance of a radiator etc. (or by some interpretations of it, just being within touching distance of a velux itself).

 

[rich.250]

The velux??
That doesn't need bonding either.... Lol.
I can see your point about people hanging out the window and holding the rad for support. But like you said if the array isn't bonded then there's no chance of them coming to harm as it is on the roof with no path to earth.