array earthing / bonding

[maffa ent]

evening all, I'm after a definitive answer regarding array earthing please. links to information would be fine?

I read the 'decision tree' on the dti guide, and decided that, using a transformerless inverter, the array would need a separate 10mm bond to a dedicated rod.

The supplier then confused me by saying that, if used in conjunction with a type b rcd (presumably he meant an rcbo?), the separate bond would not be required if the array was outside the equipotential zone (which, on the roof, it will be).

Laugh at the question if you must, but I'm a newbie with pv, just getting set up for it, so it may be a simple problem to you, but I'd like to start off doing things correctly.

thanks in advance.

 

[SolarCity]

Always good to ask if you're unsure.

You will still need to bond the roof regardless of the type b rcd. A minor voltage would be present on the array frame if it is not bonded and this would exist regardless of the RCD, whether it switched on or not.

You only need to use a dedicated rod on a PME (TNCS) system.

 

[maffa ent]

thanks for the reply mate. i take it if i use an inverter containing a safety isolating transformer, then i dont need to do anything with earthing or bonding the array framework?

 

[SolarCity]

That's right. Just leave it floating if you're using a standard string inverter.

 

[malcolmsanford]

Maffa welcome to the forum and good luck with your venture, but I would be less than diligent to ask, do you feel you are ready to do a project in someone's home. On another post on the forum you are asking about what size cable would be needed when piggy backing an Enclosure onto an existing CU, without a thought to design ie loading, type of cable erection method.

On here you have rightly pointed out that for a transformless inverter a B type RCD is required. You put do they mean a B Type RCBO, do you know how a B type RCD differs from a B type RCBO ?

I have a feeling you think that the B is the inrush characteristics of the device, which is not the case of a B type RCD.

 

[SolarCity]

On here you have rightly pointed out that for a transformless inverter a B type RCD is required. You put do they mean a B Type RCBO, do you know how a B type RCD differs from a B type RCBO ?

I have a feeling you think that the B is the inrush characteristics of the device, which is not the case of a B type RCD.

To be fair, I doubt that the majority of electricians would know the difference. Judging by my correspondence with suppliers, I would say that the confusion is industry wide.

 

[VMan]

To be fair, I doubt that the majority of electricians would know the difference. Judging by my correspondence with suppliers, I would say that the confusion is industry wide.

That's quite worrying then!! ,

RCD's alone won't protect you if basic protection failed, eg, short circuit, only a fault to earth!!

RCBO's will protect against Basic and fault protection.

 

[malcolmsanford]

To be fair, I doubt that the majority of electricians would know the difference. Judging by my correspondence with suppliers, I would say that the confusion is industry wide.

That Biggs must be a sign of the woefully poor training that is now being adopted industry wide. Fingers can be pointed at 5 week wonder courses and the such, but it goes deeper IMO than this. It is OK in my opinion to have guys that may not know, and not want to know protection device characteristics, they are told to run cable here, connect to this and that and they are happy, good electrician mates or this ridiculous new scam the "Improver".

But when your designing an installation as Maffa is doing then I would expect the designer to have the expertise to know what a B type RCD is, what a B type RCBO is, especially in the PV industry. Granted an electrician not within the PV industry is unlikely to encounter any RCD other than a bog standard AC type.

 

[SolarCity]

RCD's alone won't protect you if basic protection failed, eg, short circuit, only a fault to earth!!

RCBO's will protect against Basic and fault protection.

While I've no doubt that all electricians (and I use the term tightly - I'm not referring to 5 week wonders) know the difference between an RCD and an RCBO, when you thrown in the 'type B' part it tends to confuse people. I'll be honest and admit that I'd never even heard of 'type B' RCDs before I moved into PV installations.

 

[SibertSolar]

In fairness to the likes of Maffa, it can be a bewildering array of options and it is a dynamic industry with new technologies presenting new challenges on an almost weekly basis. AC, A and B Type RCDs, B and C Type RCBOs, RCMUs, Type B, C & D MCBs, Type 1 and Type 2 AC and DC surge/lightning protection, time-delay, surge-resistant, etc the list goes on......it's difficult enough as a supplier so we empathise with engineers who are not only trying to keep up with the PV technology but also having to correctly interpret and keep to the electrical code too!

One of our primary objectives is to try to understand as much as we can and to share information with our peers and client base wherever possible. If there's anything we can do to help further then my door is always open,

Andy

 

[malcolmsanford]

I hope that this is not an avocation of design by telephone.

I have no problems with suppliers advising on technology or the advances in them, as the days are gone I think when a supplier sold someone an item of equipment and their obligation was over as soon as money exchanged.

What I'm saying is that perhaps someone that is not sufficiently experienced enough or technically informed enough to realise what a component is, and it's use, should not be designing PV, or for that matter any type of electrical. installation.

IMO it should not a matter of someone being able to call a supplier and saying

"Oh I read this inverter catalogue and I need ......hmmm a B type RCD ?"

And said supplier

" Yes we have them 50 pound+ vat (SIC)"

To me that is papering over a very large crack in someone's training and education, and is what is systematically wrong in the industry.

 

[maffa ent]

I have to admit, I have no experience regarding type b rcd's, but I'm all ears!

I do understand plenty of what you're saying, as it's been my job as a spark since my apprenticeship finished in 1990 to understand. Throughout that time it's always been my intention to do a good job, and nothing's changed.

Lack of experience is my problem with pv, and my customer understands that, and has had it reflected in the price of his job. I've done plenty of electrical work for him in the past, and he's always been happy.

I totally appreciate what you're saying - I've said the same about some inexperienced installers of various things in the past - the difference is I will get it right. It may take me lots of second guessing, and lots of time, and lots of questions on places like this!!, but the job will be right.

It's the only way to do it right?

 

[maffa ent]

by the way malcolmsanford, myself - maffa ent - and maff (who asks about piggybacking a unit) are two different people.

 

[Graeme Harrold]

When I did my PV course the instructor did mention the type B RCD and also mentioned that it was an elusive beast that none of the major wholesalers knew existed never mind carried. From a technical aspect it is quite confising for someone entering the PV market to automatically think of a type B RCD as one having B curve inrush characteristics, so I would blame the naming convention for inducing the confusion. Only recently, and Im talking weeks have Type B RCD, been pushed by suppliers (thanks to Sibert) that have seen this hole in the market. I for one was advised to fit transformered inverters UNLESS the manufacturer had stated they were not required due to their DC leakage prevention characteristics e.g. SMA.

To point out I come from a Domestic installer background, and therfore one of the 5 week wonders......I did my PV course with time served sparks who were cluelless about DC, then again I have 20 years experiance in DC and found it a piece of cake, but that did not give me the right to poke fun at the 3 day PV wonders!!! We all have to learn

 

[Ramjam]

If you use a sunny boy inverter such as a SB4000TL then you will need to earth the array frame. This I was lead to understand was because the array frame may provide a tickle to anybody working on the roof and that it would not cause injury itself it could cause them to fall...

This is only part of the story. We installed an SB4000TL and accepted an earth electrode resistance of 190ohms (I know less than 200 for regs, less than 100 for good practice!).

The inverter caused the domestic 30mA RCD to trip around every hour. It turns out that the earthing of the array frame for TL inverters is not just a matter of good housekeeping but failure to provide an escape path for the leakage means that it will travel through the inverter causing nuisance tripping of the RCD.

The solution from SMA is to use only 100mA RCDs for TL inverters as upto 80mA is possible. This is fine in Germany but not here in the UK. Apparently lots of installers simply don't RCD protect the circuit and don't earth the array frame, hard to believe but apparently quite common!

Moral of the story, fit TL inverters but put a good earth rod in or you'll get nuisance tripping.

Oh and by the way there is a 100mA type B RCD fitted internally in Sunny Boy Transformerless inverters so there is no need for an external one. I don't know about other makes because we only fit Sunny Boys!