Weird?! Neutral-earth fault

[ADelectrical]

Hi was recently at a property and was installing a Solar PV system. was doing my testing and i came across a weird result when doing the insulation resistance stage.

testing between Neutral and earth I got a dead short. so split the circuit and narrowed it down to a 1ft long length of cable bewteen the DB and a generation meter. everything else was clearing no issue but this seciton was still dead short.

now heres the thing when testing just the cable i.e taken out of the bus bars. the cable cleared but when they where connected in it was a dead short.

at the time this was on the RCD side of the a split load board. the solar was using and MCB and the RCD had been changed for an A type the neutral was installed on the RCD neutral bar and the earth was installed in the rest of the earth bus bar.

any ideas what might have made this happen?

in the the end I installed the solar circuit onto it own RCBO and the fault was cleared but would like to know any suggestion as to why i might have been

 

[timhoward]

I seem to recall that the anti-islanding protection operates within about 5 seconds (could be wrong about exact length of time allowed), so certainly it would be well beyond the permissible disconnection times of an RCD.

It seems to vary a lot between inverters. The first time I encountered this I’d been working on a circuit and did RCD tests which failed at about 450ms. ( There was no signs of PV at all at the CU and no two sources of supply sticker which didn’t help)
I’ve known other ones (Solar edge) be very snappy but it still is against the regs and an instant C2.

As you say it’s more normal to split the tails and feed from own CU. I think RCDs both ends would still contravene the reg that prohibits a shared RCD and multiple sources of supply but I’m not looking it up tonight so could be wrong.

 

[rewireIT]

Wow you guys, i've seen hundreds of installs just installed directly on the RCD. They try to do them on RCBO's but sometimes they would bang it in on the RCD because it was a lot of council work where the council wouldn't pay for an RCBO board, so they'd just buy a breaker and match what was already there. Will be giving the company in question an email later to explain this to them.

So for future reference - solar has to be on an RCBO or have its own dedicated RCD?

 

[timhoward]

Wow you guys, i've seen hundreds of installs just installed directly on the RCD

I see it all the time too, and frighteningly companies are still fitting them like this. It tends to be when there isn’t room for another board or a board change would be difficult, and it’s never a straightforward fix but I always find some way.
The biggest problem is usually a 5 way rcd main switch board on TT earthing in a tiny cupboard.
Yes, either installed so RCD protection isn’t required, or an RCBO, or split tails and own CU.
So NOT ever sharing an RCD. I’ll dig out the reg this afternoon.

What gets me particularly is that the cost of PV is high and sorting out the CU isn’t significant in the context of materials for the whole job.

 

[rewireIT]

I see it all the time too, and frighteningly companies are still fitting them like this. It tends to be when there isn’t room for another board or a board change would be difficult, and it’s never a straightforward fix but I always find some way.
The biggest problem is usually a 5 way rcd main switch board on TT earthing in a tiny cupboard.
Yes, either installed so RCD protection isn’t required, or an RCBO, or split tails and own CU.
So NOT ever sharing an RCD. I’ll dig out the reg this afternoon.

What gets me particularly is that the cost of PV is high and sorting out the CU isn’t significant in the context of materials for the whole job.

Awesome thanks for the explanation - like the other poster i've done tonnes of these installs and just done them as instructed by the main contractor. Will refuse to do that from now on. Should have been a red flag when said company's 'QS' found a fault on a circuit and fixed it by changing the MCB type from B to C without doing a single calculation.

 

[timhoward]

Regs-wise I think the most relevant area is section 551 (even though it's called "LV Generating Sets" it includes PV cells according to 555.1.1). Reg number and my attempt at a plain English version below:
551.4.2 - any RCDs anywhere in the installation needs to work properly whether the supply is from grid, PV, or both. (This can't be assumed to be the the case with a shared RCD)
551.4.1 - fault protection needs to work properly for either supply (relevant in the case of an inverter messing up disconnections times with TT earthing)
551.7.1 (ii) - if the final circuit to the inverter needs RCD protection it needs to disconnect live and neutral so if an RCBO is used it needs to be SP+N not single pole.
551.7.2 - It's very simple if the inverter is connected on the supply side of the final circuits. But If connected on the load side the OCPD for a final circuit the requirements are more complex regarding conductor size, and ensuring the disconnection time for the final circuit still complies, taking into consideration the time taken for the inverter output to fall to under 50v.

 

[littlespark]

Was it ever allowable on previous editions of regs?
I mean, for PV to share RCD with other circuits.
My own house, I had an MCS accredited installer for PV, and its just a breaker tagged onto the end of a split board. And ive seen many a number of customers boards done the same way.... as well as EV charge points just added into DB as well.... where ever there was space.

 

[rewireIT]

Was it ever allowable on previous editions of regs?
I mean, for PV to share RCD with other circuits.
My own house, I had an MCS accredited installer for PV, and its just a breaker tagged onto the end of a split board. And ive seen many a number of customers boards done the same way.... as well as EV charge points just added into DB as well.... where ever there was space.

Yeah, interested to know the answer to this as well since i thought it was acceptable.

 

[davesparks]

Was it ever allowable on previous editions of regs?

No, whilst it has not been specifically prohibited in the past it falls foul of the fundamental principles detailed at the start of the regs book.

 

[littlespark]

No, whilst it has not been specifically prohibited in the past it falls foul of the fundamental principles detailed at the start of the regs book.

Must be thousands of installs like this. No wonder so many company’s jumped on the solar bandwagon and just threw them in.

I’ll get mine changed when I eventually upgrade to an all RCBO board.