Zs @ DB very high

[peeko]

Hi all,

Went out to fit some LED lights last Friday for a customer who's taken on a small workshop to be mostly used as storage. All fairly straightforward until I came to test for the minor works cert - got a Zs reading at the Db of 2.24 ohms - had a quick look and it's only got a 6mm supply with the SWA used as the CPC. Not a major problem for the lighting as it's a B6 breaker, but they do have a power circuit coming off that board that they intend to use. Did a quick direct Zs from the sockets and it's around 2.5 ohms on a B32.

It's on a farm and I'm guessing it's a submain bodged from somewhere, but I can't get access to the other units yet (farmer doesn't live there).

What's everyone's thoughts towards fixing it - it's a bit higher than I'd be comfortable with just adding an RCD/RCBO. I thought about TT-ing it & adding a front end RCD or should I be looking at pulling in a decent sized armoured with some copper in it? Or would that depend on whether the whole installation is already a TT or not?

Apologies if these seem like obvious questions, but I've always avoided farms because the few I've quoted on in the past have been a s**t-tip & owned by farmers who lived up to the stereotype of not being the easiest chaps to part with their money.

Cheers, Charlie
[automerge]1591628107[/automerge]

walk away. yous probably get paid with a sheeps head and a dozen eggs anyway.

Hahaha it's tempting. If the company renting it wasn't an existing customer I wouldn't be going near it. I think they'll pay as long as it's not a daft amount & if it is, that's up to them to argue with Farmer Giles about and not my problem.
Could do with the eggs though, there's still bugger all in my local Sainsbury's at the moment.

 

[radiohead]

You'll need to test the R1+R2 reading on the sub and the Ze before deciding how to proceed

 

[westward10]

Test first or repent later. Should have done some investigating initially to evaluate the earthing system so you can gauge circuit protection requirements.

 

[peeko]

What was the R1+R2 reading on the sub? And what was the Ze.

Don't know, can't access it at present it comes in on a cat wire from a different building.

 

[radiohead]

Don't know, can't access it at present it comes in on a cat wire from a different building.

I don't see how you can proceed without verifying Ze and R1+R2. If it's currently RCD protected then it will meet disconnection times as the max permitted Zs will be 1667 ohms for a 30ma RCD. But access will be required to verify if there is an issue. Have you verified the earthing type? It may even be TT now.

 

[peeko]

Test first or repent later. Should have done some investigating initially to evaluate the earthing system so you can gauge circuit protection requirements.

Agreed. He collared me without my MFT about the lights, the power circuit has been isolated, they've been notified of the danger & I've been paid, so I'm not quite at the repent stage yet, but I would like to sort it for them (ideally without too much of a ballache).

 

[pc1966]

Was there a big difference between the PFC and PSSC? Was the ratio roughly what you might expect for the SWA cable (i.e. is it simply rather long, or is there something more serious somewhere like a badly rusted section of armour, corroded joint, etc).

If it is just a long cable then an RCD (maybe a 100mA / 300mA delay as an incomer will do) and no need to TT as you get a better earth via the supply than adding a local rod.

 

[westward10]

It isn't just final circuits within the workshop which need addressing, it is likely the supply cable doesn't satisfy fault protection requirements. The only way to address and proceed with this is to access the source of supply.

 

[peeko]

Was there a big difference between the PFC and PSSC? Was the ratio roughly what you might expect for the SWA cable (i.e. is it simply rather long, or is there something more serious somewhere like a badly rusted section of armour, corroded joint, etc).

If it is just a long cable then an RCD (maybe a 100mA / 300mA delay as an incomer will do) and no need to TT as you get a better earth via the supply than adding a local rod.

To be honest PC I didn't do them - MWC doesn't request them, I've arranged access later this week and I'll have a look and should be able to trace the cable back then & do an R1+R2 & try and find the supply to do a Ze. Gut feeling is long, but I'll try the PFC/PSSC trick just to make sure I'm not missing anything.

 

[pc1966]

To be honest PC I didn't do them - MWC doesn't request them, I've arranged access later this week and I'll have a look and should be able to trace the cable back then & do an R1+R2 & try and find the supply to do a Ze. Gut feeling is long, but I'll try the PFC/PSSC trick just to make sure I'm not missing anything.

A good chance to check on it. As westward10 pointed out, it might fail on cable disconnection time at that sort of Zs. But until you find out if it is bad (or no) design, or some fault that has developed, then not much planning on rectification work can be done.

 

[peeko]

It isn't just final circuits within the workshop which need addressing, it is likely the supply cable doesn't satisfy fault protection requirements. The only way to address and proceed with this is to access the source of supply.

Yes I 100% get what you're saying, but there's probably a lot on the site that needs addressing. The supply is the farmer's problem & up to my customer whether they want to argue the ---- with him. Even if they're happy to pay it's not theirs to start messing with so I've got no power to start remedial works until it's agreed. The immediate danger and the priority is with the final circuits.