Zs a little high at main CU, what to do?

[bumcrack]

We have installed some PV at a customers house, and this feeds into a sub board, all this is ok regarding protection, allowable Zs etc.....

However, the main house CU is fed from a remote metering chamber 50 mts away...

The cable that feeds the main house CU is 2 core SWA, and looks like 16mm2. It's PVC by the looks of it, red&black, and the insulation on the cores forms semicircles in shape.

Anyway, earthing between metering and main CU is armouring only, ( earth strap around armour near meter ) and also somewhere in the house before the CU the cable changes from SWA to T&E.

Zs at the main CU is 0.42 ohm. Max Zs for the REc cut out 100A BS1361 is 0.36 Ohm x 0.8...so effectively about 0.29ohm to work to. There is also in circuit a WYLEX DP switch with integral fuse, I imagine this is BS88 and again 100A. Max Zs for this is 0.42 ohm x 0.8, so we are still a bit out with our measured 0.42 ohm.

I imagine over the years the Zs has crept up due to corrosion on the armor, and bad joints through the system. Nobody seems to know where the SWA changes to T&E, so checking this is a non starter..


Some questions if you guys would be so kind that have more experience in this than me..

Can I fit and RCD on the supply end of the SWA to get around the Zs being a bit high, also would 100mA be ok?

Secondly, assuming the SWA is 16mm, and is PVC, the best current carrying capacity of this according to 4D4A is 78A using ref method D. Obviously a 100A fuse on the source end of this circuit is too high, however would fitting an 80A fuse in place of the 100A be dodgy? I can check the size of the SWA by measuring individual strands with a vernier if I have to, its a local job.



Also, where do we as a contractor stand regarding this? Is it a case of advise only? What are the customers obligations to act on this advice?

Thanks in advance....

 

[johnboy6083]

you need to confirm the size of the cable. look atb the cable sheath, and it should listthe size, or download bs 6346, and measure the oveall diameter to find the size. measuring the armour diameter will be fruitless due to accruing error.
fitting and RCD at the supply end of the submain will do the trick, but will only kill the symptoms, not the root cause of the problem.
You could install supplementary bonding to lower your Zs too. I would deifnately do this aswell as install an RCD.
If your going to require discrimination between RCD's, then i would install an S type at the supply end.

if you fit an 80A fuse, then your still going to have a non compliant installation.

IMO as a contractor, it is your duty to informand advise, but if the customer says that they won have the remedials done, then you would be foolish to install onto that.

to be honest, and i think youll agree, it was foolish to install into that property, without checking the main earthing and bonding anyway.
At least yopull know for the next one.

John

 

[bumcrack]

Cheers John..

I have installed only one new circuit, and upgraded an MCB on another circuit to allow discrimination for my new circuit. All Zs measurements for both the new circuit, and MCB upgrade ( this was to replace a 16A with a 32A for a 16mm T&E to sub-board ) fall well within allowable Zs values.

I see what you are saying, but how many sparks turn a job down or measure Ze/Zs at time of quoting?

 

[Deleted member 26818]

With such a high Zs reading, disconnection times in the event of an earth fault will not be met for a 100A supply.
Are you certain that the fuse is 100A?
I'd contact the DNO if it is.

 

[bumcrack]

I have worked the Zs calc backwards, and the time to disconnect is just over 10 seconds for a 100A BS1361

230v / 0.42 ohm = 547A. This lines up in fig 3.1 to just above the 10 second mark, so yes, well out of the park..

The DNO's fuse is not the protective device, its a BS88 downsteam of this in a wylex SW fuse...

If this BS88 is 100A then its ok, as 0.42 Zs will allow 5s, however thats without derating allowable Zs by 0.8...

With the calculated current of 547A, an 80A BS88 fitted into the wylex Sw fuse will give me a disconnect of about 1.5 secs, does this sound about right?

 

[johnboy6083]

Sounds right to me, but now in bed and It would be weird to have regs with me! Lol

 

[bumcrack]

What kind of spark are you man!

You should be ashamed!

 

[Deleted member 26818]

Are you using the tables in the ammended edition?
Remember that those values only apply to new types of fuses, not old types.
547A would be just outside of 5s for a BS 88-2.2/6 fuse.
The other problem, is that the supply voltage could be as low as 216.2V (according to ESQCR), which would result in an earth fault current of just under 515V.

 

[bumcrack]

To the kind chaps that helped with this thank you. The reason I didnt respond sooner was I wanted to confirm cable size and fuse rating.

It's BS88 100A ( BUSSMAN OSD100 gG )
Cable is 16mm sq, I had to measure a strand with a vernier !

I will calc VD based on 100A load, and advise the customer, also about high Zs, etc...

No doubt the recommendations will get chucked in a drawer and forgotten about, but I have fulfilled my duty of care!

Cheers...:icon14: