CU change and sub main upgrade

[Ash26]

Hi there - I’m changing the CU in my two bed terraced for my ELECSA assessment and just want to make sure that I’ve got everything right.

ExistingME, 60A service head, 16mm tails to meter and 16mm to 60A BS88 isolator/switchfuse, 8metre 16mm T&E sub-main to 30 yr old Wylex 60A. Main earth is via the T&E, 10mm bonding to gas/water. Ze at CU is 0.41.

Calculated maximum loading to be about 45A

Plan to upgrade to Wylex 9 way split load, (due to width restriction of 300mm) with 100A main switch, 1x80A 30mA RCD with 4 MCB and 2xRCBO’s, main earth to 16mm, change T&E to 16mm XLPE SWA (for additional protection, and future capacity).

Using 16mm SWA restricts capacity to 89A (Note 1 on table 4E4A refers to 512.1.5, so used 4D4A ref method C), so do I need to change 100A main switch in the CU and the isolator/switch fuse to 80A and get DNO to upgrade meter tails and main cut out ?

Many thanks for your help.....

 

[Guitarist]

I would be interested to know if the DNO would consider 0.04 ohms something to come out for. I'm assuming that your meter is calibrated and that you are making a very good contact when carrying out the test.
Also, are you testing on high-current or the low-current "no-trip" setting (if you even have it, that is)?

 

[Engineer54]

At both the cut out and fuse switch = 0.39. Should I contact the DNO.

No harm in contacting them, but try and get through to the area duty engineer, or you could doing a Simple Simon's. At least then you have covered your own arse. My betting is, that your looking at a older TN-S cable in progress of being PMEd. ....With a Ze of 0.39 ohm your only what, 0.04 ohm off the mark ....lol!!

 

[Ash26]

Guitarist - yes, calibrated in March, good contact and using 'loop Hi' on merger 1552. Eng - I'll try and see where I get to, thanks.

Other than that, any other comments about my proposed upgrade in original post ? If not thanks v much for your assistance, most useful guys....

 

[GLENNSPARK]

is it the same test leads you use when in low resistance ohmmeter mode?...if so how do they hold?...do they null out ok?

 

[Ash26]

Glen, yes same leads and null ok.

 

[GLENNSPARK]

ok then if its still high...then as others have already suggested... contact DNO...make a note of it (time date etc)...get them to make a note of it n all over the phone.....this way you`v covered your arse....

 

[GLENNSPARK]

oh, and if they start the `youl have to pay for it or its a TT` card...then tell em that as the install is using their supply as the earthing at the time of this.......then they have a duty to maintain it ash......dont put up wuth any bull they may try to throw at you....if they start this lark....hold your ground.....its their aperatus....their duty to maintain it.....simple as...

 

[BP Electrics]

Whilst your Ze is higher than the norm, the requirement is that Zs measures low enough for the circuit protective device to disconnect within the maximum disconnection time as specified in the regs. If it passes, it passes - regardless of Ze.

 

[GLENNSPARK]

Whilst your Ze is higher than the norm, the requirement is that Zs measures low enough for the circuit protective device to disconnect within the maximum disconnection time as specified in the regs. If it passes, it passes - regardless of Ze.

well this is all very well....but i would not like to think that possibly extranious conductive parts were the only thing preventing high Zs....

 

[GLENNSPARK]

Ze testing is to prove you have a reliable earthing external to the install....as the O/P has posted...its high....OK so only by 0.04 ohm....but nether the less...its still high...

 

[BP Electrics]

Sounds like a reliable earth to me. I wouldn't think the DNO will entertain digging up the road over a matter of 40 milli-ohms, but as mentioned in a previous post, it certainly would be interesting to know how they respond.

 

[Ash26]

Hi all, thanks for all your input. Will be ringing the DNO tomorrow and will let you know the outcome. Cheers.

 

[Ash26]

Update: Called DNO (UKPN) and they put me through to a very nice young lady (young to me anyway !) she asked where I was getting my readings from and said that it was out of spec and logged it as a fault and an engineer will be on-site within 24hrs and will call me when he's on his way.... and no Glenn I'm not joking.... she also gave me a ref number and a freephone number if I needed to call her back !... that was at 9:45 this morning. Lets see when they get here...

 

[Guitarist]

Good to hear Ash. Keep us updated.