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.....

 

[DPG]

100A is the max carrying capacity of the main switch, not the trip rating, as it is not an MCB. Daz

 

[Ash26]

Daz - ok, thanks for your reply. Will I need to upgrade the 60A isolator/switch fuse and meter tails or can these be left as is. Thanks.

 

[Guitarist]

How are you disconnecting supply while you carry out CU change? My advice is to find out if the DNO will fit you a DP isolator for free (EON do). That way, you can isolate and do the job at your convenience rather than have the fuse pulled by the DNO in the morning and reconnected in the afternoon.
If your service fuse is 60A then your in-line 60A isolator/switchfuse is fine.

 

[DPG]

Sounds like he's already got an isolator. Daz

 

[Guitarist]

You are so right Daz. Was thinking about if he was upgrading the tails, but of course no need if 16mm are staying

 

[Ash26]

Yes it's good there is an isolator already in place. So just to confirm, is it
ok to leave tails as 16mm ?

 

[telectrix]

16mm is fine on a 60A fuse. Ze looks high though, for TNC-S. should be under 0.35.

 

[Engineer54]

16mm is fine on a 60A fuse. Ze looks high though, for TNC-S. should be under 0.35.

He should try taking the Ze from the supply side of the switch fuse, close to the service cut-out. That T&E earth could be the problem, along with any loose connections in the earthing system...

Id say 16mm tails are good for up to a 80A service fuse.

 

[Ash26]

Thanks Telectrix and Engineer54 - I'll take a reading from the supply side of the switch fuse. It looks like 10mm earth from the service head to the switch fuse.....

 

[Engineer54]

Thanks Telectrix and Engineer54 - I'll take a reading from the supply side of the switch fuse. It looks like 10mm earth from the service head to the switch fuse.....

Better still take the earth measurement from the PME connection block on the cut-out, using the fuse switch for your line connections. May also be prudent to check the tightness of any earth connections at the cut-out PME terminal block. You can then start checking connections back to the CU if improvement is found!!

You may also be on a supply cable that's in the process of being PME'd, in which case you can well have a slightly higher Ze value. This will then be more of a TNC-S supply rather than a full PME supply!!! lol!!!

 

[Ash26]

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

 

[SimpleSimon]

Have you checked for any loose connections?
If so best give the DNO a call and prepare to spend the next 3 hours trying to explain to someone who don't have a scooby doo what your talking about.

 

[GLENNSPARK]

He should try taking the Ze from the supply side of the switch fuse, close to the service cut-out. That T&E earth could be the problem, along with any loose connections in the earthing system...

Id say 16mm tails are good for up to a 80A service fuse.

Yep eng....Ze being the external ......so as close as to the origin of supply (DNO cutout)......

 

[Ash26]

Couldn't see how to get to the earth connection at the cut-out PME block to check, but gave it a tug and appeared ok.....But Ze is 0.39 there....

 

[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.