calculation check before i rip the flat to bits.

[telectrix]

hi folks, got a CU change to do. flat rewired circa '70's. wylex 3036 fuses.

my concern is earthing conductors. at present they are 6.0mm. TN-S.

Pefc is 700A. not sure of main fuse size, so have taken worst scenario at 100A.
referring to tables, @ 700A the disconnect time for a 100A BS1361 is 2 secs. so the adiabatic gives a S of 6.91 for the MEC.

thoughts. please.

 

[Octopus]

Hum ... Tricky one. Really need to know the dno fuse sizes and tails

 

[telectrix]

tails from isolator to flat CU are 10mm. can only check DNO fuses if the seal fairy comes.

and there's no sign of damage over last 30 odd years. flat is minimal load. 1 cooker cct. 1 socket cct, 2 lighting. shower is off boiler which is communal. total load is probably < 20A.average.

 

[Octopus]

If the tails are only 10 mm I'd say crack on once the fairy has told you the fuse size.

Common sense applies

 

[telectrix]

if the fuse is 60A, then the adiabatic gives a MEC size of 2.19mm.so well in with 6.0mm. as you say, murdoch, common sense prevails.

 

[HappyHippyDad]

Gosh Tel... I think in this case I'd be very, very tempted to employ the seal fairies. It sounds like a lot of upheaval for the customer based on the assumption that it's a 100A fuse.

 

[telectrix]

even at 100A for the fuse.the 6.0mm is borderline, so i'm going to go ahead and leave it with an observation on the EIC.

 

[PEG]

Come on,Tel,surely you have a little box somewhere with 60A stickers in it...

 

[Wilko]

An old TNS supply with 10mm tails will almost certainly have been no bigger than 60A fuse, in my humble opinion.

 

[mhar]

The fuse is almost bound to fall out when you do the cu change, why not make sure it is only a 60A?
I never seek authorisation to reduce a fuse rating, only to increase it.
As a matter of course my DNO replaces 100A fuses with 80s whenever they come accross them, often the meters are only rated at 80A anyway.

 

[Lucien Nunes]

Q: Has anyone EVER seen thermal damage to a 6.0mm² EC on a TN-S install of 100A or less? Or even to a 7/.036 for that matter?

 

[Andy78]

Q: Has anyone EVER seen thermal damage to a 6.0mm² EC on a TN-S install of 100A or less? Or even to a 7/.036 for that matter?

Never seen any thermal damage on any earthing conductor, and seen a lot smaller than 6mm.

 

[Octopus]

Sometimes it seems the seal fairy has changed the DNO fuse ....

 

[HappyHippyDad]

Sometimes it seems the seal fairy has changed the DNO fuse ....

I bet you thought twice before writing that Murdoch
I wish there was a 'brave/risky/but we all agree' button!

 

[Midwest]

Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.

 

[mhar]

Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.

Meter fitters do not have the authority to uprate a fuse.
There have been issues with 100A fuses in cutouts. The fuses will take a sustained overload for a considereable period of time and if subjected to such an overload the fuse carrier gets very warm, some of the fuse carriers in the newer cutouts were unable to cope with this heat and became damaged.

 

[telectrix]

Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.

no way. meter monkeys aren't allowed to do anything that technical.

 

[PEG]

Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.

Yep,they'll jam as big a fuse in as they can....they don't want it blowing,and lose all that lovely data...

 

[PEG]

Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.

Yep,they'll jam as big a fuse in as they can....they don't want it blowing,and lose all that lovely data...

 

[Paul P]

We had a job a while back it had a bit of wet string (ok it looked about 2.5 but in old money)from the incoming cable sheath to the earth terminal we asked the DNO to replace it. They flatly refused say that the checked Ze and it was only 0.24 ohms so was ok.