Discuss calculation check before i rip the flat to bits. in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

telectrix

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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.
 

Murdoch

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

telectrix

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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.
 
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Murdoch

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

Common sense applies
 
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telectrix

telectrix

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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

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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.
 
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telectrix

telectrix

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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.
 

Wilko

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

mhar

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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

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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?
 

Murdoch

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

mhar

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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.
 

PEG

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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

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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

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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.
 

stevels4

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“Do you feel lucky punk, we’ll do you? go on then make my day and pull that cut out fuse”
The seal restorer will be around one day.
Seriously if you do pull cut out fuses please use something to prevent them being pulled out again once refitted, OK they are generally a good tight fit but the last thing anyone wants is a curious child who was watching you pulling it back out and sticking their fingers in there!

FD8A0F6C-ABDE-46AE-8E00-EB998E5E4579.jpeg
 

stevels4

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Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.
Doesn’t happen, not on my watch anyway, anything below 60A gets reported to the DNO, 60A rewirables get replaced with 63A, all others are left as found.New connections get an 80A left by the DNO.
 

Midwest

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Well, not sure what came first chicken or the egg, but I had someone ring me up last week, after having a smart meter fitted. The 'engineer' told my potential customer to get their tails upgraded to 25mm, 'cos they now have a 100amp main fuse.
 

stevels4

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Cut out to meter tails get upgraded to 25mm2 as standard provided the cut out terminals will accept them, load tails get left as they are if they are 16mm2 minimum or if below 16mm2 an isolator is fitted and the customer issued a warning notice recommending they contact a qualified electrician.
 

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