Flat supply wiring and fuse protection.

[rattlehead85]

Any ideas on this one?
I am doing a survey/eicr on an intake serving 9 flats belonging to a local housing authority. The cables are early 70's 16mm/6mm T&E run through the building structure. I have assumed the maximum draw is around 30A per flat and checking the Ze values and distance it appears the cpc is of adequate size using the adiabatic equation. The issue is each flat runs through a 60A switch fuse isolator housing 60A 3036 fuse before terminating into a 3 phase ryefield housing what appears to be a bs88 Gg 60 70's fuse per flat.
Which fuse do i use for the adiabatic calc and am i correct in saying i have a discrimination issue between the fuses here? code 3 on the eicr?

 

[MDJ]

This is where I get my balls busted on this forum, I think the 6mm2 earth wire in the 16mm2 T&E is not adequate regardless of the adiabatic equation, thats if it does actually pass which I doubt,There is a thing as good practice in my view and rollocks to the regs, seriously rollocks to it, I always run a 10mm2 earth wire with a 16mm2 Twin and Earth for the main supply to a flat and usually connect it via a KMF switch via an 80 amp max fuse, a 60 even better if there is minimal load, you get slated on here for suggesting running earths with sub-mains haha, but I have always done it this way and always will.

 

[rattlehead85]

Well i did the adiabatic calcs using both fuses allowing an estimated cable run of approx 55m and the cpc comes out as adequate. I would tend to agree MDJ on using a separate cpc but seeing as this is an existing install i have to give an accurate report on if their is an existing safety or fire risk. As it is an existing installation surely applying the adiabatic reg over the reg stating sizing the earth in accordance with the supply neutral is the one to use?

 

[MDJ]

Hi-Fella, welcome to the forum by the way, out of interest have you checked out the mains water, and if evident even gas if any to the flats? some flats have blue plastic water pipes to them from the main incommer which can be in the basement for example and then it turns to copper from individual stop taps in the flats, if so how have you considered the bonding?

 

[rattlehead85]

The bonding within the flats is done at the stopcock hatch using 10mm cable as this was upgraded as part of a kitchen/ bathroom refurb programme. Somehow the main cables were not inspected or verified hence the request for me to do this. Any thoughts on the discrimination issue at the intake in the basement?

 

[MDJ]

well I hate 3036 fuses myself, others love them I'm sure, and combined with the 6mm2 earth in the submains I would attempt to talk the client into some improvements, the 3036s may not blow even if you cut through the cables with a disk grinder lol. Joking aside the mains were not verified by the last spark who installed the earthing/ bathroom upgrade due to the lack of main earths, the fact the sub-mains are now 40 years old, and the fact 3036s are protecting the cables, are the dbs in the flats modern?

 

[rattlehead85]

well I hate 3036 fuses myself, others love them I'm sure, and combined with the 6mm2 earth in the submains I would attempt to talk the client into some improvements, the 3036s may not blow even if you cut through the cables with a disk grinder lol. Joking aside the mains were not verified by the last spark who installed the earthing/ bathroom upgrade due to the lack of main earths, the fact the sub-mains are now 40 years old, and the fact 3036s are protecting the cables, are the dbs in the flats modern?

17th edition wylex units as far as i know.

 

[Billynm]

This is where I get my balls busted on this forum, I think the 6mm2 earth wire in the 16mm2 T&E is not adequate regardless of the adiabatic equation, thats if it does actually pass which I doubt,There is a thing as good practice in my view and rollocks to the regs, seriously rollocks to it, I always run a 10mm2 earth wire with a 16mm2 Twin and Earth for the main supply to a flat and usually connect it via a KMF switch via an 80 amp max fuse, a 60 even better if there is minimal load, you get slated on here for suggesting running earths with sub-mains haha, but I have always done it this way and always will.

Me too !

 

[rattlehead85]

Flats protected by a 3030 60A and then a 60A bs88 Gg at the ryefield.

 

[Guest55]

Which fuse do i use for the adiabatic calc
the one most likely to blow first in a fault , which is almost certainly the bs88

and am i correct in saying i have a discrimination issue between the fuses here?

well if the the 88 & 3036 are both 60a , you'll have no discriminstion at all.

code 3 on the eicr?

sounds fair.

...............

 

[MDJ]

17th edition wylex units as far as i know.

Thank god for that, sounds like they are at minimum dual RCD, at least the flat owners/ tennants have protection to the shower and socket circuits via RCD protection and not 3036s, you did mention upgrades there, so at least something seems modern, sorry to sound negative but 6mm2 earth main cables, 3036 fuses and 40 year old cables just make me want to get hold of the customer and talk him into an upgrade to improve things there, okay it may pass according to the regs, but as mentioned earlier IMO, rollocks to them there is a minimum standard and good practice IMO.

 

[rattlehead85]

...............

Yeah exactly what i was thinking. So as a minimum i will advise the 3036 switch fuses are replaced with MEM 60 isolator switches to eliminate this issue. However it appears that the supply cables do not warrant upgrading as the cpc calculates out ok and there are no signs of deterioration or insulation damage.

 

[rattlehead85]

Thank god for that, sounds like they are at minimum dual RCD, at least the flat owners/ tennants have protection to the shower and socket circuits via RCD protection and not 3036s, you did mention upgrades there, so at least something seems modern, sorry to sound negative but 6mm2 earth main cables, 3036 fuses and 40 year old cables just make me want to get hold of the customer and talk him into an upgrade to improve things there, okay it may pass according to the regs, but as mentioned earlier IMO, rollocks to them there is a minimum standard and good practice IMO.

I agree but trying to justify the cost when a regs book can be pulled on me as back up to their argument would make me look like i don't know what i'm talking about.

 

[MDJ]

Sorry my friend, last question, what earthing system is in these flats? is it TNCS, TNS or TT?

 

[MDJ]

I agree but trying to justify the cost when a regs book can be pulled on me as back up to their argument would make me look like i don't know what i'm talking about.

I of course understand your point, you could add a "rec" though to the report suggesting improvements, unless you do not want the work of course.

 

[rattlehead85]

The earthing system is a TN-S. It is a copper brace strap tagged to the carcass of the ryefield from the head cable sheath.

 

[MDJ]

Oh Okay, readings are good then?

 

[rattlehead85]

Oh Okay, readings are good then?

Yes 0.26 Ze on each phase at the ryefield bars. Psc of 0.9ka and PFC 1.25Ka / 2.5Ka P-P.

 

[MDJ]

sounds good, fair enough then.

 

[rattlehead85]

Thanks for all the advice much appreciated.