Replacement board TNS supply and TT

[Jimmyb]

Hello, just after people thoughts, I have been asked to replace a consumer unit (original is plastic) and the cable feeding the house is supplied from a switched fuse 60meters away with no rcd protection, the first 18meters is 16mm twin and earth and the rest is 16mm 4 core with the 2 cores doubled up and the steel used as cpc. (the house has barely any load as has oil boiler, arger cooker and no showers ect)
They had someone else a few years ago pick up on the fact that the earth is undersized so he installed a rod and made the house TT. all good and well at the time.
Now I want to/have to install a metal board and was wondering which is the correct way to go about it

1. Gland the swa into a separate enclosure before the consumer unit and run the core of the swa though conduit and into the cu. (zs too high for switched fuse so a no go)

2. Gland swa straight into consumer unit (cant do due to undersized cpc so again a no go)

3. Add 100ma T/D RCD enclosure before consumer unit then conduit into consumer

I don't really like the sounds of option 3 due to the cost of a T/D rcd but I cant think of another way


Many thanks

Jimmy

 

[Spoon]

Hello, just after people thoughts, I have been asked to replace a consumer unit (original is plastic) and the cable feeding the house is supplied from a switched fuse 60meters away with no rcd protection, the first 18meters is 16mm twin and earth and the rest is 16mm 4 core with the 2 cores doubled up and the steel used as cpc. (the house has barely any load as has oil boiler, arger cooker and no showers ect)
They had someone else a few years ago pick up on the fact that the earth is undersized so he installed a rod and made the house TT. all good and well at the time.
Now I want to/have to install a metal board and was wondering which is the correct way to go about it

1. Gland the swa into a separate enclosure before the consumer unit and run the core of the swa though conduit and into the cu. (zs too high for switched fuse so a no go)

2. Gland swa straight into consumer unit (cant do due to undersized cpc so again a no go)

3. Add 100ma T/D RCD enclosure before consumer unit then conduit into consumer

I don't really like the sounds of option 3 due to the cost of a T/D rcd but I cant think of another way


Many thanks

Jimmy

I am most probably reading this wrong, but are you saying that the electric meter for the house is 60 meters away from the house and the cable between the meter (ok the switched fuse next to the meter) and the CU is "16mm twin and earth and the rest is 16mm 4 core with the 2 cores doubled up and the steel used as cpc"

 

[Jimmyb]

sorry if I written it bad, it start at service head, 25mm tails into the meter then 16mm tails into switch fuse, from there it 18meters of 16mm twin and earth upto a joint box and then it swa cable for the out side run into the house
its a farm which has 4 house all feed from this area, they said it all was twin and earth but had to replace it where it was outside due to damage and they thought it best to put a bigger cable in, I did ask why not all the way back to the switched fuse

 

[Leesparkykent]

You could use an insulated gland.

 

[davesparks]

Why can't the TNS earth connection be used? Have you done the adiabatic calculation to confirm that the cpc is not suitable?

 

[Jimmyb]

it isn't big enough, its only 4mm in the twin and earth.

 

[davesparks]

it isn't big enough, its only 4mm in the twin and earth.

Really? Are you sure it's not 6mm?
What size cpc does it need?

Could you not just replace that last section of T&E?

 

[jackhammerJIM]

16 mm Twin and Earth should have a 6mm cpc .

Sounds like a rite lash up !

 

[Jimmyb]

sorry it prob will be 6mm, I'm not 100% of the size that the steel works out to be and also there is bonding to take in account so when I thought it was 4mm I didn't think its up the job and neither did the last man.

it actually works out at 3.21mm so might be ok, I just wonder why the last guy has tt the install

 

[Jimmyb]

What is the copper equ of 16mm 4core ??