Concern over TN-S system connections between meter & CU

Hello All

This is my first post so here goes. I have purchased a new house with a basement. After having a flood & the power going out. The dno replaced the cut out from 60A to a new 100A and upgraded the tails between it and the meter to 25mm. I now have 16mm tails going from the meter to a wylex 60A fused switch. My concern is that from this switch runs a 16mm two core pvc cable through the basement and up into the utility room a cable run of just over 30ft to a wlylex CU that has a 100A isolator and a range of MCB's with no RCD protection. Also have noticed that from the Main earthing terminal 10mm cable is used for the main protective bonding of water pipes and metal gas pipe, but the earth connection at the CU is not even 10mm, possibly as small as 4mm.

A friend ( not an electrician) advised that they thought the earth should be 16mm to the CU especially over such a distance and that the cable between the meter and the CU via the fused Switch should be 25mm tails. Also mentioned that he thought the 60A fused switch should be replaced be some kind of 100A rcd device incase the cable is punctured as runs exposed at head height in the basement for 24 ft before going up through floor into utility room.
Having looked at the forum postings i thought it sensible to ask what you all think. Should I also have Rcd's added to CU to protect final circuits?

Really appreciate any advise. I have attached a picture of the basement setup which includes other bits that have been switched off for now.

The earthing conductor which goes from the DNO cable sheath to the main earthing terminal looks bigger than 4mm in your pic, this ought to be 16mm for a 100A supply.
The tails supplying the switch fuse don't really need to be 25mm, 16mm will be adequate.
Your main earthing terminal (MET) isn't up to scratch, it ought to have two screws per terminal.
The wiring from the RCD looks like a bit of a lash up.
The CPC in the 16mm from the switch fuse is fine providing it isn't being used for protective equipotential bonding of gas pipes, water pipes etc. The main PEBs should in your case connect directly to the MET in 10mm cable.
The cable from the switch fuse to your consumer unit doesn't require RCD protection providing it isn't unprotected and concealed in a wall or partition <50mm from the surface.

I'm beginning to understand why BAS is oft so curt, please read properly before you reply

"but the earth connection at the CU is not even 10mm, possibly as small as 4mm."

not the section between meter and cutout

Whats the blue flexes do?

I'd recommend a full periodic, especially as you have just purchased the house, and as it has had water penetration.

This will highlight any remedial work required.

cozycats said:

I'm beginning to understand why BAS is oft so curt, please read properly before you reply

"but the earth connection at the CU is not even 10mm, possibly as small as 4mm."

not the section between meter and cutout

Click to expand...

He hasn't posted a pic of his CU so can't comment on that bit.

There is no earth between the meter and cutout

londonboy said:

Whats the blue flexes do?

I'd recommend a full periodic, especially as you have just purchased the house, and as it has had water penetration.

This will highlight any remedial work required.

Click to expand...

After the flood, the DNO ran 4mm tails from the meter to the old black switch as shown in the pic, which was then wired to the RCD to enable heaters to be powered as a temporary measure to dry out the basement hence the blue flexes. I have call the DNO to have it cleaned up, they haven't done so so far, to do it myself i would have to cut the ties on the meter and cut out which i understand i am not allowed to do so I will have to contact them again.

I agree with you regarding a full periodic, then will have idea what needs doing, great idea cheers

cozycats said:

I'm beginning to understand why BAS is oft so curt, please read properly before you reply

"but the earth connection at the CU is not even 10mm, possibly as small as 4mm."

not the section between meter and cutout

Click to expand...

Many thanks - your on the ball

Spark123 said:

The earthing conductor which goes from the DNO cable sheath to the main earthing terminal looks bigger than 4mm in your pic, this ought to be 16mm for a 100A supply.
The tails supplying the switch fuse don't really need to be 25mm, 16mm will be adequate.
Your main earthing terminal (MET) isn't up to scratch, it ought to have two screws per terminal.
The wiring from the RCD looks like a bit of a lash up.
The CPC in the 16mm from the switch fuse is fine providing it isn't being used for protective equipotential bonding of gas pipes, water pipes etc. The main PEBs should in your case connect directly to the MET in 10mm cable.
The cable from the switch fuse to your consumer unit doesn't require RCD protection providing it isn't unprotected and concealed in a wall or partition <50mm from the surface.

Click to expand...

The earth conductor is 16mm from the DNO cable sheath to the MET, when said the earth conductor is 4mm thats from the MET to the Consumer unit. Should this be 16mm as well as supply is now 100A? Great spot on the MET - I will get it replaced asap. The lash up on RCD is exactly that, it was the DNO's doing as a temp measure to enable heaters to be run in the basement to dry it out. have asked them to come and remove it as they connected tails directly from the meter to the black switch to power the RCD. I understand not allowed to cut wire on cut-out or meter to tidy this up myself. so must wait All PEBs is wired in 10mm and connects directly to the MET had this renewed when the cut out was replaced. the earth from the MET to the CU is about 4mm, so this will definately need upping to 16mm as i understand.

The 16mm pvc cable from the switched fuse to the consumer unit is clipped at head height running for 24ft , there is no protection by means of conduit or trunking its just clipped in place. as it goes through the floor & rises 8ft the cable is hidden under plaster and where it appears by the CU it is presented in plastic conduit so have assumed it is within this for the 8ft that the cable is hidden.

Is 16mm cable undersized to run from the meter via a fused switch to the CU on a 100A supply? or is the purpose of using a 60A fuse in the fused switch to safely allow the continued use of the existing 16mm 30ft run of 16mm to CU ? any thoughts?

Spark123 said:

cozycats said:

I'm beginning to understand why BAS is oft so curt, please read properly before you reply

"but the earth connection at the CU is not even 10mm, possibly as small as 4mm."

not the section between meter and cutout

Click to expand...

He hasn't posted a pic of his CU so can't comment on that bit.

There is no earth between the meter and cutout

Click to expand...

Sorry no Pic of CU, i am working away at moment and dont have only to post. However I can confirm that the supply cable L/N are only 16mm and that the earth is approx 4mm, THe earth seems very undersized considering the distance from MET of 30 Ft and i beleive it should be 16mm.

The CU is a wylex 10 way flexible high integrity unit Model NHR10SSLHI with a 100A Isolator and contains the following:
3 x 6A MCB lighting circuits
3 x 32A MCB - three ring, upstairs, downstairs and Kitchen
1 x 40A MCB - Cooker.

I know its been cobbled together as research shows that this unit should have a 100A isolactor and two 80A 30mA RCD's. There are no RCD's in unit and whoever put it in has had to link the Neutral busbar's together. very messy but left the details out as have little money to remedy at moment, more concerned with the earth to CU being only 4mm and the 30ft tails to CU that are only 16mm. and to why there is a 60A fused switch in system with 100A cut-out and 100A isolator in CU. I understand that it isolates the whole house for safe working with need to touch the main cut-out but why is it 60A? is it to protect the 16mm 30ft tails to CU?

Many thanks in advance to comments and pointers

Please note I have only skimmed over your posts.

I read that the submain (feeding your Wylex CU) is partially under plaster.

Because Twin and Earth is suseptable to damage, this needs to be RCD protected at 30ma.

However fitting a 30ma RCD to cover your entire property isn't (fully) compliant with the regulations, because a lightbulb blowing etc would cut the power to everything.

What I would suggest, would be to run (or have run) a 25mm 3 core armoured from a new Wylex 100a switch fuse (in cellar) to your existing Wylex board, and have the two 30ma RCD's fitted back into your consumer unit.

Because armoured has a metallic sheath it doesn't require the RCD protection that Twin and Earth does, its more tolerant to abuse

Obviously this is fairly substantial work, if you're on a budget you could consider running the armoured yourself (under electricians supervision) and leave him to terminate, test, and install switch fuse.[/i]

The 60A wylex switch fuse is there to protect the 16mm twin and earth cable supplying the consumer unit.
As I said before, providing the main protective bonding is connected to the MET the fact that the earth (CPC) in the 16mm T&E is 4mm is matterless.
If you want to you can work out the minimum required CPC for the circuit by measuring the fault current at the CU using a PFC tester, look up the time taken for a 60A fuse to disconnect at that current in the regs (must be less than 5secs), the K value from the regs for a copper protective conductor inside a cable and plug it into the adiabatic equation. Alternatively you could accept that the people who make cable have already done this for you.
The 100A main switch in the consumer unit is only a switch, the maximum current it is rated to carry continuosly is 100A, as you have a 60A fuse upstream this is not a problem.
The 17th edn regs require any unprotected concealed cable such as twin and earth which is buried in a wall or partition <50mm is protected by a 30mA RCD. At a guess yours will have been installed pre 17th edn, the regs are non-retrospective hence I'd leave well alone. If you really want it to be up to 17th edn regs then you could unbury the cable and surface mount it in trunking.

Spark123 said:

The 60A wylex switch fuse is there to protect the 16mm twin and earth cable supplying the consumer unit.
As I said before, providing the main protective bonding is connected to the MET the fact that the earth (CPC) in the 16mm T&E is 4mm is matterless.
If you want to you can work out the minimum required CPC for the circuit by measuring the fault current at the CU using a PFC tester, look up the time taken for a 60A fuse to disconnect at that current in the regs (must be less than 5secs), the K value from the regs for a copper protective conductor inside a cable and plug it into the adiabatic equation. Alternatively you could accept that the people who make cable have already done this for you.
The 100A main switch in the consumer unit is only a switch, the maximum current it is rated to carry continuosly is 100A, as you have a 60A fuse upstream this is not a problem.
The 17th edn regs require any unprotected concealed cable such as twin and earth which is buried in a wall or partition <50mm is protected by a 30mA RCD. At a guess yours will have been installed pre 17th edn, the regs are non-retrospective hence I'd leave well alone. If you really want it to be up to 17th edn regs then you could unbury the cable and surface mount it in trunking.

Click to expand...

Spark 123, really appreciate the response given. reegarding the 16mm cable, i am going to leave well alone as your suggestion, until i'm ready to gut the room and will update to 17th edn regs at that point. Going to focus on getting the CU rcd protected be using the high intergity box as one and not as just a main switch unit, have purchased the nessessary N and L connectors and have picked up a couple of rcd's to in couple of wks time going to test all the circuits as required and then reconfigure CU after fixing an issues.

Many thanks once again