Upgrade Tails & Bonding?

[hightower]

Say you were going to change a CCU, and it is an 80A fuse with 16mm tails (10mm earth). Bonding is in place in 6mm. What would you do with regards the tails and bonding? Been thinking about this scenario all morning and it's bugging me now. I'll walk you through my thinking, would appreciate a mentors thoughts.

Assuming reference method C 16mm is good for 87A according to table 4D1A, so the tails are acceptable.

Now assuming a good Ze of 0.10 and plugging those figures in to the adiabatic:

230/0.10 = 2.3kA

2300 x 2300 x 0.1 = 529000 (sqrt) = 727.32 / 143 = 5.09mm

Assuming a poorer Ze of 0.35:

230/0.35 = 657A

657 x 657 x 1 = 431649 (sqrt) = 657 / 143 = 4.59mm

So my thinking is the earthing conductor is up to scratch with regards adiabatic, and being TNCS would need to be a minimum of 10mm anyway - that's what is installed.

Next, bonding. Table 54.8 states that bonding should be 10mm minimum. We have 6mm installed. Not acceptable.

Now, this is what I think I would do in this situation, but would love to be told if I'm thinking correctly or not by the mentors. If the tails and earthing conductor were in good condition, I would leave them in place and reuse them in the new CCU. If not in a good condition, or if it they were easy to replace (not ripping walls apart) I'd replace them and probably stick 25/16mm tails/earth in - may as well if making a mess anyway?

As for the bonding, I'd upgrade it to 10mm. Now I've seen the arguments for both sides of the story in that if 6mm is installed and shows no sign of thermal damage etc then it can be reused. To me though, it just wouldn't feel right.

So am I thinking along the right lines, that it's okay to reuse the tails etc if they meet the requirements?

And that leads on to a slight change of scenario - everything the same, except a 100A fuse now. I've seen the arguments again that it's okay to use the tails if they show no signs of damage even if they are undersized now.

 

[Richard Burns]

Your thinking is correct here.

Unless the tails are placed in trunking or otherwise derated then they are suitable for an 80A load.

For a TNCS system the minimum size for a main earthing conductor is 10mm² as it is also a bonding conductor and so is sized in table 54.8 based on the csa of the incoming neutral of the supply (for domestic generally 35mm² or less).

Therefore the main earthing conductor and the tails are suitable for reuse.
Obviously if they are damaged/ too short or suchlike then changing them to 16mm² tails and 10mm² EC would be acceptable, but there is nothing to say that you cannot upsize them if you want.

For a 100A fuse the tails are not suitably sized to comply with BS7671 and should be uprated to 25mm².
It is a possible consideration that the design current will be less than 87A and so the tails will not be overloaded but this is not really something you can say with confidence as the loading of a house can vary and the size of cables will not be checked by the house owner.
Though there are often supplier tails at 16mm² on a 100A fuse which is annoying.

On bonding the csa does not comply with the BS7671 required minimum of 10mm² and so should be replaced with the required csa (though another 6mm² could be run along the same route in parallel and then comply, just as an off the wall thought, normally it is easier to replace a cable than add another).
The guidance on this is a bit variable, stating both that the 6mm² is utterly unacceptable and that if a 6mm² cable shows no sign of damage that it can remain in place.
Replacement is the preferred option and will then provide the required protection for the installation.
Because of the guidance information I would say that if the level of damage required to replace an undamaged and correctly connected 6mm² bonding conductor is well beyond the level of the existing work that it may be acceptable to leave it in place, but every effort should be made to upgrade where possible.

 

[hightower]

Thanks Richard, first time I've been on the computer since last week (although I read your reply on Friday on my phone) - just wanted to say thanks, very helpful response and I appreciate you taking the time to do so.