Distribution board question

leebut · Mar 27, 2019

hi guys

Question for the knowledgeable on here, I went to look at a possible board change last night after the couple had experienced the ring final circuit going down when the fuse wire burnt out due to s faulty socket outlet had burnt out

The socket face had been replaced by the customers insurance companies emergency electrician, my question is why would the distribution head unit have overheated and melted quite a bit of black tar out the bottom edges

See the photo i have enclosed

Regards

Lee

[ElectriciansForums.net] Distribution board question

davesparks · Mar 27, 2019

leebut said:
16mm earth on any installation where the tails are up to 35mm so being either 4 or 6mm is way undersized

Nonsense, where did you get that from? The size required is calculated using the adiabatic equation and subject to a minimum of 4mm if I remember correctly.
16mm would be the size you install if you choose to select the conductor size rather than calculate, but you cannot use this method to assess an existing installation as undersized.

Bobster · Mar 27, 2019

Vortigern said:
As a matter of urgency I would have called out the DNO on the spot. I am astonished the "emergency electrician" did not do so or that he would blithely connect a new socket without addressing the earthing and checking that is ok before doing any remedial work. I think I would offer to talk to the insurers for the client to discuss the matter. I am concerned the insurance company is allowing such a cavalier approach to electrical safety. If as you say tar had melted out of the main supply cable then DNO immediately!

What is wrong with the earthing to prevail an emergency?

Also a small amount of bitumen is quite common to leak from these heads, so again what would you reasoning behind the emergency?

There is not enough information provided to say the emergency electrician has acted inappropriately.

leebut · Mar 27, 2019

This is what regulations I work to

leebut · Mar 27, 2019

The homeowner has phoned Electricity North West who are their DNO they’re going today to look at the head unit, customer is getting back to me once they’ve been today

davesparks · Mar 27, 2019

leebut said:
This is what regulations I work to

View attachment 48533

That is not the regulations, that is a guide to to the regulations. Also if you read it properly you will see that it says quite clearly that the adiabatic equation can be used to calculate the size of earthing conductor required. If you carry out this calculation you will find that 6mm I slamost always sufficient for domestic supplies.

Bobster · Mar 27, 2019

leebut said:
This is what regulations I work to

An now that you have read them I'm sure you see where your blanket statement is incorrect.

leebut · Mar 27, 2019

Customer has just had DNO at his house, they’re changing the Head unit tomorrow afternoon and changing the earth connection

leebut · Mar 27, 2019

So is there anything wrong with fitting larger earth cables than a calculation says rule of thumb better to be safe than sorry

Bobster · Mar 27, 2019

leebut said:
So is there anything wrong with fitting larger earth cables than a calculation says rule of thumb better to be safe than sorry

Absolutely nothing wrong with fitting larger than necessary.

I don't like blanket statements though, and following these "rules of thumbs" dumbs down the trade. There are rare instances where following that table will provide the wrong size. An so it is always better understanding the theory behind it to confirm.

leebut · Mar 27, 2019

The latest from the customer electricity north west have already changed the head unit this afternoon

Now that’s quick service he only rang them at 10.30 this morning

davesparks · Mar 27, 2019

leebut said:
So is there anything wrong with fitting larger earth cables than a calculation says rule of thumb better to be safe than sorry

There is nothing wrong with fitting a larger earth than required. What is wrong is stating that an installation is incorrect because it has a correctly sized earthing conductor rather than the oversized one you normally fit.

The Ghost · Mar 27, 2019

Rob said:
What is wrong with the earthing to prevail an emergency?

It is not the earthing I was on about it was the bitumen leaking as the OP stated

leebut said:
distribution head unit have overheated and melted quite a bit of black tar out the bottom edges

If that is the case I would definitely phone the DNO. In any event the cut out would not pass muster for me. I do not like the earth and if I did an adiabatic on that I have doubts it would pass. That is the earth on the TNS outer armouring. The earth floating looking like it should have a home is also somewhat suspect as I am not there I cannot say for sure but it looks like it has come off of the MET above it. In the state the picture shows the installation/cut out to be in I would be doubtful about replacing anything until I had satisfied myself that the earthing/bonding was as it should be. NOT as a matter of emergency just diligent application of regs and best practice

leebut · Mar 27, 2019

Hi Voltigern

I wasn’t happy with the size and condition of the earth on the armour, the bonding was disconnected from the MET when I went to view the job but was connected to the gas pipe and connected securely in the consumer unit so not a worry for the minute, the brass retaining bolt was missing or I would have reconnected.

My main worry was the amount of fresh tar that had oozed out of the joint hence the post, as per my message earlier the DNO must have had concerns over its safety as they initially said it would be replaced in the week, then it was being replaced by the end of this week and eventually they came back and replaced it today.

All now ready for me to go in and replace the consumer unit with an rcbo board after initial testing the installation

Regards

Lee

leebut · Mar 27, 2019

This is my take on why I was saying the earth cable wasn’t up to spec I was meaning the earth to the incoming sheathing shows in all parts of the on-site guide to be 16mm plus in table 4.4 (i) and 4.4(ii) all say minimum cross section of earthing conduct should be 16mm

Andy78 · Mar 27, 2019

leebut said:
This is my take on why I was saying the earth cable wasn’t up to spec I was meaning the earth to the incoming sheathing shows in all parts of the on-site guide to be 16mm plus in table 4.4 (i) and 4.4(ii) all say minimum cross section of earthing conduct should be 16mm

View attachment 48543

View attachment 48544

As stated previously, that table shows one way of calculating the size of a protective conductor, the method that errs on the side of caution and simplicity of instruction. That's why this method is shown in the on site guide.

The table references 543.1 next to it which you should read to understand why the information in the guide is just that, a guide.