Small Earth question.

[Dairyspark]

Hi everyone

So I had a thought the other day. The old style T+E which had 2.5mm² conductors and a 1mm² Earth which doesn’t satisfy the Earth size etc. and I know that if you find it on an EICR it’s a sad day for the customer etc etc.

However, my question is, If you fitted an RCBO to the circuit containing the 2.5/1 cable would that not limit the current flow on the earth under fault conditions to 30mA and then make it ok?

Orrr have I got that wrong in all areas

Bobby.

 

[UNG]

Hi everyone

So I had a thought the other day. The old style T+E which had 2.5mm² conductors and a 1mm² Earth which doesn’t satisfy the Earth size etc. and I know that if you find it on an EICR it’s a sad day for the customer etc etc.

Why is it a sad day for the customer

 

[Dairyspark]

Why is it a sad day for the customer

I was under the impression that it didn’t comply and there for had to be rewired? Which equals a sad day for the customer who now needs to pay for such things to happen

 

[Fleeting]

A 1.0 cpc is likely to satisfy modern requirements. I believe the 1.0 was upgraded to 1.5 in T&E cables due to the likelihood of failure where BS3036 30A, semi enclosed fuses are used for circuit protection.

 

[binky]

I was under the impression that it didn’t comply and there for had to be rewired? Which equals a sad day for the customer who now needs to pay for such things to happen

it's not an automatic rewire.

 

[SPECIAL LOCATION]

I was under the impression that it didn’t comply and there for had to be rewired? Which equals a sad day for the customer who now needs to pay for such things to happen

If the circuit meets the Max Zs values why would it not comply?

A 1.0 cpc is likely to satisfy modern requirements. I believe the 1.0 was upgraded to 1.5 in T&E cables due to the likelihood of failure where BS3036 30A, semi enclosed fuses are used for circuit protection.

As per wot sir Fleeting said....

If you look at the Max Zs values a 3036 30A is 1.04 ohms..
whereas a 60898 type B 32A is 1.37 ohms..

so you have an extra 0.33ohms to play around with if 60898's..

Consider some hypothetical older properties with single ring circuit of 100m...
a 2.5mm with 1.0mm CPC has r1 & r2's of 0.74 & 1.81 so R1+R2 = around 0.64
a 2.5mm with 1.5mm CPC has r1 & r2's of 0.74 & 1.21 so R1+R2 = around 0.48
((r1+r2)/4)

If the above circuits are on TNS or TN-C-S installations the Zs values could be plus 0.35 or 0.8..

So the 1.0mm CPC's could have potential Zs of 0.99ohms up to 1.44ohms?
Which may or may not be compliant depending upon the actual Ze and/or if 3036's or 60898's used as protective devices?

To my (limited) knowledge there is NO regulation stating that T&E, 2.5mm+1.0mm is non-compliant...
BUT.... providing the disconnection times can be met, what is the danger or reason for non-compliance with BS7671?

{may have to double-check my numbers later,
as I have had a few
But the concept is correct.. If meets max Zs what's the problem?}

 

[Phoenix]

However, my question is, If you fitted an RCBO to the circuit containing the 2.5/1 cable would that not limit the current flow on the earth under fault conditions to 30mA and then make it ok?

Protective devices (inc RCBOs) do not limit fault current, they limit fault duration and through (mainly) that fault energy. Whats the Zdb at the board?

If its a 60898 B32 and the Zdb is not *lower* than 0.09 then a fag packet calc suggests its easily OK

 

[Andy™]

only an issue with 3036 fuses, the let through energy may be higher than the CSA of the cpc can handle (temp rise may excedd permitted values). if its 3036 fuses then change to 60898 and you're all good

Zs is not the problem

 

[Dairyspark]

Well…..that answers that then….thanks everyone

 

[Fleeting]

The ring final in my house of which there is only one incorporates 1.0 cpcs.

 

[Murdoch]

The estate I live on is all 1.0 mm CPC's except ours!

 

[UNG]

The estate I live on is all 1.0 mm CPC's except ours!

So are you the customer who had a sad day

 

[Murdoch]

So are you the customer who had a sad day

We bought the house after the incoming water main had split and filled up the house with mains pressure water

 

[SPECIAL LOCATION]

Well…..that answers that then….thanks everyone

In my opinion it is always better to get a question from someone who is open and up-front about how they understand a problem.. (even if they have misunderstood something.)
Rather than someone waffling and spouting rubbish thinking they know stuff that they don't...

So always willing to try and help someone who lays their cards on the table at the beginning,
so we all know where we are coming from, and at what level to aim our answers.

There are to many myths and crap flying around in this industry...
So keep asking when anything crops up that you need clarifying.

 

[Fleeting]

To be completely clear, especially if the are putting it into a technical document the word earth should be referred to as cpc.

 

[Dairyspark]

In my opinion it is always better to get a question from someone who is open and up-front about how they understand a problem.. (even if they have misunderstood something.)
Rather than someone waffling and spouting rubbish thinking they know stuff that they don't...

So always willing to try and help someone who lays their cards on the table at the beginning,
so we all know where we are coming from, and at what level to aim our answers.

There are to many myths and crap flying around in this industry...
So keep asking when anything crops up that you need clarifying.

You are absolutely right, I have no issue with asking questions that might seem obvious for some but totally alien to others. “He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”. Again it doesn’t help when others are passing on information like that as I’m 100% sure I read it somewhere which is why I’ve got it noted on my EICR help list on my phone. I always do appreciate the amount of help and knowledge that this forum has and have had many questions answered over the years of being on here. So I will continue to ask as there will always be someone who will know the correct answer.

 

[Dairyspark]

To be completely clear, especially if the are putting it into a technical document the word earth should be referred to as cpc.

You are right, it’s hard not to continue to use the laymen’s terms lol

 

[SPECIAL LOCATION]

You are right, it’s hard not to continue to use the laymen’s terms lol

I got a text message today from a customer asking about extending a new circuit from the second "fuse box" with three "fuses" by the back door....

So often its just easier carry on talking their language, so they understand what you are on about, rather than going on about the consumer unit with three MCB's and an upfront RCD.
Which just makes their eyes glaze over!!

 

[Andy™]

I got a text message today from a customer asking about extending a new circuit from the second "fuse box" with three "fuses" by the back door....

So often its just easier carry on talking their language, so they understand what you are on about, rather than going on about the consumer unit with three MCB's and an upfront RCD.
Which just makes their eyes glaze over!!

go into a house and ask where the consumer unit is and you often get a blank confused look. ask for the fusebox and they know where it is. so ask for fusebox, its usually easier

got to remember youre dealing with people who dont know much about electrics most of the tim. if they did know all the technical terms you wouldnt be doing the job because they would be doing it themselves

 

[Murdoch]

And if they are American ask for the switch panel