Borrowed earth

[SGG123]

I have found that the garage light is on some original 2 wire from the 1970s. I could change the fitting to a class 2 but would like to know why I cannot borrow an earth from the nearby ringmain with 4mm. All domestic earths are bonded together independent of MCBs or RCBOs, so there cannot be any potential difference in the earth circuitry should anything fail.

 

[westward10]

Nothing wrong with doing this.

 

[Tom256]

I have found that the garage light is on some original 2 wire from the 1970s. I could change the fitting to a class 2 but would like to know why I cannot borrow an earth from the nearby ringmain with 4mm. All domestic earths are bonded together independent of MCBs or RCBOs, so there cannot be any potential difference in the earth circuitry should anything fail.

First thing this is all offered towards full scope of discussion - that is all!

The questions that relate (for me) would be:
1. Is it compliant with BS 7671 if not can I be happy noting the departure
2. Is it potentially dangerous and (3) as an inspector...
3. What is my engineering judgement during an EICR set against the guidance that is BS7671

BS7671 and the regulation that relates is 543.6.1. :
'Where overcurrent devices are used for fault protection, the protective conductor shall be incorporated in the same wiring system as the live conductors or in their immediate proximity'

A twin and earth cable is as it is 'a wiring system' by definition.
Singles in containment is a wiring system.
A shared CPC within containment or indeed the containment itself can be the shared cpc for multiple circuits - Regulation 543.1.2 . The singles and the containment becoming 'the wiring system'.

With T & E (wiring system) in this example the earth is separate (It might be proposed) to the wiring system so is it in 'the immediate vicinity' of throughout (?)
Interesting of note from 543.6.1 - if Fault protection by over current devices.

Your 4mm sizing is noted - 543.1.1

Case by example: I picked up on a similar situation on an Industrial inspection and under the circumstances present I considered it was C2 'potentially dangerous'.

Two separate DB's (for simplicity = DB1 & DB2) within the building, each just meters apart in a large workshop.
DB1: From DB1 a final circuit to a small goods lift. Wired in 3 core SWA (3 phase load) to a (way too small - picture) plastic isolator within a packaging area some distance away and in a separate area - this final circuit came off DB1. The swa armours were earthed just fine at DB1

DB2: From DB2 a small 6 way consumer unit had been wired on a SWA from DB2 - picture. This consumer unit was in the same packaging area and what-you-know just meters away from the isolator on that lift circuit on DB1.

Anyway, who ever wired the isolator may well have done so on a Friday afternoon. They could not get a cpc connection to the swa gland for the outgoing circuit from the isolator, no room - a right bodge then an idea...
I can imagine the moment and they thought F*** it. They ran a 2.5mm single to the consumer unit fed from DB2 - conveniently only a few meters from the isolator you may recall. Genius let's go home it's 1pm and Friday.

This lift circuit was fed from DB1 but its earth via the consumer unti was from DB2. With the Protective Earth removed at the consumer unit there is no earth (confirmed) for the lift motor / outgoing side of the isolator.

Now what is a realistic situation: Someone working on DB2 and removing the Earth would take the earth from the lift circuit on DB1 - and have no way have knowing this had happened or could expect for such. This removed protective earth could be off for unknown length of time.
Would notices clear and permanent at DB1 and DB2 have changed this position ?

Question: Would it have been better had this been well documented and labelled to the degree required?
CDM Regulations are statutory and Regulation 9 sets out the duty to eliminate, so far as is reasonably practicable, foreseeable risks to the health or safety of any person maintaining the installation.
Could the person defend the install as is (?) - not for me to answer.

Which takes me to point 3. In my case using my engineering judgment (and that is the essence of inspections) I C2 the issue and the duty holder discussed and we agreed. The isolator was replaced and an earth nut fitted to the gland for the cpc.
My observations from the report (for sake of thread) read:
1. 2.5mm Protective Conductor taken from DB 15/17/03/01Y to lift Isolator on circuit 15/17/02/RYB15 to provide the means of Earthing for the lift (Isolator for lift below 15/17/03/01Y). Regulation 543.6.1 = C2
2. 2.5mm Protective Conductor taken from DB 15/17/03/01Y to lift Isolator on circuit 15/17/02/RYB15 to provide the means of Earthing is without mechanical protection and as such is undersized. Regulation 543.1.1 , 543.3.1 = C3

Dependent on circumstances I could see the case for a C2 or a C3 or indeed No code at all. Could easily make a case for each subject to circumstances.
Why this trade is a skilled trade and an inspection becomes the engineering judgment of the inspector that they must, as required, be able to justify. Regulation and pictures greatly assist to this end.

The example the OP posts is different circumstances to the industrial example I have outlined. I think I would note such at the consumer unit but as for any more... (?)

To end and I repeat: all offered towards full scope of discussion.

 

[DefyG]

I have found that the garage light is on some original 2 wire from the 1970s.

Would it not be just as easy to replace that 2 wire with T&E cable? Or is it a case that the whole lighting installation is wired in twin without a cpc and not just the garage light?

4mm seems unlikely for a cpc in a 'ring main'!