How to conduct safe isolation on this main switch ?

Rob678 · Oct 17, 2023

Hi all, if I cannot access the line side of a crabtree 100A DP main switch. Would someone enlighten me how to conduct safe isolation on this installation. Would the main fuse need to be removed ? Thankyou.

rich7469 · Oct 17, 2023

https://www.----------------------------/media/blak1wkt/best-practice-guide-2-issue-4.pdf

westward10 · Oct 17, 2023

What is it you are wanting to do?

Rob678 · Oct 17, 2023

westward10 said:
What is it you are wanting to do?

Conduct safe isolation of the consumer unit. However once I have proved my my voltage meter works. How would I be able to conduct L-N, and L-E on the consumer side of the main switch ? The face of the screw is covered by plastic

westward10 · Oct 17, 2023

Safe isolation to replace it, work on it?

Rob678 · Oct 17, 2023

westward10 said:
Safe isolation to replace it, work on it?

Neither in this instance. Just purely interest. So far in my apprenticeship we have always been able to conduct the safe Isolation tests on L-E L-N and N-E. As I can use my voltage detectors on the non-supply side of the main switch. I don’t understand how I would be able to conduct safe isolation on this main switch to ensure the board is safe to work on if I was to remove an mcb or RCBO ? I understand if I needed to remove the main switch DNO would need to be contacted as that involves work on the supply side.

Rob678 · Oct 17, 2023

westward10 said:
What is it you are wanting to do?

For example in one photo I can get a probe onto the Line side in the other photo I can’t. On the one I can’t how would I conduct SI ?

snowhead · Oct 17, 2023

Ah, so your enquiry relates soley to the pictured Isolator, with missing terminal to test at.
What make is it?

[ElectriciansForums.net] How to conduct safe isolation on this main switch ?

westward10 · Oct 17, 2023

That is an isolator for a Crabtree Starbreaker CU. The only way you can prove the outgoing side is dead is at the protective device terminal as the live terminal from the isolator plugs onto a busbar.

Rob678 · Oct 17, 2023

snowhead said:
Ah, so your enquiry relates soley to the pictured Isolator, with missing terminal to test at.
What make is it?

View attachment 111110

Yes 100%, the product is a Crabtree star breaker 100A DP main switch disconnector.

davesparks · Oct 17, 2023

Presumably a crabtree starbreaker CU where everything plugs into the shrouded busbar?

I think you would have to use the outgoing terminals of the protective device(s)

Rob678 · Oct 17, 2023

davesparks said:
Presumably a crabtree starbreaker CU where everything plugs into the shrouded busbar?

I think you would have to use the outgoing terminals of the protective device(s)

Yes Dave that is correct. That’s the only solution in my head I could come up with. But I thought surely there must be another route ? Have looked for many sources online but couldn’t find an answer.

davesparks · Oct 17, 2023

Rob678 said:
But I thought surely there must be another route ?

Why would you need another way?

This is a genuine question as there is an argument you could make as to a, very unlikely, scenario which could make this method unsuitable.

Rob678 · Oct 17, 2023

davesparks said:
Why would you need another way?

This is a genuine question as there is an argument you could make as to a, very unlikely, scenario which could make this method unsuitable.

I suppose you wouldn’t. I meant that question more in the sense I felt like I’d be doing something wrong as I have always completed SI from the Main switch in this scenario

davesparks · Oct 17, 2023

Rob678 said:
I suppose you wouldn’t. I meant that question more in the sense I felt like I’d be doing something wrong as I have always completed SI from the Main switch in this scenario

OK, so the very unlikely scenario is the MCB going faulty at the same time as you open the main switch, this could give a false result on your safe isolation test.

This is an incredibly unlikely thing to happen and you can mitigate it by testing at multiple protective devices just to be sure.