Commercial sub mains exceed max zs of mccbs

[jackhammerJIM]

Thoughts required on todays scenario .

Commercial premises fed from 100 amp 3 phase TNS supply ZE 0.40

feeds 80 amp 3 phase switch fuse (max zs 0.57)

feeds 400 amp mainswitch protek panel in 35mm swa 16mm earth

125 amp mccb (0.24 max zs) submain exceeds max zs at 0.57

Had this down as a code 2 but max zs of 80 amp switch fuse will disconnect in under 5 seconds ....

Dogs dinner of a set up , any one in agreement of my code 3 ????

 

[Nigel]

What is the Zs at the load end of the circuit protected by the 125?

 

[jackhammerJIM]

What is the Zs at the load end of the circuit protected by the 125?

0.57

 

[westward10]

Are you testing with the high current range on your tester?

 

[jackhammerJIM]

Are you testing with the high current range on your tester?

Yes

 

[westward10]

Even if your Ze was 0.10 you wouldn't achieve selectivity. I don't suppose you did a line/neutral reading at source of supply.

 

[jackhammerJIM]

Even if your Ze was 0.10 you wouldn't achieve selectivity. I don't suppose you did a line/neutral reading at source of supply.

not yet . Im going in early tomorrow so will test L N .

The main panel and sub mains were installed 5 years ago with no paperwork and not a lot of thought in design .

In your opinion does it warrant a code 2 ?

 

[westward10]

Fault protection is provided at the 80A switch-fuse the problem you have is selectivity by possible poor design and a high Ze.

 

[Nigel]

0.57

I am really confused. So what is the Zs at the 80A switch fuse?

 

[jackhammerJIM]

I am really confused. So what is the Zs at the 80A switch fuse?

0.40 . what is confusing you ?

 

[westward10]

So you have 0.40 at the 80A switch-fuse which supplies a switch panel at which Zs is 0.57 thus, the Zs for the 125A device at this panel will be excessive due to the 0.57?

 

[Nigel]

0.40 . what is confusing you ?

You op. It ia not very clear. You said your Ze was 0.40 now your Zs is 0.40 at the 80A switxh fuse????

 

[jackhammerJIM]

You op. It ia not very clear. You said your Ze was 0.40 now your Zs is 0.40 at the 80A switxh fuse????

They are the same because they are next to one another

 

[westward10]

Sorry but I am lost, thought I had it. Draw it out and post a pic.

 

[Nigel]

Ok so I would say a C3 then also.