Discuss Commercial sub mains exceed max zs of mccbs in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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jackhammerJIM

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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 ????
 
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.
 
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 ?
 
Fault protection is provided at the 80A switch-fuse the problem you have is selectivity by possible poor design and a high Ze.
 
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?
 
You need to be clear in your Report as we have been confused.

Read the first post again it is not that confusing .

The report will state that the 125a sub mains from the 400 amp panel board exceed maximum permissible zs values but the disconnection time will be met by the 80 amp switch fuse at source
 
It isn't confusing to you because you can visualise it but to someone reading you first post it is not clear.
 
Read the first post again it is not that confusing .

The report will state that the 125a sub mains from the 400 amp panel board exceed maximum permissible zs values but the disconnection time will be met by the 80 amp switch fuse at source
This statement to me gives the impression is all okay and maybe it is as fault protection is provided but the issue of selectivity is not clearly addressed. There is no selectivity between x and the supply protective device y whereby fault protection is provided by y. Or something along those lines.
 

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