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.

 

[jackhammerJIM]

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?

0.57 zs is on the outgoing sub mains from the 400 amp panel . sub mains are protected by 125 amp mccbs

 

[westward10]

You need to be clear in your Report as we have been confused.

 

[jackhammerJIM]

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

 

[westward10]

It isn't confusing to you because you can visualise it but to someone reading you first post it is not clear.

 

[westward10]

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.

 

[davesparks]

Are the 125A mccbs set at full or have they been adjusted down to a more sensible level?

 

[jackhammerJIM]

Are the 125A mccbs set at full or have they been adjusted down to a more sensible level?

Hi Dave this type is not adjustable

 

[jackhammerJIM]

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.

I have commented on the poor discrimination between devices and made them aware of the inconvenience a fault may cause .

cheers lads thanks for your input ....

 

[westward10]

Selectivity these days

 

[jackhammerJIM]

Selectivity these days

person receiving the report won't under stand either term

 

[arashabb]

Hi Guys,
in an installation with one main Distribution board and several submain-DBs, do when you measure Zdb on a submain-DB incomer? and do we need to verify the Zdb value with the up stream protection device?( for example, a 125A, MCCB on the MDB outgoing). If yes, how can I get the max Zs values for MCCBs ?

 

[westward10]

Hi Guys,
in an installation with one main Distribution board and several submain-DBs, do when you measure Zdb on a submain-DB incomer? and do we need to verify the Zdb value with the up stream protection device?( for example, a 125A, MCCB on the MDB outgoing). If yes, how can I get the max Zs values for MCCBs ?

Hello Arashabb this is an old thread so you need to create a new one.