High measured Zs values

[londonlec]

I have some remedial works to do on a fairly large commercial installation.
The majority of the submains from the main panels are protected by 63 and 100A MCCBs, and as a results of a high Ze (0.24), a lot of the submains Zs’s don’t meet the regs figure.
Replacing the cables is not an option, and won’t reduce the Zs down enough anyway.
So I was going to refeed the affected circuits via BS1361/BS88 fuses, which have a much higher (almost double) permitted Zs.
Does anyone else have any better ideas?!
Many thanks in advance.

 

[Flanders]

What make of MCCB are they? the Max Zs vary with different makes 100 amp Square D max zs is 0.36 with out applying rule of thumb, what are the max demands of the submains, Some MCCBS can be turn down to 80 % of the current value which may get you inside the disconnection time.

 

[Paultrim123321]

I think Merlin TMD MCCB modules even when turned down will have no effect on Max Zs. It was a while ago I researched that but it sticks in my mind.

 

[Lucien Nunes]

What's the incomer rating and type of earthing? For a supply with lots of 63A & 100A subs a PFC of 1kA seems rather wimpy. Is that Ze definitely accurate?

 

[44ALLAN]

decimal point error perhaps.................or maybes 2 depending where taken from

My main board shows pfc as 24 zillion amps.................well it is only 50 paces from our trannie
And I mean tx before we go off on another tangent!!!

All I said was soggy mars bar and we have war and peace elsewhere fgs lol

 

[Engineer54]

I have some remedial works to do on a fairly large commercial installation.
The majority of the submains from the main panels are protected by 63 and 100A MCCBs, and as a results of a high Ze (0.24), a lot of the submains Zs’s don’t meet the regs figure.
Replacing the cables is not an option, and won’t reduce the Zs down enough anyway.
So I was going to refeed the affected circuits via BS1361/BS88 fuses, which have a much higher (almost double) permitted Zs.
Does anyone else have any better ideas?!
Many thanks in advance.

Do you actually know the max Zs of these MCCB's, as every manufacturers figures differ, even between model numbers for a given rating from the same manufacturer??

Check before even contemplating changing out, or backing up with HRC fuses!!

 

[londonlec]

Thanks for replies. Yes the MCCBs are Merlin Gerin NSX type (from memory), I cannot adjust them down as 1. they are operating at near full current, and 2. according to Merlin the Max Zs is the same regardless of adjustment (as someone has said). I make the Max Zs for these breakers as 0.24. The measured Zs is 0.46.