MCCB Zs

[kendo57]

Got a Ze of 0.6 ohm on a TN-S.
Circuit protected by a 250 amp mccb with an operating Zs of 0.073 ohms. Any ideas what I can do? Any advise much appreciated.

 

[Lister1987]

Got a Ze of 0.6 ohm on a TN-S.
Circuit protected by a 250 amp mccb with an operating Zs of 0.073 ohms. Any ideas what I can do? Any advise much appreciated.

How would we normally go about improving Zs? We can't improve Ze so we're either going to be upgrading cable size, upgrading breaker size or smaller circuits/runs; any of these a possibility?

 

[pc1966]

Investigate why the Ze is that high, assuming it is a supply > 250A then the PSCC ought to be 5kA or more so it must be related to the earth arrangement.

If there is no reasonably way to fix that, look at the RCD add-on for the MCCB, set to 1-5A or whatever supported trip current with delay 0.5s or so and you meet earth fault disconnection as well as selectivity with any down-stream boards using 300mA S-type RCDs..

 

[kendo57]

Investigate why the Ze is that high, assuming it is a supply > 250A then the PSCC ought to be 5kA or more so it must be related to the earth arrangement.

If there is no reasonably way to fix that, look at the RCD add-on for the MCCB, set to 1-5A or whatever supported trip current with delay 0.5s or so and you meet earth fault disconnection as well as selectivity with any down-stream boards using 300mA S-type RCDs..

Think the rcd add on is the only option, DNO said Ze is what it is, helpful as usual