Max Allowed ZS for Schneider mccbs

zs? Do you mean Zero Sequence current? Do your MCCBs have ZS protection built-in, as in having external core balanced Zero Sequence Current Transformers (ZSCT)? If not, ZS is a non-issue, a strandard MCCB will not be capable of detecting nor reacting to a Zero Sequence current siutation directly. Even if you were referring to solid state MCCB trip units, they will typically have Residual Current detection for ground (earth) fault detection, ZS is an option because it requires that external ZSCT.

Assuming that's what you mean by "zs".

PS: We can't read your mind and acronyms have lots of different meanings, even in the same context.

Maximum values for Zs are in BS7671-2008. They aren't manufacturer specific. Or contact the manufacturer.

So Tony, what is Zs in this instance? Source impedance?

zs-> earth fault loop impedance

http://pws.schneider-electric.co.uk/internet/pws/literature.nsf/luAllByID/NKIY-5JUHSL/$file/MGD5032_section2.PDF/

page-6........................

Zs= Uo/Ia

Where

Zs = Maximum earth fault loop impedance.

Uo = Nominal phase voltage.

Ia = Current ensuring automatic operation of the protective device.

Ia for circuit breakers is the mean tripping current as follows:

Type B = 4 (typical)1 x rated current

Type C = 7.5 (typical) x rated current

Type D = 12.5 (typical) x rated current

Ia for fuses are appropriate mean values from AS 60269.1