MCCB required but Ze too high

Question

One of my old customers has deciding whether to buy a big compressor that requires a 160A TP supply. He asked me to help him work out what is needed for the job to see if it is a worth doing. The site TNCS Ze is 0.23 so this will put the Zs above any 160A MCCB max limit. So I think we would have to go down the route of having earth fault protection on the MCCB to cover this. 
 My question is that as the compressor is inverter driven, am I going to be having issues with the earth fault tripping all the time? Or is there an alternative route I should be looking at?

broadgage · Accepted Answer

One possibility would to install an MCCB with earth fault protection, but not too sensitive, to avoid tripping from leakage currents. 
 Earth fault protection on small circuits is often at about 30ma as this gives protection against dangerous electric shock. For large circuits such as 160 amps, much less sensitive earth protection can often be used, set at several amps.

mapj1 · Answer

You will need either an MCCB with an earth fault sensing that can be set to 300mA or higher, or the equivalent of a separate programmable RCD - the so called 'earth fault relay' 
 Take a seat before looking at the prices though. 
 Mike.

Dbat · Answer

Yes that's the route I was thinking I was going to have to go with the inverter. Shame it's going to bump the cost up but not really any other way round it I don't think. I'll have to have a play with the setup once installed to see what the leakage looks like so we can find a reasonable setting that doesn't trip all the time

MCCB required but Ze too high

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