what next!

sparky1ste · Jan 30, 2014

My max permitted Zs for a circuit is 0.05 - my calculated Zs = 0.07 - 240mm swa cable on 630a MCCB (adjustable)ZE at origin 0.01 (substation otherside of wall) - got this as a code 2 but no way of fixing this excessive earth loop impedence so a satisfactory report can be achieved. any thoughts! Ps. I know MCCB is oversized but when adjusted it is still over the max!

danzor · Jan 30, 2014

calculated?

pepparz · Jan 30, 2014

change the mccb to one with a higher allowable zs for ur needs

Engineer54 · Jan 30, 2014

What does it say in the manufacturers data sheet that came with the 630A MCCB?? Why are you calculating rather than measuring. Depending on your test equipment, you may have to implement IQ's supplementary resistance method, (see Sticky's above) to obtain a realistically accurate measurement... Anyway, ....check manufacturers data first.

Guest55 · Jan 30, 2014

its a tough call
when the difference between pass and fail of such low values can be just 1 / 100th of an ohm , can you rely on the resolution of standard test equipment to get the right readings so close to the Tx ?
unlikely tbh.

and as you say there aint no quick & easy fix beyond adjusting the mccb.
if they really want a followed up pass remedial , downrating the mccb & thus the circuit also , may have to be considered , or improve the earth path even.

Engineer54 · Jan 31, 2014

biff55 said:
its a tough call
when the difference between pass and fail of such low values can be just 1 / 100th of an ohm , can you rely on the resolution of standard test equipment to get the right readings so close to the Tx ?
unlikely tbh.

and as you say there aint no quick & easy fix beyond adjusting the mccb.
if they really want a followed up pass remedial , downrating the mccb & thus the circuit also , may have to be considered , or improve the earth path even.

Yep, not many here will have a high resolution ELI tester, that's for sure!! Improving the earth path is going to be a hit and miss affair too, short of connecting a suitably sized separate CPC from the the substations MET.

Manufacturers will generally only give a max Zs figure, not a Zs value for every setting on a adjustable MCCB, which is why i advised the OP to check the data sheet first, or better still contact the manufacturers technical dept direct. Replacing the 630A MCCB for a smaller one to suit the connected load, could turn out to be an expensive exercise, so best left as a last resort.