Earth fault loop impedance

[Sean McD]

Morning all. So just trying to refresh my grey matter but having a biff moment. Earth fault loop impedance with circuits protected by BS EN60898 type B circuit breakers. I know the tabulated values are in table 41.3, but for the life of me I’ve forgotten how to calculate if acceptable or not. So for example; 40A circuit breaker, measured Zs 0.5, maximum tabulated value 1.09. How do I get the corrected values and see if acceptable? Any help for my non firing brain would be greatly appreciated. TIA.

 

[Strima]

As in multiply the tabulated by 0.8 to account for increased resistance on the cable under load conditions?

 

[H]

The values in table 41.3 BS7671 need to be multiplied as above (0.8), the values within the on site guide have already been adjusted to reflect the 0.8

 

[Sean McD]

Strima, H, many thanks for that. I just could not for the life of me remember today.

 

[telectrix]

and don't forget Cmin. that takes another 5% off the max. allowed value. however. I'm skeptical about the 0.8 correction esp. in a domestic. the 0.8 assumes that cables will be running @ 70 deg.C. that is loaded to the max. If you go with the design current Ib, rare you'll see cables getting that warm, only on things like showers.