Slightly high Zs on large ring circuit

PB1 · Dec 1, 2011

Hi all, I did a PIR yesterday at a largeish house (150m2). there is only 1 ring final circuit for all house sockets. Zs was 1.25ohms, vs 1.16 permitted maximum value. there is an RCD so additional protection in theory although the RCD did not disconnect at 5x 30 ma test. I will obviously recommend replacing the RCD, but wondered what to say if anything about the size of the ring as the length of the cable is presumably the reason that the EFLI is too high. Given that RCDs do fail sometimes the circuit will not meet the disconnection time for the B32 breaker that is protecting it. Any advice greatfully appreciated.

MarkieSparkie · Dec 2, 2011

If the RFC was additionally protected by a tested and fully functional 30mA RCD the maximum Zs for the final circuit would be 1667 Ohms (BRB, Table 41.5), so your measured Zs of 1.25 Ohms easily complies with BS7671 in this respect. On a TN system in most cases the absence of fully functional additional RCD protection would warrant a PIR Code 4, however if the RCD also covers final circuits a special location (e.g. bath or shower) and the bonding requirements of BRB 701.415.2 are not satisfied, or the supply of mobile equipment outdoors can reasonably be expected, a PIR Code 2 would be appropriate. You should recommend to your customer that the RFC should be split in to 2 or more smaller final circuits, ring or radial as appropriate. The guidance from the OSG Appendix 8.2 is that an A1 RFC should not serve a floor area greater than 100m[SUP]2[/SUP], the reasoning behind this is that the greater the floor area served by a RFC the greater the chance of unbalanced current distribution of the total load in each leg of the RFC. Depending on your assessment of likely distribution of high current using appliances on the RFC and degree to which BRB 433.1.5 is infringed, this may warrant a PIR Code 2 or 4 accordingly.