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Not long passed my test but was wondering why it’s okay for a circuit to be over the max zs if it’s protected by an RCD?
Discuss Why is a too high zs reading on a circuit okay if it’s protected by an Rcd? in the UK Electrical Forum area at ElectriciansForums.net
not really relevant as even with a high Zs, the L-N loop impedance is independentof the Zs and a fault L-N shoud still trip the OCPD.Simply put really it is not ok. The problem with relying on RCD is it does not really address faults between L-N. In certain situations that can be a problem. Before opting for such a means to say "cover" high Zs one must consider the implications of what would happen in a L-N fault. Could someone be harmed or a fire, in say a combustible environment?
Yes, it would still trip the mcb and NOT the rcd, but the time to trip may or may not be acceptable.Thanks for all the replies, just trying to get my head around it, so my understanding that even with a zs (earth fault loop) reading that’s too high a live to neutral short circuit would still trip your MCB due to the size of the fault current created??
Yes, if the high Zs is due to the earthing arrangement, and not due to a stupidly long/thin cable.my understanding that even with a zs (earth fault loop) reading that’s too high a live to neutral short circuit would still trip your MCB due to the size of the fault current created??
Err...
You could make up a pathological case of, say, a 50W LED flood lamp at the end of several hundred meters of 1mm cable. It meets the VD for its fixed load of around 0.22A, and with, say, a 10A MCB the 1mm cable is overload protected, but it might not trip on a L-N fault in any sort of reasonably time-scale.
As you said "bad design knows no limits..."And here was me making it up to show an example!
I think you meant to say "I suspect the swa is the size it is because it's the cheapest they could get."I suspect the swa is the size it is because it's the smallest they could get.
(Two core, poorly installed to boot)
This is where experience and common sense kicks in (hopefully)....if such a reading was obtained on a TN system clearly alarm bell should ring despite the presence of an RCD, and if a TT was in place you would hope an experienced person would verify that the high reading was due to the Ra and not a wiring issue.Interesting, A while ago I had conduit CPC fail on 2 circuits with 50-150ohm Zs, I pulled through a new CPC for one of them, and for the cooker ran a new bit of T&E as was close to CU.
Although I believe this is the best way to fix it as now has a nice low Zs and meets disconnection time for the MCBs, But in theory if the circuits were on an RCD or upgraded to be on an RCD then could have been left like this as the 150ohms is enough to meet disconnection due to the RCD.
If it was to later have an EICR though I cant imagine many people passing it without a code as the r1+r2 would be high
Reply to Why is a too high zs reading on a circuit okay if it’s protected by an Rcd? in the UK Electrical Forum area at ElectriciansForums.net