Why is a too high zs reading on a circuit okay if it’s protected by an Rcd?

[Welsh_sparky_21]

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?

 

[Julie.]

...
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.

Err

That's the stuff I come across sooooo.. often!

Pre covid, a lot of my work was hospitality rebuilds/renovations frequently in the highlands, almost all have long private approach roads with a few led lamps, but hundreds of metres away from the supply.

I would say over 90% have been wired in 1.5mm^2 swa because the load is so small, - even with the small loads the voltage drop is more than allowed, never mind meeting any trip times whatsoever

 

[pc1966]

And here was me making it up to show an example!

 

[Julie.]

And here was me making it up to show an example!

As you said "bad design knows no limits..."

Although use of the word "design" in these cases would be a new definition of the word!

Just no thought to what was being done at all.

I suspect the swa is the size it is because it's the smallest they could get.

(Two core, poorly installed to boot)

 

[pc1966]

I suspect the swa is the size it is because it's the smallest they could get.

(Two core, poorly installed to boot)

I think you meant to say "I suspect the swa is the size it is because it's the cheapest they could get."

 

[marcuswareham]

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

 

[radiohead]

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

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.
Experience electricians recognise when a problem exists regardless of the regulations.