Earth Fault Loop Impedance Test for lighting circuits

[pc1966]

Means even if the Zs value is higher than values mentioned in the BS7671 Table, the RCD will disconnect the earth fault . Am I wrong?

The RCD will disconnect quickly at impedances well above the Zs needed for the MCB, so in that sense you are right.

But equally if the Zs values is higher than what was designed (or reasonably expected for the type of circuit and size of installation) then it points to something going bad. Could be a loose joint, cable near the point of fracturing, bad repair with seriously under-spec cable, etc, which should be investigated.

 

[Taylortwocities]

So practically, if there is any earth fault, the RCD would detect and remove the fault much quicker than MCB

Yes, practically

BUT

an RCD is only an additional protection device. The circuit should be designed so that an MCB/fuse will disconnect the supply in the event of an earth fault. So the circuit must correctly meet the EFLI criterea.

 

[pc1966]

an RCD is only an additional protection device.

This is a very important point.

An RCD (and the equivalent section in an RCBO) is an order of magnitude more complicated than a MCB and far more likely to fail "unsafe", hence the periodic testing that is specified (though I doubt most folk do it).