Earth Fault Loop Impedance Test for lighting circuits

[arashabb]

Hello guys,
Do you do Earth fault loop impedance test for lighting circuits during periodic inspection ? do you measure it at the lighting point? what is the percentage of lights you test?
Thanks alot in advance

 

[cliffed]

Yes you do like all circuits,@ furthest light & several to check readings.
% depends on client,usually on a domestic it’s reasonable to check all circuits.

 

[multimick]

i just do R2 on lights

 

[arashabb]

i just do R2 on lights

yeah, I am not sure it if it necessary to to it. on the GN3 guideline, it has been mentioned the recommended points to be tested are at:
- origin of the installation
-distribution boards
-accessible socket outlets
-extremity of radial circuits
[automerge]1571029063[/automerge]
On the GN3 guideline, it has been mentioned the recommended points to be tested are at:
- origin of the installation
-distribution boards
-accessible socket outlets
-extremity of radial circuits

 

[telectrix]

the last one..... the extremity of a radial includes a lighting circuit. so it would appear that recommended is all accessible sockets (on a ring) or last point on a radial (socket, lights.or other radial).)

 

[baldelectrician]

For lights we use the R2 lead on all acessible points and do an r1r2 at the highest reading.

 

[arashabb]

The reason for doing this test is to verify that Zs is low enough and at the time of earth fault, current would be high enough to trip to breaker(MCB).Correct? Where I work ( in Dubai) we have 100mA RCDs for all lighting circuits. So practically, if there is any earth fault, the RCD would detect and remove the fault much quicker than MCB and it would not let the current reaches to the MCB tripping point. So I am not sure how useful this test is!

 

[Mark Wright]

If you do your R1 and R2 as normal and your Ze as normal you have your Zs Don't you?

 

[Risteard]

it would not let the current reaches to the MCB tripping point.

An RCD will not limit the current, but simply provide disconnection where the rated residual operating current (well actually between half and all of the rated residual operating current) is exceeded.

 

[arashabb]

An RCD will not limit the current, but simply provide disconnection where the rated residual operating current (well actually between half and all of the rated residual operating current) is exceeded.

I agree but the purpose of testing Zs on lighting point and checking the values with Zs of protection device ( MCB in our case) is useless because the RCD will detect and remove the earth fault much quicker than MCB. 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?

 

[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).