EFL on TN-S with RCD

[devon1]

I'm doing 30mA & 100mA RCD resting on a TN-S system

TN-S earth fault loop impedance max is .8 ohms
TT earth fault loop impedance max is 200 ohms

Regs state approx 1500 ohms for RCD on TT system

What should the maximum Zs be on a TN-S system when testing an RCD?

 

[JUD]

I'm doing 30mA & 100mA RCD resting on a TN-S system

TN-S earth fault loop impedance max is .8 ohms
TT earth fault loop impedance max is 200 ohms

Regs state approx 1500 ohms for RCD on TT system

What should the maximum Zs be on a TN-S system when testing an RCD?

On a 230V supply Max Zs can be worked out like this:

Max Zs = 50V ÷ IΔn


So for a 30mA RCD the max Zs is 1666.66Ω (50 ÷ 0.03 = 1666.66) and for a 100mA RCD it is 500Ω (50 ÷ 0.1 = 500)

 

[caddy]

The maximum a 30mA can protect at is 1666 as Jud has said however, it should still be below 200 Ohms on a TT and irrespective of RCD protection or not the max ZS on an a TN-S should still be around 0.8

 

[devon1]

irrespective of RCD protection or not the max ZS on an a TN-S should still be around 0.8

Do you know of any documentation to back that up? i just need it for a work situation.

 

[mnmwill]

the max ZS on an a TN-S should still be around 0.8

The max Ze, not Zs, should be 0.8 on a TN-S.

 

[mnmwill]

Do you know of any documentation to back that up? i just need it for a work situation.

Have a look at p.11 OSG m8.

 

[JUD]

The maximum a 30mA can protect at is 1666 as Jud has said however, it should still be below 200 Ohms on a TT and irrespective of RCD protection or not the max ZS on an a TN-S should still be around 0.8

That would be Ze, not Zs.

A Ze or Ra of more than 200Ω on a TT is considered unstable as Caddy says.

The maximum Zs of circuits on a TN system should be taken from Tables 41.2 to 41.4 on pages 48 & 49 of BS7671.

 

[ezzzekiel]

Do you know of any documentation to back that up? i just need it for a work situation.

er BS7671 Onsite guide Gn3

 

[JUD]

devon1, I've just noticed your qualifications in your signature.

With those qualifications you really should know all this

 

[devon1]

devon1, I've just noticed your qualifications in your signature.

With those qualifications you really should know all this

yeah i just need to explain it to the electrical engineer on site! The engineer is telling us that the Zs max is 1500 ohms on the TN-S