Does anybody know of this new regulation that omits the ZS test.

sham · Sep 30, 2018

On another forum about an high ZS reading about a fused spur feeding a boiler, someone replied that in the new 18th regs if the RCD breaks the circuit within time under test you do not need to check ZS, just R2 continuity test for CPC continuity would be good enough. You just put N/A on the test form for Zs. He even posted an article from Stroma.
This was a response to a question about the Zs reading with his Instrument was 100 ohms higher than his calculation.
Does anybody know more about this, and is it because of rcds and rcbos causing high reading on live ZS redings ?

[ElectriciansForums.net] Does anybody know of this new regulation that omits the ZS test.

Octopus · Sep 30, 2018

Which is the “other” forum ?

sham · Sep 30, 2018

It's a forum on facebook
Electrical Installation Condition Report (EICR) - https://www.facebook.com/groups/1040643346015311/

Strima · Sep 30, 2018

It's no different from Measuring Ze then using measured R1+R2 to confirm the circuit complies to satisfy disconnection times.

To say you do not need to check Zs is wrong, you still need to confirm that the OCPD will operate within the required times either by measurement or calculation.

Ian1981 · Sep 30, 2018

sham said:
On another forum about an high ZS reading about a fused spur feeding a boiler, someone replied that in the new 18th regs if the RCD breaks the circuit within time under test you do not need to check ZS, just R2 continuity test for CPC continuity would be good enough. You just put N/A on the test form for Zs. He even posted an article from Stroma.
This was a response to a question about the Zs reading with his Instrument was 100 ohms higher than his calculation.
Does anybody know more about this, and is it because of rcds and rcbos causing high reading on live ZS redings ?

View attachment 44579

This is in regards to a TT system where the regulations now state a continuity test of the cpc and any applicable exposed conductive parts and an rcd test is acceptable and a zs test is not applicable

Octopus · Sep 30, 2018

Ian1981 said:
This is in regards to a TT system where the regulations now state a continuity test of the cpc and any applicable exposed conductive parts and an rcd test is acceptable and a zs test is not applicable

Why only TT systems ?

sham · Sep 30, 2018

Murdoch said:
Why only TT systems ?

Good question.

sham · Sep 30, 2018

Ian1981 said:
This is in regards to a TT system where the regulations now state a continuity test of the cpc and any applicable exposed conductive parts and an rcd test is acceptable and a zs test is not applicable

That is what he said in the forum.

Ian1981 · Sep 30, 2018

Murdoch said:
Why only TT systems ?

Because it’s pointless verifying the zs of the circuit when the OCPD offers no earth fault protection and is reliant on the rcd for earth fault protection
The Ra must be measured, or an external earth fault loop impedance test carried out and the effectiveness of the rcd for ads

Ian1981 · Sep 30, 2018

sham · Sep 30, 2018

But I have

Ian1981 said:
View attachment 44581

Ian1981 said:
Because it’s pointless verifying the zs of the circuit when the OCPD offers no fault protection and is reliant on the rcd for earth fault protection
The Ra must be measured, or an external earth fault loop impedance test carried out and the effectiveness of the rcd for ads

On TN systems, if there is a high Zs, Is it ok because it is protected by RCDS. Can it apply here as well ?
I would think so.

Flanders · Sep 30, 2018

sham said:
But I have

On TN systems, if there is a high Zs, Is it ok because it is protected by RCDS. Can it apply here as well ?
I would think so.

Technically i would say its ok but would be a bad design as you are just relying on the rcd to protect from fault current.

Octopus · Sep 30, 2018

sham said:
But I have

On TN systems, if there is a high Zs, Is it ok because it is protected by RCDS. Can it apply here as well ?
I would think so.

You should be investigating why .......... not fudging it IMHO

Flanders · Sep 30, 2018

Murdoch said:
You should be investigating why .......... not fudging it IMHO

Yes i agree but could be obvious reason for the high Zs for example long cable run , so the only reason for the high Zs is bad design . If i did not do the actual Zs test i would put the caculated reading on the report instead

HappyHippyDad · Sep 30, 2018

I can't see where any new regulation states that max Zs figures do not have to be reached on a TN system?

I have thought on occasion 'why' is an RCD not deemed acceptable on TN system for fault protection when it is fine for a TT, but I came to conclusion that I was thinking about it in the wrong way.....

I'm pretty rubbish at analogies but lets say a TT system is like a classic car without a seat belt (pre 1966) and a TN system is a modern car.

Sticking to the speed limit is the RCD and wearing a seatbelt is ADS (i.e meeting max Zs figures).

Both cars can stick to the speed limit (have RCD) but that's no reason not to wear a seatbelt (ADS). The only reason you don't wear a seatbelt legally is if you don't have one fitted and it's pre 1966, but that's no excuse to not wear one in a modern car.

Yep, that all makes perfect sense