Discuss Does anybody know of this new regulation that omits the ZS test. in the Electrical Forum area at ElectriciansForums.net

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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 ?

Does anybody know of this new regulation that omits the ZS test. 42806519_1911455935588038_6381880673786920960_n - EletriciansForums.net
 
It's a forum on facebook
Electrical Installation Condition Report (EICR) - https://www.------------/groups/1040643346015311/
 
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.
 
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
 
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 ?
 
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
 
But I have

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.
 
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.
 
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
 
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 o_O:confused::)
 
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 o_O:confused::)
Come again??o_O
 
So you should wear a seat belt when doing a Zs test?
That made me laugh quite a lot :).

I've just checked your profile Sparkdog. You must have been one of the first to join this site! 2008, not even Telectrix had joined then.
 
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.....
:)

In the 17th A3 and 18th regulations it is deemed acceptable for a TN system, though there has been a slight change in the new regulations as the max Zs is changing.

Previous versions of the regulations prohibited the use of an RCD as the sole means of fault protection, which effectively means your Zs had to comply for the ocpd to provide fault protection.

I think this was mainly based on reliability as RCDs were not necessarily as reliable as the available OCPDs. Also there will have been the usual fear of change with an element of clinging to the past and tradition. Just look at the uproar each time a new regulation is brought in.

RCDs were acceptable as fault protection on TT and I think this was partly because they were the only devices commonly available which could do the job with the higher Zs of some TT installations.
Also there is the historical requirement for the use of RCDs and the previous VOELCBs on TT systems, the regulations have required them since at least the 1950’s

Another factor which may have something to do with it is that fault currents are often a lot lower in a TT system than a TN system.
 

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