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

[HappyHippyDad]

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.

Dave, can you point me towards the reg number showing it may not be necessary to meet max Zs figures for a TN system if there is an appropriate RCD in situ?

 

[telectrix]

Yup, been on here since Tel was a boy.

that makes your age around 110 years.

 

[Deleted member 26818]

All seems weird, a Zs test is what I use to confirm CPC continuity when conducting periodics.

I wonder if this is connected to the NICEIC’s wish to stop live testing?

 

[davesparks]

Dave, can you point me towards the reg number showing it may not be necessary to meet max Zs figures for a TN system if there is an appropriate RCD in situ?

There is no such regulation as it is always necessary to meet the max Zs figures for the method of fault protection. If the method of fault protection is a 30mA RCD then the max Zs is currently 1667ohms (50/0.03) when the 18th edition comes in to effect it will be 7667ohms (230/0.03)

 

[Ian1981]

There is no such regulation as it is always necessary to meet the max Zs figures for the method of fault protection. If the method of fault protection is a 30mA RCD then the max Zs is currently 1667ohms (50/0.03) when the 18th edition comes in to effect it will be 7667ohms (230/0.03)

Where does it say 7667 ohms?
Looking at table 41.5 for a 30mA rcd it’s max zs is still 1667 ohms.
Also see regulation 411.4.204

 

[Marti]

There is no such regulation as it is always necessary to meet the max Zs figures for the method of fault protection. If the method of fault protection is a 30mA RCD then the max Zs is currently 1667ohms (50/0.03) when the 18th edition comes in to effect it will be 7667ohms (230/0.03)

I make that spot on. Table 41.5 on page 64 leads to the conclusion that safe touch voltage divided by the RCD trip current is 1667 ohms so if 240v x C-min of 0.95 is 230 divided by 0.030amps we have a happy Zs of 7.6 Kohms! That can't be right......
(promise I'll learn how to work the sub-script font thingy)

 

[Deleted member 26818]

So wait, all this time we’ve struggled to get the Zs down to 1667 ohms, and now it turns out we never needed to?

 

[Strima]

I make that spot on. Table 41.5 on page 64 leads to the conclusion that safe touch voltage divided by the RCD trip current is 1667 ohms so if 240v x C-min of 0.95 is 230 divided by 0.030amps we have a happy Zs of 7.6 Kohms! That can't be right......
(promise I'll learn how to work the sub-script font thingy)

I don't read it that way. I read it as circuits with a Uo of 230 v.

For RCDs have a look at 411.5.3 and it's still Ra x In <= 50 V.

 

[sham]

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

It does make sense, mate.

 

[Octopus]

I wonder if this is connected to the NICEIC’s wish to stop live testing?

More evidence they want to de-skill the trade

Live testing is perfectly safe when conducted by competent people with the relevant testing experience PLUS the proper equipment and adaptors available today.....

Just saying

 

[sham]

MCBS are for short circuits. There has been cases of Live and Neutral touching and it just trips the MCB and not the RCD. It seems that the regs are saying we can't rely on the zs being low enough to help trip the MCB in time which may cause the cable to be permanently damaged or appliance, but we still have protection of persons and livestock by the use of RCDs. This is my thoughts at the moment.

 

[sham]

MCBS are for short circuits. There has been cases of Live and Neutral touching and it just trips the MCB and not the RCD. It seems that the regs are saying we can't rely on the zs being low enough on an TT system to help trip the MCB in time which may cause the cable to be permanently damaged or appliance, but we still have protection of persons and livestock by the use of RCDs. This is my thoughts at the moment.

 

[davesparks]

Where does it say 7667 ohms?
Looking at table 41.5 for a 30mA rcd it’s max zs is still 1667 ohms.
Also see regulation 411.4.204

I don’t have a book to hand but I’m pretty sure that for a TN system they’ve changed it from 50V to 230V giving 7667ohms.
TT systems have remained at 50V

 

[davesparks]

. There has been cases of Live and Neutral touching and it just trips the MCB and not the RCD. t.

Well yes, RCDs don’t work on L - N faults

 

[Deleted member 26818]

Well yes, RCDs don’t work on L - N faults

I think Sham is saying, that although the MCB won’t trip, there will still be RCD protection if anyone comes into contact with electricity due to the L to N fault.

 

[sham]

I think Sham is saying, that although the MCB won’t trip, there will still be RCD protection if anyone comes into contact with electricity due to the L to N fault.

Thankyou

 

[sham]

I

Well yes, RCDs don’t work on L - N faults

I'm saying that the Regs when it comes to TTs, it that we can't always garantee the disconnection of the mcb circuit on time and that it may damage property but as soon it leaks to livestock or people , it must trip within the time and the continuity of CPC must be sound. And results must be recorded.