UK High Max Zs

[JXYM89]

Undertaking an EICR and there was a high measured Zs on the B30 MCB Ring Main circuit (TN-C-S) - No RCD protection on any circuit. Lighting circuit for building extension is spurred off ring main, max R1+R2 recorded was 1.84 (Zs 80% = ~1.17). As I understand it, though not ideal, the remedial work required to make good would be to put the circuit on an RCD/RCBO to achieve the disconnection time? - can anyone confirm deny/am open to suggestions. Thanks in advance.

 

[Pretty Mouth]

Lighting circuit for building extension is spurred off ring main, max R1+R2 recorded was 1.84 (Zs 80% = ~1.17).

Is this via a fused spur? If so, then the protective device for this part of the circuit would be the BS1362 fuse, which has a much higher max permitted Zs, e.g. 15.6 ohms for a 3A fuse. It probably already meets disconnection times. See table 41.2.

But back to your question: Yes. A 30mA RCD has a max permitted Zs of 1667, and can be used to meet disconnection times. This applies not just to TT, but also TN-S and TN-C-S earthing arrangements.

 

[JXYM89]

Is this via a fused spur? If so, then the protective device for this part of the circuit would be the BS1362 fuse, which has a much higher max permitted Zs, e.g. 15.6 ohms for a 3A fuse. It probably already meets disconnection times. See table 41.2.

But back to your question: Yes. A 30mA RCD has a max permitted Zs of 1667, and can be used to meet disconnection times. This applies not just to TT, but also TN-S and TN-C-S earthing arrangements.

Hey Pretty Mouth, thanks for the reply. Yes it is on a fused spur, 1mm cable on a 13A fuse. I think I'm right in understanding, that though permitted it is not advisable that the max measured Zs is exceeded on a TN earthing system.

 

[mainline]

Hey Pretty Mouth, thanks for the reply. Yes it is on a fused spur, 1mm cable on a 13A fuse. I think I'm right in understanding, that though permitted it is not advisable that the max measured Zs is exceeded on a TN earthing system.

What's the Zs on the ring final.

 

[Pretty Mouth]

think I'm right in understanding, that though permitted it is not advisable that the max measured Zs is exceeded on a TN earthing system.

By this, do you mean that it is not advisable that the max permitted Zs of the overcurrent device be exceeded, instead using an RCD to meet disconnection times, on a TN earthing system? (Sorry to be pedantic over wording, but it's to avoid confusion). Where did you hear this?

I doubt there is any truth in it. I don't ever recall reading this in any IET guide or the regs, and the on-site guide has a list of standard circuits in section 7, many of which rely on an RCD for fault protection where there is TN earthing.

 

[JXYM89]

What's the Zs on the ring final.

Hey Mainline, Ze = 0.1, r1 = 0.29, rn = 0.30, r2 = 0.47, Zs on the main part ~0.19 to 0.24, though in the extension on average ~0.50.

 

[mainline]

Hey Mainline, Ze = 0.1, r1 = 0.29, rn = 0.30, r2 = 0.47, Zs on the main part ~0.19 to 0.24, though in the extension on average ~0.50.

The protection is your fused spur. This is what you should base your calculations on.

 

[JXYM89]

The protection is your fused spur. This is what you should base your calculations on.

That's absolutely grand, thanks for clarifying that for me.

 

[JXYM89]

By this, do you mean that it is not advisable that the max permitted Zs of the overcurrent device be exceeded, instead using an RCD to meet disconnection times, on a TN earthing system? (Sorry to be pedantic over wording, but it's to avoid confusion). Where did you hear this?

I doubt there is any truth in it. I don't ever recall reading this in any IET guide or the regs, and the on-site guide has a list of standard circuits in section 7, many of which rely on an RCD for fault protection where there is TN earthing.

Not a problem at all. As I understand it, it is not advisable, or considered 'poor design' at least (What if a circuits Max Zs is exceeded but the circuit is protected by an RCD? | on ElectriciansForums - https://www.electriciansforums.net/threads/what-if-a-circuits-max-zs-is-exceeded-but-the-circuit-is-protected-by-an-rcd.171676/), that the max permitted Zs on a TN earthing system is exceeded, relying on an RCD/RCBO to achieve the desired disconnection time.

I guess another angle for this would be, the installation of the RCD/RCBO would make calculating max measured Zs a redundant exercise (aside from ascertaining R2 at the EOL), on the most part, as most domestic properties are never likely to ever come close to the max Zs stated in Table 41.5?

 

[mainline]

That's absolutely grand, thanks for clarifying that for me.

I would add:
Where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.

 

[Pretty Mouth]

Not a problem at all. As I understand it, it is not advisable, or considered 'poor design' at least (What if a circuits Max Zs is exceeded but the circuit is protected by an RCD? | on ElectriciansForums - https://www.electriciansforums.net/threads/what-if-a-circuits-max-zs-is-exceeded-but-the-circuit-is-protected-by-an-rcd.171676/), that the max permitted Zs on a TN earthing system is exceeded, relying on an RCD/RCBO to achieve the desired disconnection time.

The opinions expressed in that thread aren't, to the best of my knowledge, official guidance.

I guess another angle for this would be, the installation of the RCD/RCBO would make calculating max measured Zs a redundant exercise (aside from ascertaining R2 at the EOL), on the most part, as most domestic properties are never likely to ever come close to the max Zs stated in Table 41.5?

That is pretty much the size of it. If a circuit is protected by an RCD*, it isn't necessary to measure a Zs for the circuit. As long as the Ze is proven, and the RCD tests ok, then ADS is confirmed by a continuity of protective conductors test. See the notes to 643.7.1 (a) and (b).

*can't use RCDs to BS7288 for fault protection apparently, but they can be used for additional protection.

 

[Pretty Mouth]

This article goes into more detail:

Verification of automatic disconnection of supply

This article discusses the significance of the change introduced by the 18th edition of BS 7671 to the verification required for the protective measure automatic disconnection of supply (ADS).

tinyurl.com

 

[Tom256]

This is not a comment specific to this situation more by way of observation.
I am certainly seeing a trend where the default position becomes 'fit an RCD' if ever a Zs reading exceeds a limit number.
Other checks / tests are seldom carried out... "sorry sir if it is not on the schedule as required it is not being done". No other calculations performed against the expected 'as-installed' figures to identify how [we] arrived at this position requiring this approach for fault protection within ADS.
BS7671 is the minimum requirement, the floor not the ceiling. RCD blinding by appliances plugged in... "it'll be fine".
Good design is a requirement of an Electrical Installation so is it designed that way or is it more it ended up that way? If so why?

Human nature often defaults to the easiest path, the easirst path becomes the preferred path. Such a path well walked requires little effort, little effort requires little knowledge, which has a tendency to 'dumb-down' the game and yes that is my opinion only.

 

[Pretty Mouth]

This is not a comment specific to this situation more by way of observation.
I am certainly seeing a trend where the default position becomes 'fit an RCD' if ever a Zs reading exceeds a limit number.
Other checks / tests are seldom carried out... "sorry sir if it is not on the schedule as required it is not being done". No other calculations performed against the expected 'as-installed' figures to identify how [we] arrived at this position requiring this approach for fault protection within ADS.
BS7671 is the minimum requirement, the floor not the ceiling. RCD blinding by appliances plugged in... "it'll be fine".
Good design is a requirement of an Electrical Installation so is it designed that way or is it more it ended up that way? If so why?

Human nature often defaults to the easiest path, the easirst path becomes the preferred path. Such a path well walked requires little effort, little effort requires little knowledge, which has a tendency to 'dumb-down' the game and yes that is my opinion only.

This is a good point, however I think it conflates different issues which, while there may be some overlap in some cases, should be looked at separately. Often circuits can have a Zs well within the limits for the overcurrent device yet still have problems with loose connections etc. Likewise, some circuits can exceed the Zs for the OCPD yet be perfectly healthy. Loose connections should be checked for regardless of Zs.

 

[JXYM89]

I would add:
Where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.

Thanks for the heads up. Please could you help unpack that some more for me or provide a reference for further reading.

 

[newfutile]

Most socket outlets should be on (RCD) Residual Current Device especially if they could be used outside, the maximum ZS for the lighting should be less than 1.84 ohms for a 13 amp bs1362 fuse,though a 3 or 5 amp may be better if loads permit

 

[mainline]

Thanks for the heads up. Please could you help unpack that some more for me or provide a reference for further reading.

The Rcd doesn't provide over current protection.

 

[timhoward]

I would add:
Where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.

Thanks for the heads up. Please could you help unpack that some more for me or provide a reference for further reading.

This is one of my bug bears as it's very easily overlooked with temporary installations unless they are properly inspected and tested under BS7909. I'm happy to unpack.

I'll turn up at a site and find lots of distribution boxes strung together with long cable between them, normally starting at a 63amp socket that is part of the fixed wiring. Sometimes a generator.

(production manager) "Every distro has an RCD, it will be fine"....
(me) "We'll see"

The most recent case was a distro box near the end of a chain that had a Zs of 0.99 ohms and a Line to Neutral loop impedance of 1.14 ohms.
The protective device was a C32 MCB.

So as a tester I'm mainly bothered about 3 things:
1 - line to earth faults clearing
2 - line to neutral faults clearing
3 - loading and volt drop

1 - The Zs (0.99 ohms) exceeds 0.68 ohms which is the max Zs of a C32 MCB for ADS to occur using the MCB. So 'normal' ADS is out of the window. But there are multiple RCD's between here and the supply, and these will take care of line-earth fault protection. Not first choice, but safe.

2 - L-N loop impedance is 1.14 ohms. The prospective short circuit current (PSCC) is 230/1.14 = 202 amps. Can you see the problem with this, using the graph at the back of the regs book:

(Note it isn't a given that the nominal voltage, 230v is actually present due to volt drop of long runs, so in reality it might be even lower fault current)

This is why BS7909 certificates usually have a specific column for PSCC:

The point @mainline is making is that an RCD doesn't do anything at all to help with line to neutral faults. If the impedance is too high will take a long time to trip the MCB and things will get toasty; about 8 seconds of toasty-ness in the example above.

Solutions here would be include changing the protective device (a B32 would be ok), finding a nearer supply point. or using higher rated cables to lower the cable resistance.

Hope that helps.
Tim

 

[JXYM89]

This is one of my bug bears as it's very easily overlooked with temporary installations unless they are properly inspected and tested under BS7909. I'm happy to unpack.

I'll turn up at a site and find lots of distribution boxes strung together with long cable between them, normally starting at a 63amp socket that is part of the fixed wiring. Sometimes a generator.

(production manager) "Every distro has an RCD, it will be fine"....
(me) "We'll see"

The most recent case was a distro box near the end of a chain that had a Zs of 0.99 ohms and a Line to Neutral loop impedance of 1.14 ohms.
The protective device was a C32 MCB.

So as a tester I'm mainly bothered about 3 things:
1 - line to earth faults clearing
2 - line to neutral faults clearing
3 - loading and volt drop

1 - The Zs (0.99 ohms) exceeds 0.68 ohms which is the max Zs of a C32 MCB for ADS to occur using the MCB. So 'normal' ADS is out of the window. But there are multiple RCD's between here and the supply, and these will take care of line-earth fault protection. Not first choice, but safe.

2 - L-N loop impedance is 1.14 ohms. The prospective short circuit current (PSCC) is 230/1.14 = 202 amps. Can you see the problem with this, using the graph at the back of the regs book:

View attachment 119310
(Note it isn't a given that the nominal voltage, 230v is actually present due to volt drop of long runs, so in reality it might be even lower fault current)

This is why BS7909 certificates usually have a specific column for PSCC:
View attachment 119311

The point @mainline is making is that an RCD doesn't do anything at all to help with line to neutral faults. If the impedance is too high will take a long time to trip the MCB and things will get toasty; about 8 seconds of toasty-ness in the example above.

Solutions here would be include changing the protective device (a B32 would be ok), finding a nearer supply point. or using higher rated cables to lower the cable resistance.

Hope that helps.
Tim

Hey Tim,

Thank you so much for taking the time to explain this, that has really helped clarify the matter for me.