Zs on ring main high

[cliffed]

NICEIC visit the other day… pulled up on length of RM circuit …. Zs 1.30 ohms… RCBO 32amp B type…thoughts on this

 

[pc1966]

Anything odd about the end-end values? I.e. high r2 w.r.t. ratio from r1 or rN, or indicating a very long length?

If it is RCD protected and just an unfortunate combination of high, but reasonable, TN-S case Ze and long, but sane from VD point of view, RFC installation then it is not an installation fault and still meets ADS.

I might be tempted to put in 20A RCBO or whatever if RCD old and unlikely to get much client testing.

 

[Dustydazzler]

ZS above 1 ohm is quite high for a domestic Ring main , what was the Ze ?

 

[Pretty Mouth]

If the circuit was too long (>106m for a 2.5mm2 ring), it might not meet voltage drop requirements.

If the circuit had loose connections, then this would be a problem for obvious reasons.

If the Ze was unusually high, then this would indicate a problem with the installation earthing.

However, if the only issue is that the Zs exceeds that for the OCPD, then there is no problem. It's a compliant circuit, as the disconnection times are met by the RCD.

 

[westward10]

My thoughts are you should give us more info @cliffed

 

[cliffed]

Circuit too long according to OSG…resistance’s on conductors high due to length… how many RM are going to be over the length OSG…suggest’s….

 

[timhoward]

Was it a circuit you completely installed or did you extend it?

We're trying to work out if it is high due to sub-optimal design (and everything terminated correctly etc.) or high due to mis-adventure and the DIY pixies having played.
If you tell us the breakdown of the tests, Ze, r1, rn, r2, R1+R2 it would help.

 

[pc1966]

Circuit too long according to OSG…resistance’s on conductors high due to length… how many RM are going to be over the length OSG…suggest’s….

A bit more info as requested by @timhoward would help.

Dropping the OCPD to 25A is a "fix" only in the sense the Zs, and (probably) VD figure allowed, become consistent with the circuit implemented. But it is not really satisfactory as such a long RFC suggests it is something like a whole house on one circuit, and so total load would be high and might make 25A trip-prone.

Is there any point where you could break the RFC and link up two more cables as legs to make two RFCs? Taking great care of course not to have them cross-linked!

 

[cliffed]

Was it a circuit you completely installed or did you extend it?

We're trying to work out if it is high due to sub-optimal design (and everything terminated correctly etc.) or high due to mis-adventure and the DIY pixies having played.
If you tell us the breakdown of the tests, Ze, r1, rn, r2, R1+R2 it would help.

ze 0.34 R1 1.12 Rn 1.13 R1+ R2 0.76zs1.00…r2 1.96

 

[westward10]

What is your r2?

 

[cliffed]

A bit more info as requested by @timhoward would help.

Dropping the OCPD to 25A is a "fix" only in the sense the Zs, and (probably) VD figure allowed, become consistent with the circuit implemented. But it is not really satisfactory as such a long RFC suggests it is something like a whole house on one circuit, and so total load would be high and might make 25A trip-prone.

Is there any point where you could break the RFC and link up two more cables as legs to make two RFCs? Taking great care of course not to have them cross-linked!

There are 3 RM in the house… this RM travels a distance from the DB before looping into the power points….in most cases being RCD protected this would not be flagged up

 

[cliffed]

What is your r2?

soz r2 1.96 R1+R2 0.76

 

[Pretty Mouth]

r2 ever-so slightly high, although not by much really. Otherwise too long for theoretical voltage drop limits then. What is this circuit likely to power?

 

[timhoward]

What are people's thoughts on adding a parallel 4mm separate CPC to the first point in each direction? Mathematically that would probably get the Zs under the bar. Do regs allow that?

 

[Pretty Mouth]

What are people's thoughts on adding a parallel 4mm separate CPC to the first point in each direction? Mathematically that would probably get the Zs under the bar. Do regs allow that?

I think the Zs is well under the permitted 1667 ohms as it is

 

[timhoward]

I think the Zs is well under the permitted 1667 ohms as it is

I know what you mean, but this is NICEIC.....we can't expect them to know that.

 

[cliffed]

r2 ever-so slightly high, although not by much really. Otherwise too long for theoretical voltage drop limits then. What is this circuit likely to power? Table lights, TV, Chargers normal house equipment

 

[cliffed]

I know what you mean, but this is NICEIC.....we can't expect them to know that.

Exactly the inspector actually calculated the length by the resistances…we can all do that…under normal circumstances

 

[Pretty Mouth]

I know what you mean, but this is NICEIC.....we can't expect them to know that.

I'm so glad I'm not registered, I don't have to go through any of this

 

[cliffed]

What are people's thoughts on adding a parallel 4mm separate CPC to the first point in each direction? Mathematically that would probably get the Zs under the bar. Do regs allow that?

Not really possible now… not too sure on the outcome on this one… waiting for Niceic response

 

[pc1966]

There are 3 RM in the house… this RM travels a distance from the DB before looping into the power points….in most cases being RCD protected this would not be flagged up

It looks like its about 150m long.

Are the legs routed such that you could potentially replace them with 4mm to reduce both VD and Zs to get past RFC design goals?

 

[cliffed]

It looks like its about 150m long.

Are the legs routed such that you could potentially replace them with 4mm to reduce both VD and Zs to get past RFC design goals?

No it was a barn conversion only a grd floor building…High ceilings etc

 

[Pretty Mouth]

No it was a barn conversion only a grd floor building…High ceilings etc

We may be able to make this circuit compliant with the minimum of fuss.

What is this circuit likely to power? Is it kitchen, utility, heaters? Or just general purpose sockets?

Also: What is the reference method for the cable?

 

[DPG]

We may be able to make this circuit compliant with the minimum of fuss.

What is this circuit likely to power? Is it kitchen, utility, heaters? Or just general purpose sockets?

Also: What is the reference method for the cable?

I think you're thinking the same as me.

 

[westward10]

What did the design on the back of the cigarette box say.

 

[cliffed]

We may be able to make this circuit compliant with the minimum of fuss.

What is this circuit likely to power? Is it kitchen, utility, heaters? Or just general purpose sockets?

Also: What is the reference method for the

Lounge area general Power Points… ref method C

 

[Pretty Mouth]

Lounge area general Power Points… ref method C

Would a design current of 16A be adequate for this space?

 

[cliffed]

Would a design current of 16A be adequate for this space?

I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…

 

[cliffed]

These are the Zs values I’m getting 1.00 ohms…so compliant..but not in the length suggested by OSG…

 

[DefyG]

What is the uplift value due to the RCD? (Before and after Zs values) as that can easily cause an otherwise good circuit to appear incorrect.
Although the relatively high readings (post #11) show it to be a long circuit, as others have indicated VD being a possible issue?

 

[Pretty Mouth]

I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…

Yes, the volt drop is why this circuit does not comply. And the volt drop depends on two things: 1) The circuit resistance, and 2) the design current.

The circuit resistance we can't do much about, without at least some rewiring. We don't want to do that, cos it's a tonne of hassle, so let's try and change the design current:

The standard 2.5mm2 ring final from the OSG uses a design current of 26A, and is 106m long, the limiting factor being voltage drop.

Your circuit, at just over 151m in length, has a volt drop of 17.8V for a design current of 26A (calculated using the basic VD calculation, with no adjustment factors). Way over what it wants to be.

If we lower the design current for your circuit to 16A, the volt drop is just 10.9V. Here is the calculation, based on the whole 16A design current being at the furthest point, midway round the ring:

half the length of the ring X half the design current X V/A/m

76 X 8 X 0.018

= 10.9V

This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy. This will also mean the circuit Zs meets the maximum for the OCPD, just in case your assessor was grumbling about that too (which he shouldn't have been, but there you go.)

 

[timhoward]

This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy.

I agree that would be perfectly safe.
The problem I see is that it only works if the 16A is shared between 2 legs, i.e. it needs to be a ring.
If the NICEIC guy was being a pedantic pr**k. he could say that 433.1.204 specifies a 30 or 32A protective device.
I still reckon it's worth a try though.

 

[cliffed]

I agree that would be perfectly safe.
The problem I see is that it only works if the 16A is shared between 2 legs, i.e. it needs to be a ring.
If the NICEIC guy was being a pedantic pr**k. he could say that 433.1.204 specifies a 30 or 32A protective device.
I still reckon it's worth a try though.

Thanks we’ve got 28 days to adjust this circuit & yes he was more interested in the resistance on the RM …& the volt drop…

 

[pc1966]

The standard 2.5mm2 ring final from the OSG uses a design current of 26A, and is 106m long, the limiting factor being voltage drop.

I thought it was 32A as for the MCB?

This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy. This will also mean the circuit Zs meets the maximum for the OCPD, just in case your assessor was grumbling about that too (which he shouldn't have been, but there you go.)

Based on the measured r1 & rN of 1.12 & 1.13 ohms the worst case R1+RN resistance is 0.563 ohms at the mid-point, so for 5% = 11.5V drop the max current = 11.5 / 0.563 = 20.4A and so that makes 20A MCB OK. While 16A is perfectly OK, I would prefer the extra margin for diversity that 20A offers.

Zs limit for 20A B-curve is 1.75 ohms so that box is also ticked as above 1.3 ohms measured.

Recent versions of the regs all give the RFC supply OCPD as 30A fuse / 32A MCB but I have seen a few older systems with 20A rewirable fuses, I guess as the Ze was a touch too high at the time? In any case it is perfectly safe, even if not your "text book" RFC, as 20A protects the 2.5mm cable without worrying about current share between the two legs.

 

[Pretty Mouth]

I thought it was 32A as for the MCB?

You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.

Based on the measured r1 & rN of 1.12 & 1.13 ohms the worst case R1+RN resistance is 0.563 ohms at the mid-point, so for 5% = 11.5V drop the max current = 11.5 / 0.563 = 20.4A and so that makes 20A MCB OK. While 16A is perfectly OK, I would prefer the extra margin for diversity that 20A offers.

Those figures are assuming 20 deg, at 70 deg you'd only be allowed 17A.

Actually, the ring final conductors won't run at 70 deg with the worse case of the full load at the mid-point, even if the cables are installed with a CCC of 20A. There is a correction factor to allow for this (P429 of the brown book), but even with that applied, this circuit just about misses the 20A mark.

 

[pc1966]

You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.

Good point.

Those figures are assuming 20 deg, at 70 deg you'd only be allowed 17A.

Actually, the ring final conductors won't run at 70 deg with the worse case of the full load at the mid-point, even if the cables are installed with a CCC of 20A. There is a correction factor to allow for this (P429 of the brown book), but even with that applied, this circuit just about misses the 20A mark.

Interestingly most of the Table 7.1(i) are showing about 80% working load compared to OCPD, much the same as the 80% factor for cable resistance over nominal temperature range.

Still, I would put in 20A MCB and call it a 20A circuit, then 16A design load, etc.

It is not addressing what folks will try to use it for, which depends on the area it covers and expected use-case, but it becomes a circuit consistent with safe CCC, VD, and Zs all being met without dependency on RCD for ADS.

 

[ebillthekid]

A most interesting thread.
Having absorbed and agreed with most of the conclusions, I have another EICR case in point - I spoke to NAPIT some years ago about an excessively long 30A RFC within an historic church building which was fine after a great many years of service.
The tech suppt. guy suggested that if the total load usually running is quite small, (eg. a fridge, the odd little charger and occasional hoover or kettle), then nothing to worry about - based on observation and common sense; note it and give it a C3. But then NAPIT are apparently less finicky than NICEIC!

 

[HappyHippyDad]

What is your r2?

I think that's at the end... 1.96ohms

 

[Pretty Mouth]

A most interesting thread.
Having absorbed and agreed with most of the conclusions, I have another EICR case in point - I spoke to NAPIT some years ago about an excessively long 30A RFC within an historic church building which was fine after a great many years of service.
The tech suppt. guy suggested that if the total load usually running is quite small, (eg. a fridge, the odd little charger and occasional hoover or kettle), then nothing to worry about - based on observation and common sense; note it and give it a C3. But then NAPIT are apparently less finicky than NICEIC!

I come across ring finals of this sort of length too from time to time, they don't seem to be a problem. I'll be honest, I don't tend to take VD for this sort of thing too seriously, at least when working on existing circuits. The theoretical volt drop calculation is based on a nominal voltage of 230V, whereas in reality the supply voltage is usually in the 240-250V range. A large margin of error for the rare occasion that the circuit is fully loaded.

 

[cliffed]

Feedback from the NICEIC… no improvement required… basically there is a VD which is minimal.. & the RM loading for that area it serves is minimal… The equipment being used have voltage tolerances & so would not be effected by VD…. Oh I say…

 

[HappyHippyDad]

Feedback from the NICEIC… no improvement required… basically there is a VD which is minimal.. & the RM loading for that area it serves is minimal… The equipment being used have voltage tolerances & so would not be effected by VD…. Oh I say…

Eh? Common sense?

Although, I'm not sure how you will sleep at night having a Zs of 0.2ohms higher than than that allowed.