Discuss Zs on ring main high in the UK Electrical Forum area at ElectriciansForums.net
A bit more info as requested by @timhoward would help.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….
ze 0.34 R1 1.12 Rn 1.13 R1+ R2 0.76zs1.00…r2 1.96Was 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.
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 upA 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!
soz r2 1.96 R1+R2 0.76What is your r2?
I think the Zs is well under the permitted 1667 ohms as it isWhat 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 know what you mean, but this is NICEIC.....we can't expect them to know that.I think the Zs is well under the permitted 1667 ohms as it is
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
Exactly the inspector actually calculated the length by the resistances…we can all do that…under normal circumstancesI 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 thisI know what you mean, but this is NICEIC.....we can't expect them to know that.
Not really possible now… not too sure on the outcome on this one… waiting for Niceic responseWhat 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?
It looks like its about 150m long.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
No it was a barn conversion only a grd floor building…High ceilings etcIt 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
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?
Lounge area general Power Points… ref method CWe 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
Would a design current of 16A be adequate for this space?Lounge area general Power Points… ref method C
I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…Would a design current of 16A be adequate for this space?
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.I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…
I agree that would be perfectly safe.This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy.
Thanks we’ve got 28 days to adjust this circuit & yes he was more interested in the resistance on the RM …& the volt drop…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.
I thought it was 32A as for the MCB?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.
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.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.)
You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.I thought it was 32A as for the MCB?
Those figures are assuming 20 deg, at 70 deg you'd only be allowed 17A.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.
Good point.You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.
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.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.
I think that's at the end... 1.96ohmsWhat is your r2?
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.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!
Eh? Common sense?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…
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