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