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?