Zs of circuit higher than maximum values allowed?

[LittleHelper]

Hi there,
I have a ring circuit where the Zs reading taken is higher than maximum allowed values.
the circuit is protected by a 32a RCBO type C device, Zs = 1.43.
I have currently have tightened terminations which has brought it to its current value.

as i have been out of the game for a while i wondered if the rcbo can overide maximum values is sufficient Zs is not achieved.

many thanks in advance!!

 

[NDG Elecs]

Even if you changed it to type B you would still be 0.33 ohms out or thereabouts. Why is it type C? What is the Ze and earthing arrangement? A possible solution may be splitting the RFC into two radials on 16/20A MCBs, depending on the R1R2s and consequent Zs's when the cct is split.

I personally would not be overly happy relying solely on the RCD for fault protection if there is a suitable work around. Better to have the RCD as additional protection.

 

[Flanders]

I have come across this quite a lot even in new installation that have been designed using Type C RCBOs although Zs results have been more around 0.80 ohms an although as the Gay owl suggest its not a good idea just to relay on RCD protection to disconnect the supply in the event of a fault I have been guided if carrying out an ECIR to code 3 this is as long as the RCD is found to be operating corrrectly , It sound like the circuit in your case as been poorly designed and may have other issues such as a high volt drop so you may need to be looking at a 20 amp type B RCBO

 

[Amp David]

Is this a measured or calculated value?

 

[Richard Burns]

You have to look at the design of the circuit and see if the values you have measured are in the right sort of range for that circuit.
If it is a ring circuit with only six sockets over a 10m ring then your Zs should be 0.05Ω greater than your Ze, but a 100m ring should be 0.5 plus your Ze, by pure calculation.
With all the connections and so on you can expect a reasonable variation from this but only by a few tenths of an ohm.
If your circuit should be OK then it is time to look for the fault, resistance in the RCBO, loose connections, etc.

If the ring has been installed such that the Zs would be exceeded by calculation then this is a badly designed ring and it would be preferred to resolve this bad design rather than relying on the RCD EFLI limits.

 

[Octopus]

What are your end to end readings for the ring?

 

[LittleHelper]

Hi guys thanks for your help and input.
I believe the circuit may be too long hence the values ill be in the apartment tommorow and will post end to ends of the ring.
so far i have only done calculated readings as i did a QA of the install before walls were put so have verified the ring visually.
What i will do is go around again tommorow and tighten all terminations as i believe i could do it better. the sockets are module type where you have to link out the modules so quite a few terminations. There are about 12 sockets on the ring including floor boxes. Hopefully i can get r1+r2 towards or beneth 1.20ohms which will allow a type B Rcbo provided i can get 1.80ohms or lower L-L and 3.00 ohms or lower cpc.

 

[Octopus]

My crystal ball sees good end to end readings so the high zs being caused by the rcbo

 

[NDG Elecs]

Or a carp Ze possibly.

Twelve sockets on a RFC is not that many, although there is a good few terminations as you say. Don't forget bonding will very likely bring the actual recorded Zs down lower. Need to get testing.

 

[Brightspark2]

If it's only calculated values then wait till you take measured values, you'll probably find its lower.

Sent from my E5823 using Tapatalk

 

[TJ Anderson]

Take a Zs on the busbar terminal of RCBO and one on the L Load of RCBO and you will see what it is adding. If it is the unit adding it to your reading then make a note on cert and put 1667 as maximum Zs for that circuit only.