Too large a ring main

[Boaky]

I recently tested a new build (mostly new build) and found 45 sockets on a large ring main in an office environment.
My r1 was 1.6 ohms rn 1.6 ohms and r2 was 2.7 ohms which made my r1+r2 1 ohm roughly
my zs on the circuit was also 1 ohm although I didn't test all of them.
the protective device is a c32 MCB, common sense tells me this is wrong but can anyone tell me why and how to explain it to a spark who has done this twice aslong as me.
No Rcd needed as it's a managed building and all surface work although I would of used a RCBO

 

[Farmelectrics]

Don't forget to apply 80% value if out of the reqs

 

[SJD]

For r1=1.6, r2 = 2.7, then (r1+r2) = 1.08 Ohm.
If Zs at DB origin = 0.2 Ohm, then Zs for RFC = 1.28 Ohm.
From table B6 in OSG, max permitted Zs for C32 = 0.58 Ohm (the derated value).

I think if you split to 2x 20A radials, you'll exceed max Zs for C20 of 0.93 Ohm

 

[HandySparks]

Split the circuit turn it into 2 radials and change breakers to 20amp...

This might increase the maximum load capacity, but this won't sort the Zs issue.

 

[jaydee81]

whats the circuit supplying could you not fit b 32

 

[Boaky]

Thanks for all the great answers
i originally told them 3 ring mains was the answer but when they questioned me I couldn't really back myself up.
Would a RCBO solve the problem as the max zs would then become 1666

 

[Lucien Nunes]

Problem is, in office environment, total leakage might be too high either for single RCD/RCBO or for a circuit without high integrity CPC. Even if you can get the breaker to stay in, it's bad design.

 

[telectrix]

no. that's the cowboy solution. you might just get away with 2 radials on B20 or B16 MCBs.

 

[HandySparks]

For r1=1.6, r2 = 2.7, then (r1+r2) = 1.08 Ohm.
If Zs at DB origin = 0.2 Ohm, then Zs for RFC = 1.28 Ohm.
From table B6 in OSG, max permitted Zs for C32 = 0.58 Ohm (the derated value).

I think if you split to 2x 20A radials, you'll exceed max Zs for C20 of 0.93 Ohm

Indeed, if you split it in the middle, your (R1+R2) will double. If split asymmetrically, one radial will be better than that, and one worse.

 

[Pete999]

Look on youtube "testing a ring circuit" cant understand your r1,rn,r2 r1 is the resistance of the line conductor r2 the resistance of the neutral conductor, in a RFC wired in 2.5mm singles the R1+R2 reading should be approx 0.5 of your r1 reading, I think you may be testing your RFC incorrectly

 

[SJD]

An alternative way to reduce the Zs might be to provide some additional path(s) to earth (i.e. supplementary bonding), if the installation lends itself to that?

 

[jaydee81]

am i missing something from the original post the measured zs was 1 ohm max zs for b 32 is 1.15 ohms ?

 

[Boaky]

Pete r2 is the resistance of the cpc

 

[telectrix]

am i missing something from the original post the measured zs was 1 ohm max zs for b 32 is 1.15 ohms ?

it's on a C32.

 

[SJD]

Look on youtube "testing a ring circuit" cant understand your r1,rn,r2 r1 is the resistance of the line conductor r2 the resistance of the neutral conductor, in a RFC wired in 2.5mm singles the R1+R2 reading should be approx 0.5 of your r1 reading, I think you may be testing your RFC incorrectly

In 2.5mm T&E with 1.5mm CPC, then r2 (for CPC, not neutral) is approx. 1.67x r1.

 

[jaydee81]

i know but why cant it be changed to a b type