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

 

[Rockingit]

Forget 'roughly', you need to deal with 'exactly'.

So - what, exactly, was your measured Zs on the circuit? And, what does the BGB say the maximum Zs for a C32 needs to be?

 

[jaydee81]

just out of interest what was your ze

 

[Rockingit]

I can foresee many potential issues with this circuit, btw, but let's start with what we know......

 

[Boaky]

The feed to the DB is 25mm 4 core. Armour used as earth
zs of this board is 0.2
what is BGB. I know I need to be exact but sometimes I not sure to trust my meter even know it's was new this year

 

[Farmelectrics]

Split the circuit turn it into 2 radials and change breakers to 20amp or re wire

 

[telectrix]

all depends on the loading of the circuit. worst can happen is the MCB will trip. bad design though. although i don't know of a reg. stipulating max. no. of sockets on a RFC, i consider any more than 15 is too many. should have been 3 circuits, IMO.

 

[SJD]

... not sure to trust my meter even know it's was new this year

Monthly check of MFT, for Zs measurement use known circuit/outlet ?

 

[Octopus]

The feed to the DB is 25mm 4 core. Armour used as earth
zs of this board is 0.2
what is BGB. I know I need to be exact but sometimes I not sure to trust my meter even know it's was new this year

BGB - Big Green Book - latest version of BS7671

 

[jaydee81]

maximum floor area for a ring is 100m2 tho can see why its not been put on a rcd with that amount of I T equipment on it

 

[Boaky]

I think the max as for a c32 MCB is 0.78 can anyone guarantee this for me

 

[Octopus]

I think the max as for a c32 MCB is 0.78 can anyone guarantee this for me

OSG or BGB?

 

[Boaky]

I think that 100m2 is old hat now though not 100% on this

 

[jaydee81]

whats the reason for it being on a c type mcb?

 

[telectrix]

I think the max as for a c32 MCB is 0.78 can anyone guarantee this for me

0.72. and that's without temp. correction. just seen that it's a C type, hence your Zs is way over.

 

[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

 

[Pete999]

My mistake r1 line r2 cpc rn neutral,

 

[Aindowsimon]

Also not getting why no rcd protection on this large skt circuit if installation is maintaind by skilled and instructed Person . Wot about all the office staff surely not all are skilled and or instructed ?? In my opinion all SO unless labelled as so for a specific purpose ie IT hub . Potentially in youse of normal folk need rcd/rcbo

 

[Guest55]

well if its an office with accessable ceiling voids and surface wiring it cant be that much work to pull in 2 extra legs to create 2 smaller ring mains ?

 

[Guest55]

Also not getting why no rcd protection on this large skt circuit if installation is maintaind by skilled and instructed Person . Wot about all the office staff surely not all are skilled and or instructed ?? In my opinion all SO unless labelled as so for a specific purpose ie IT hub . Potentially in youse of normal folk need rcd/rcbo

im guessing you dont do much commercial design then ? :-/

 

[Aindowsimon]

You guessed Well. No not an expert , il be having a little reminder session in van tomorrow, if memory serves SO's <20a for youse of normal people need rcd protection . I know my max earth loops though and this one is over