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.