Code 2 or 3? Opinions please.

Deleted member 9648 · Sep 16, 2017

Recent EICR at a special needs care site found a built in double oven (4.2kw) connected to a 4.0mm ring final on a 30a C50 mcb. Two other double sockets on the ring for general use and an FCU for a fridge. Interested in what code people might apply to that, no need to consider RCD or anything else, only interested in the above issue. I'll keep mum on how I coded it until some other opinions come in!

telectrix · Sep 16, 2017

C3 IMO. although not ideal, I can't see a potential danger as any overload would trip the MCB before the cable was compromised. or was it on a BS1363? in which case I'd lean towards a C2 (plug/socket overload and overheating).

Wilko · Sep 16, 2017

A bit of a puzzle ... RFC from 4mm doesn't have a text book rating (that I recall) but it does not sound overloaded. Sorry feeling dense this morning, is it a 50A C type mcb? If circuit can make dis times then C3.

andyb · Sep 16, 2017

How is the oven connected to the ring?
I'm tending to agree with Tel C3, or no code.

Richard Burns · Sep 16, 2017

I would agree with Tel on the C3, it is borderline no code.
Generally heavy consistent loads should not be placed on a ring final because of the potential to overload one leg of the ring. However an oven is not a consistent load, although 18A is fairly heavy for a single point on a ring, with 4mm² conductors one leg of the ring cannot be overloaded even with a type 2/type C hydraulic MCB @ 30A.
Assuming all other factors remain compliant I cannot see it giving rise to danger but I can see it possibly tripping the circuit breaker but no more than with any socket circuit really as there are only four (six) points of use, unless the general use sockets are often used for heating loads by the nature of the installation.
I am probably pushing the justification for a C3 but feel that even an 18A oven should be on its own circuit so would recommend improvement from a design rather than a danger point of view.

ruston · Sep 16, 2017

Richard Burns said:
I would agree with Tel on the C3, it is borderline no code.
Generally heavy consistent loads should not be placed on a ring final because of the potential to overload one leg of the ring. However an oven is not a consistent load, although 18A is fairly heavy for a single point on a ring, with 4mm² conductors one leg of the ring cannot be overloaded even with a type 2/type C hydraulic MCB @ 30A.
Assuming all other factors remain compliant I cannot see it giving rise to danger but I can see it possibly tripping the circuit breaker but no more than with any socket circuit really as there are only four (six) points of use, unless the general use sockets are often used for heating loads by the nature of the installation.
I am probably pushing the justification for a C3 but feel that even an 18A oven should be on its own circuit so would recommend improvement from a design rather than a danger point of view.

I agree the thing that would lead me to a code ; providing the oven connection is sound - given that is is a care home and the oven will be used throught the day in my experience .

Lucien Nunes · Sep 16, 2017

Agree borderline C3 - unless the installation method seriously reduces the CCC of the 4.0, and subject to the connection being effectively made

is it a 50A C type mcb

C50 is the old Crabtree MCB board model number, with tbe brown bakelite BS3871 MCBs. Hence 30A not 32A and type 2/3 not B/C.

ED17 · Sep 16, 2017

I'd give it a C3

Wilko · Sep 16, 2017

Lucien Nunes said:
C50 is the old Crabtree MCB board model number, with tbe brown bakelite BS3871 MCBs. Hence 30A not 32A and type 2/3 not B/C.

Thanks !

Wilko · Sep 16, 2017

Richard Burns said:
... with a type 2/type C hydraulic MCB @ 30A ...

Thanks !

westward10 · Sep 16, 2017

Lucien Nunes said:
Agree borderline C3 - unless the installation method seriously reduces the CCC of the 4.0, and subject to the connection being effectively made

C50 is the old Crabtree MCB board model number, with tbe brown bakelite BS3871 MCBs. Hence 30A not 32A and type 2/3 not B/C.

Crabtree C50 are either 3871 type 2 or 3871 type C. Recent new styles which are now discontinued were 60898 type C. If there is no identification on the side except BS3871, assume type C. There was never a type 3.

Lucien Nunes · Sep 16, 2017

Interesting, it's a while since I spent any real time amongst C50 boards and I'm not sure I even realised that in the first place.

westward10 · Sep 16, 2017

Lucien Nunes said:
Interesting, it's a while since I spent any real time amongst C50 boards and I'm not sure I even realised that in the first place.

Somewhere amongst my junk I have a fax from Crabtree with all the info, have only ever seen one 60898/C in use though.

Deleted member 9648 · Sep 16, 2017

Thanks for that everyone. I applied a code 3 really just to highlight an unusual installation, I can see no danger or risk of overloading as it stands. There was a discussion over this between the site and another electrician following the issue of the report who stated it was unsafe and should have been a code 2.
The breaker is a non type marked crabtree C50 30a 3871 M3 (white lettering). For the purpose of Zs I assumed type C as worst case, as long as the Zs was under the max for that it would be under for other possible types. That was explained on the report.