Ring Final question - not DIY

[SUNRAY]

I gave a hand this morning with a fuse box change.

100A fused SWA feed 6x 15A feeding small machines such as pillar drill, bench grinder etc.

One circuit is a ring final using 6mm² singles E2E resistance 0.16Ω [~53m from memory] on a 45A fuse, at each of the 42 accessory points is a 60A isolator with 2 sets of singles feeding 2 DSSO's, some of these are up to 0.06Ω [~20m].

I suggested breaking it exactly in half onto 2x 32A radials

What other ideas are there?

 

[EFLImpudence]

Good memory.

Splitting it as you say seems like a good idea.


What else do you want to achieve or what problems do you perceive?

 

[EFLImpudence]

Oh, why did you say 32A?

Presumably it is in conduit, you could have two 40A - assuming it is not technically compliant at the moment.

 

[SUNRAY]

Good memory.

For some reason I remember 1+1mm² is 36mΩ/m and divide down.

Splitting it as you say seems like a good idea.
What else do you want to achieve or what problems do you perceive?
Oh, why did you say 32A?

Presumably it is in conduit, you could have two 40A - assuming it is not technically compliant at the moment.

Yes it's in conduit in an engineering training workshop with 36 benches and a few other bits on this board. Been like it for years, apparently without any issues until a few days ago when an angle grinder cable got caught on a sharp edge and 'electrocuted' [according to the w/shop manager] a trainee. He dropped it and with the grinder still running it ran across the floor and cut through another cable without blowing the fuse.
The kneejerk reaction has been to replace the fusebox with a DB with RBCO's. All good by me.
We were told 6 x 15A for machines and a ring final so went equipped with C16's and a C32 Eaton RCBO's, I suggested the split into 2 as a simple improvement and by absolute chance have a brand new one in the car but the boss preferred to return tomorrow with a C45.

Somehow a single 45A ring feels wrong for the situation, I'm sure it's technically safe but just feels wrong.

TBH I'm surprised it's not equipped with 110.

 

[plugwash]

How big are the singles from the isolators to the sockets? still 6mm?

 

[EFLImpudence]

Somehow a single 45A ring feels wrong for the situation, I'm sure it's technically safe but just feels wrong.

I have no doubt it is safe, so I suppose it must be 'allowed'.

However, a 6mm² single in conduit has a CCC of 41A so with 45A OPD, is it technically compliant?

Perhaps that is why someone made it a ring - but why not just two 40A radials in the first place?

 

[SUNRAY]

The wires and accessories are in perfect working order, there is no doubt at all of the safety of the installation. I hadn't considered the CCC. It's actually 7/0.044" in trunking which is more like 7mm² but the boss is too young to be familiar with it and insists on calling it 6mil in fact he started calling it 10mil as it's that sort of size. It's been there too long to question the original installation, If I were to do that i'd start with the whole layout, this part of the workshop is basically 3 rooms formed by angle iron frames to support the gas, water, air and electric services, black, white and notice boards etc but they're actually run around the middle section shown red. The brown, orange and green show the run between the isolators and sockets for the outside benches, personally I'd have installed 2 rings around the 2 outer sections to avoid those long runs and have the isolator for each bench beside the bench just like the middle rows. It would have been easier and use a lot less wire.
But that's all lost in history.

 

[SUNRAY]

How big are the singles from the isolators to the sockets? still 6mm?

Yes it's all 6mm² - actually 7/0.044" however each spur feeds only one DSSO so by todays standards 2.5mm² would be permissable for at least 2/3 of the spurs.

 

[SUNRAY]

The wires and accessories are in perfect working order, there is no doubt at all of the safety of the installation. I hadn't considered the CCC. It's actually 7/0.044" in trunking which is more like 7mm² but the boss is too young to be familiar with it and insists on calling it 6mil in fact he started calling it 10mil as it's that sort of size. It's been there too long to question the original installation, If I were to do that i'd start with the whole layout, this part of the workshop is basically 3 rooms formed by angle iron frames to support the gas, water, air and electric services, black, white and notice boards etc but they're actually run around the middle section shown red. The brown, orange and green show the run between the isolators and sockets for the outside benches, personally I'd have installed 2 rings around the 2 outer sections to avoid those long runs and have the isolator for each bench beside the bench just like the middle rows. It would have been easier and use a lot less wire.
But that's all lost in history.
View attachment 205834

Oh my goodness, This particular boss just phoned: "Sunray, Can you rember this job?" from 5 years ago

He explained the requirement is to change the majority of the sockets to 110V.
I forwarded this post to him and explained the position of isolators. Knowing it;s 7/0.044" I suggested changing the ring to 110V using locally protected yellow sockets rather than fully rewiring back to an additional DB and adding a new ring/radials for the few 230V sockets as required.

 

[plugwash]

Somehow a single 45A ring feels wrong for the situation, I'm sure it's technically safe but just feels wrong.

From an "electrical principles" point of view if rings with 20A cable and 32A breakers are OK, and 13A sockets on 50A cooker circuits are OK then it's hard to see how a ring with 40A cable and a 45A (or even 50A which seems more common nowadays) breaker would not be ok.

He explained the requirement is to change the majority of the sockets to 110V.
I forwarded this post to him and explained the position of isolators. Knowing it;s 7/0.044" I suggested changing the ring to 110V using locally protected yellow sockets rather than fully rewiring back to an additional DB and adding a new ring/radials for the few 230V sockets as required.

If it hasn't already been done, then it sounds like an assessment of power requirements is needed, remembering that at 110V you will need roughly double the current for a given power requirement as at 230V.

 

[SUNRAY]

From an "electrical principles" point of view if rings with 20A cable and 32A breakers are OK, and 13A sockets on 50A cooker circuits are OK then it's hard to see how a ring with 40A cable and a 45A (or even 50A which seems more common nowadays) breaker would not be ok.


If it hasn't already been done, then it sounds like an assessment of power requirements is needed, remembering that at 110V you will need roughly double the current for a given power requirement as at 230V.

It hasn't been done yet, he asked for info before going for the survey as I tend to make lots of notes and he had vague recollection. Unless major changes have taken place the power demand is fairly nominal. This is a significant engineering workshop where most work is on lathes, milling machines etc etc and the 'hand work' forms a small part. If there is ever more than half a dozen in those spaces I'd be surprised.
Years ago there would have been a hoard of apprentices filling (and filing) much but...
I think he is erring towards a 10KVA 3ph site transformer (Which I feel would be overkill) and running individual circuits to each bench, however the initial suggestion was a yellow brick on each bench.

 

[JohnW2]

From an "electrical principles" point of view if rings with 20A cable and 32A breakers are OK ....

They are considered to be 'OK' because, with the required proper circuit design (sockets location etc.) it is considered very unlikely that the current in any part of the cable of a 32A ring will exceed 20A for long periods, and ....

.... and 13A sockets on 50A cooker circuits are OK

The socket and anything downstream of it will be adequately protected by the 13A (or smaller) fuse in the plug. Whether the cable of the circuit is adequately protected by a 50A OPD depends upon its CSA - so the acceptability of a 13A socket is not very relevant in this situation. However ...

then it's hard to see how a ring with 40A cable and a 45A (or even 50A which seems more common nowadays) breaker would not be ok.

Indeed. As per the first consideration above, given sensible placement of the sockets it would be very unlikley that current in any part of the cable of a 45A (or 50A) ring would exceed 40A for long periods. In fact, even a 63A ring with 40A cable would be slightly 'better' (pro rata) than a 32A ring with 20A cable!

 

[plugwash]

The socket and anything downstream of it will be adequately protected by the 13A (or smaller) fuse in the plug - so the acceptability of a 13A socket is not very relevant in this situation.

I don't know if it's explicitly addressed in BS7671, but IMO it is reasonable to consider "what upstream protection is acceptable/reasonable for a 13A socket".

BS1362 fuses seem to have a breaking capacity of 6KA, but BS646 fuses (allowed by BS1363 for adapters, used in shaver adapters to this day, were also used historically in some BS1363 to BS546 adapters) only seem to have have a breaking capacity of 1KA and who knows what breaking capacity the internal protection in wall-warts has.

AIUI you ideally want the breaking capacity of a protective device to be higher than the prospective fault current, but if that is not practical then you at least want it to be higher than the "fast-trip" current of the next protective device "upstream". A C50 has a fast-trip current of 500A, about half the breaking capacity of a BS 646 fuse.

(breaking capacities from https://www.farnell.com/datasheets/58287.pdf I don't know for sure if they are required by the standards or just what this manufacturer specifies).