Regulations 433.2.2 and 433.3.1

[Pretty Mouth]

433.2.2
The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:

(i) It is protected against fault current in accordance with the requirements stated in Section 434.


433.3.1
A device for protection against overload need not be provided:

(ii) for a conductor which, because of the characteristics of the load or the supply, is not likely to carry overload current, provided that the conductor is protected against fault current in accordance with the requirements of Section 434.


Would the following theoretical situation fulfil either of the above regulations, and if so which one?

100A main cut out fuse supplies an 80A switch fuse via 25mm tails.
After 80A SF, 25mm tails to henley block, where the tails split. 25mm to main house CU, and 10mm to garage CU.

The 10mm tails to the garage CU would not be protected from overload by the 80A SF, but would they be protected if the sum of the breakers in the garage CU totalled less that the current carrying capacity of said tails?

 

[davesparks]

In my opinion no because a CU forms branches before the OCPDs and there is no restriction to the rating of those OCPDs.

 

[Strima]

You have potential for over current on the garage CU so no you can’t. If the garage was a fixed load with no risk of over current you could.

 

[Ian1981]

That regulation could apply to say a separate CU to a shower unit,10 mm tails under 3 meters feeding a shower CU , where the fixed load cannot overload the supply tails.

 

[Sintra]

You can however fit the over current protection for the garage CU beside the garage CU and not in the house.

 

[Julie.]

You have potential for over current on the garage CU so no you can’t. If the garage was a fixed load with no risk of over current you could.

This exactly.

 

[pc1966]

First concern is if the 80A fused switch is going to protect the 10mm from fault currents. A quick look in the OSG (table B5) has max Zs 0.38 ohms for that situation and 5s disconnection time so if on TN-S, or even the assumed 0.35 max Ze for TN-C-S, you might struggle meet it even before a long cable length is considered.

While the garage is not a fixed load as such, if the sum of MCBs there is less than the cable rating then surely the cable is already protected from overload?

A quick look at table 4D4A for 70C SWA has 10mm rated at 60A buried direct, and I doubt your garage is going to have more than that even if used as a house extension (say 32A ring & 6A lights = 38A so well below the CCC).
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In my opinion no because a CU forms branches before the OCPDs and there is no restriction to the rating of those OCPDs.

I guess your concern here is in the future more/different MCB could be fitted that would potentially violate the overall limitation of feed cable current?
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100A main cut out fuse supplies an 80A switch fuse via 25mm tails.
After 80A SF, 25mm tails to henley block, where the tails split. 25mm to main house CU, and 10mm to garage CU.

I presume the reason for not having a separate fused-switch for the garage 10mm (i.e. tails split with Henley blocks after isolator and before a pair of fused-switches) is overall space to fit it?

And I guess the 25mm to main house CU is also a run of SWA or similar needing fused-switch protection as over 3m in length?

 

[Baddegg]

Great question @Pretty Mouth, is it theoretical or something you come up against on an eicr or such?

 

[Pretty Mouth]

Thanks all for your comments.

@Baddegg , to me it's a purely theoretical question, inspired by a discussion on a FB group. A member wanted to supply a garage, the easiest route being to split the tails in the meter cupboard rather than taking it from the house CU which was some distance away. The 80A SF was already there protecting the tails to the house, but they concluded another SF was required in the meter cupboard to protect the tails to the garage.

Initially I thought that perhaps, as @Sintra pointed out, the SF could go in the garage, just upstream of the board. This would fulfil 433.2.2 (ignoring the requirements for fault protection for the time being, it's the overload theory that interests me)

Then it occurred to me that if the sum of all the breakers in the garage came to less than the rating of the cable, then why wouldn't it be protected from overload?

If, (for example) a 32A breaker protects an upstream cable with a CCC of 32A, why wouldn't 2 X 16A breakers do the same?
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That regulation could apply to say a separate CU to a shower unit,10 mm tails under 3 meters feeding a shower CU , where the fixed load cannot overload the supply tails.

@Ian1981 , in your example, supposing the 10mm tails were protected from fault current by the SF, would it now comply with 433.2.2? The device protecting the tails from overload being the shower breaker in the CU. No need to limit the length to just 3m?

 

[pc1966]

If, (for example) a 32A breaker protects an upstream cable with a CCC of 32A, why wouldn't 2 X 16A breakers do the same?

They would and from my own perspective they meet the requirement to protect the cable from overload.

Now @davesparks has pointed out that the garage CU is a branch before the OCPD device(s) which certainly appears to go against the regulation 433.2.2 aspect of:

"...position of the protective device has neither branch circuits nor outlets for connection of current-using equipment"

But I see the branch within a CU as rather different to a branch caused by a junction box / Henley block in the following aspect. When you have a junction in the cable you do not have immediate visibility of all of the loads, but in the CU you see immediately the total OCPD arrangement of MCBs/RCBOs.

While we like to think professionals would always determine the full extent of a circuit so its current load is known for any changes, we know also that frequently does not happen and that in some cases junctions and load outlets get hidden by subsequent building work.

Whereas in the garage CU it is always visible in its entirety, in particular if there is a label saying something like "total MCB not to exceed xxA for cable protection" then any competent, or even semi-competent, future changes will still respect the design aspect where the CU total was part of the cable overload protection.

 

[Pretty Mouth]

They would and from my own perspective they meet the requirement to protect the cable from overload.

Now @davesparks has pointed out that the garage CU is a branch before the OCPD device(s) which certainly appears to go against the regulation 433.2.2 aspect of:

"...position of the protective device has neither branch circuits nor outlets for connection of current-using equipment"

But I see the branch within a CU as rather different to a branch caused by a junction box / Henley block in the following aspect. When you have a junction in the cable you do not have immediate visibility of all of the loads, but in the CU you see immediately the total OCPD arrangement of MCBs/RCBOs.

While we like to think professionals would always determine the full extent of a circuit so its current load is known for any changes, we know also that frequently does not happen and that in some cases junctions and load outlets get hidden by subsequent building work.

Whereas in the garage CU it is always visible in its entirety, in particular if there is a label saying something like "total MCB not to exceed xxA for cable protection" then any competent, or even semi-competent, future changes will still respect the design aspect where the CU total was part of the cable overload protection.

I still can't quite decide if such an arrangement would comply or not. Which I suppose, in the real world, doesn't matter as it would be very easy to ensure it does. Thanks again for the comments

 

[Baddegg]

I still can't quite decide if such an arrangement would comply or not. Which I suppose, in the real world, doesn't matter as it would be very easy to ensure it does. Thanks again for the comments

I was always taught in these situations to image the worst had happened and you were stood in court explaining your design decisions to a judge and an expert.......sharpens your decisions a bit ?

 

[pc1966]

There is also a distinction between the "Is this safe?" sort of question for EICR coding and "Is this acceptable design practice?" where you are planning on a system and (at least hopefully!) that is a higher standard to be met.

So in this case if you found the cable protected by the sum-of-MCB being within range it is clearly not going to overload and so is safe. But equally if you were desiging it and following best practice you would aim for your cable protection to be at the feed end of the cable if at all possible, as not only do you guarantee overload protection but it is likely going to be a better match for fault/short protection as well given the likely end of cable Zs range.