DB to be located away from DNO equipment

[IAmSparkytus!]

Evening and happy Friday!

Does anyone regularly use or know of a suitable way to relocate a DB from its original position (adjacent to main fuse) if there is nowhere within 3m of the main fuse inside the building where a fused isolator can be fitted?

Main fuse is in an external meter cabinet, current DB position is on the other side of the wall, new DB position is to be 10+ metres away.

Is there a surface mounted external box which can be fitted in which a fused isolator can be installed inside?

Is there another way of fitting it internally which is still acceptable?

Thanks.

 

[telectrix]

can yo not fit a KMF in the meter cupboard?

 

[Rockingit]

You can usually squeeze a KMF into the meter box

 

[Dartlec]

If there's no space or the DNO are likely to be an issue, you can get external meter boxes which could fit a KMF in - I used these people a few years back when there wasn't space (3 phase head in the main box) Meter Boxes | Emiter Ltd - https://www.emiter.co.uk/products/meter-boxes/

 

[IAmSparkytus!]

There are 2 x meter setups inside the meter cabinet so no room for much else!

Looking like a external surface mounted meter cupboard could be the way forward.

Any harm in drilling through the back of the meter cabinet and through the wall, then bringing the tails through the wall, into trunking taking them up into a suspended ceiling and fitting the 80/100A fused switch up there?

Would need to be a decent sized metal switch though to take a 25mm SWA away me thinks.

 

[Rockingit]

Why 25mm? Put an 80A fuse in the KMF and run 16mm

 

[telectrix]

There are 2 x meter setups inside the meter cabinet so no room for much else!

Looking like a external surface mounted meter cupboard could be the way forward.

Any harm in drilling through the back of the meter cabinet and through the wall, then bringing the tails through the wall, into trunking taking them up into a suspended ceiling and fitting the 80/100A fused switch up there?

Would need to be a decent sized metal switch though to take a 25mm SWA away me thinks.

why SWA if in trunking?

 

[IAmSparkytus!]

Why 25mm? Put an 80A fuse in the KMF and run 16mm

100a main fuse existing, didn't want to downgrade

 

[IAmSparkytus!]

why SWA if in trunking?

Only intend on putting tails from meter cupboard up into ceiling in trunking, not the swa away from KMF

 

[telectrix]

if tails are> 3m then fusing down to 80A makes sense, protecting the DNO fuse. have you opened it to check that it is what it says on the tin?

 

[richy3333]

100a main fuse existing, didn't want to downgrade

You’ll have no choice to achieve selectivity.

 

[Rockingit]

100a main fuse existing, didn't want to downgrade

Is this a domestic?

 

[Zerax]

I usually drop the downstream OCPD to 63A for a 100A main fuse. I think 80A is too close to ensure that this will blow first. I thought there was something in the regs about this, but I can't find it with a quick flick through... maybe someone with a soft copy can find it ?

 

[IAmSparkytus!]

if tails are> 3m then fusing down to 80A makes sense, protecting the DNO fuse. have you opened it to check that it is what it says on the tin?

Yes, 100A fuse in place.

 

[IAmSparkytus!]

Is this a domestic?

Commercial, 60A will be touch and go whether it blows for all that is to be installed.

 

[pc1966]

For BS88 fuses you typically need a 1.6:1 ratio for selectivity, so basically two "steps" in the rating (e.g. 63A and 100A, or 50A and 80A, etc).

However, here the only reason for putting in the fuse is to satisfy the regulations that say it must have its own OCPD if over 3m unless the DNO agrees to their fuse being used (assuming the SWA is OK for the DNO fuse rating obviously).

So you don't need selectivity here. You are not running a supply with multiple circuits off it so they need to be able to disconnect independently, and you are not running any sort of load that can be expected to blow it (e.g. some massive commercial motor, etc).

 

[pc1966]

See reg 434.3 (iv)

The 3m value is mentioned in 433.2.2 and in 434.2.1 (i)

 

[IAmSparkytus!]

"So you don't need selectivity here. You are not running a supply with multiple circuits off it so they need to be able to disconnect independently"

What do you mean by this? Don't know if im taking what you're saying too literally but the supply will be feeding a DB with multiple circuits off it?

With Telectrix asking why SWA if ran in trunking, is there a not a limit to how far meter tails can be ran if enclosed in trunking? This would probably be easier than trying to find a KMF that will allow for a 16mm/25mm SWA to be glanced into it in a tight spot?

I've seen it before where tails are ran over a distance of 10-15m above a ceiling but running single cores over that distance doesn't seem very well protected.

 

[pc1966]

What do you mean by this? Don't know if im taking what you're saying too literally but the supply will be feeding a DB with multiple circuits off it?

I mean you are not taking multiple fused sub-mains off the DNO fuse (not a Ryefield board or similar where you would expect selectivity between multiple supplies).

The CU with multiple circuits is very unlikely to blow the DNO fuse. Typically you have assessed the expected total demand allowing for diversity and real world use. So while the sum of MCBs probably far exceeds 100A, many will never be run at maximum (e.g. lights at 6A per floor, etc), and others like RFC are rarely running at their limit.

Without the sub-main you would not be putting in a fuse to protect the DNO's fuse, so with a single sub-main simply extending it then the same surely applies?

 

[pc1966]

With Telectrix asking why SWA if ran in trunking, is there a not a limit to how far meter tails can be ran if enclosed in trunking? This would probably be easier than trying to find a KMF that will allow for a 16mm/25mm SWA to be glanced into it in a tight spot?

If the tails are mechanically protected and so unlikely to be damaged by DIY nailing of stuff to walls, etc, then you don't need SWA.

Often on here you see questions about long tails with just an isolator and not a switched-fuse, and it comes back to the same "3m rule" and if the DNO fuse can be considered acceptable OCPD for the installation.