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.

 

[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.

 

[IAmSparkytus!]

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?

This is my opinion on why you shouldn't need to downgrade the fuse.

 

[IAmSparkytus!]

Thanks for your help and advice pc1966. Have a good weekend ?

 

[pc1966]

And you!

 

[pc1966]

I guess I should point out for anyone reading that there might be other reasons to fuse down at the supply end:

• If 16mm SWA (or tails) are used for ease-of-wrangling reasons so not overload protected at 100A
• If the end of sub-main Zs is too high to allow 5s disconnection on a fault

To check if Zs is the issue, the OSG Table B5 has max Zs of 0.28 ohm for 100A BS1361 through to 0.53 ohm at 60A, though here a delay RCD of 300mA or similar would be an alternative to a smaller fuse if the CU is fully RCD/RCBO.

 

[bigspark17]

16mm ho7. Kmf with 80A fuse.

why are there 2 meters in the external box? Sounds like its a redundant meter or the old ec7 switch over. Id be doing my best NOT to have another external box.