3m rule

[beatboaby924y]

Can anyone clear something up for me was at a job yesterday and the meter had 25mm double insulated tails going from it to the main isolator about 500mm from it clipped direct, however the meter also had 6mm double insulated tails doubled up with the 25mms going about 2m away clipped surface into a garage splitter. Main fuses were marked as 100amp.

Spark I was with said this was ok because of the 3m rule but I thought the tails would have to be enclosed in trunking or conduit to give them some sort of mechanic protection.

Can anyone shed some light on what the 3m rule says?

 

[Simonslimline]

I believe DNO's require tails that exceed 3 metres to be protected by their own switch fuse.

Also 433.2 In BGB perhaps?

 

[beatboaby924y]

So because its under 3m this is ok to do, even though its not mechanically protected and has 100amp fuses protecting a 6mm tail?

Really didnt think it was allowed.

 

[Leesparkykent]

Have a look at Reg 434.2.1

 

[Plonker 3]

Is there a single point of isolation for the whole installation?

Also as Lee said regulation 434.2.1 needs to be complied with.

 

[GEOFF S]

Can anyone clear something up for me was at a job yesterday and the meter had 25mm double insulated tails going from it to the main isolator about 500mm from it clipped direct, however the meter also had 6mm double insulated tails doubled up with the 25mms going about 2m away clipped surface into a garage splitter. Main fuses were marked as 100amp.

Spark I was with said this was ok because of the 3m rule but I thought the tails would have to be enclosed in trunking or conduit to give them some sort of mechanic protection.

Can anyone shed some light on what the 3m rule says?

The three meter rule allows you to move the protective device to the far end.

So as long as the whole of 433.2.2 is met, and the protecvtive device provides overload, happy days.

Cheers

 

[i=p/u]

Shouldn't apply in domestic

 

[Darkwood]

The 3m is not a rule, your local DNO has the say on the tail length and this may vary as some are 2m .... 3m is just the average. It also comes down to how prone the tails are to getting damaged, in reality if you provide adequate protection and the mains is housed in its own room then tails can be much longer and seen it many times when coming out of a DNO fused splitter where your supplying multiple meters for say a complex of shops or flats etc.

 

[i=p/u]

Looking at 1 yesterday where builder wants to move from front window to front door, and he said he just gonna box it in, where Id say needs to be accessible ..

 

[Jonspark]

many a time ive seen distribution systems where dozens of fuse boards are fed from a busbar with no ocpd, some been upto 10 M away, just protected in steel trunking with no chance of the supply tails been overloaded.

 

[GEOFF S]

many a time ive seen distribution systems where dozens of fuse boards are fed from a busbar with no ocpd, some been upto 10 M away, just protected in steel trunking with no chance of the supply tails been overloaded.

This is the same reg as i quoted earlier, but ustilising part (i).

Cheers

 

[GEOFF S]

The 3m is not a rule, your local DNO has the say on the tail length and this may vary as some are 2m .... 3m is just the average. It also comes down to how prone the tails are to getting damaged, in reality if you provide adequate protection and the mains is housed in its own room then tails can be much longer and seen it many times when coming out of a DNO fused splitter where your supplying multiple meters for say a complex of shops or flats etc.

The reason for the length of meter tail is for protection purposes. From the service head the installation needs to meet BS7671, so the dno limit the length for disconnection purposes.

Cheers

 

[James sparks]

My take on it would be that as the upstream protection (DNO cutout) isn't sufficient to protect the 16mm meter tails you would need down stream protection so the 16mm tails aren't overloaded which from memory has to be within 3m as previously stated unless I'd be happy that the meter tails couldn't be overloaded. Ie a 2way fuseboard was fitted at the other end (maximum size MCBs available for it are 32),

I came across not long ago, a 400A busbar chamber with a 10way fuseboard tapped straight off in 6mm. It was under 1m away from the busbar chamber but sufficient load could have been connected to that fuseboard to overload the 6mm supply..