Meter tails over 3 meters

[haptism]

Hello, anyone know when the regs recommending a switch fuse for meter tails greater than 3m long was introduced ? On older installations (about 20 years ago) was a switch fuse required on tails over 3m ?

Thanks

 

[SparkyChick]

I don't believe there are any regs about this (certainly nothing in BS 7671). It's based on guidance from DNOs who insist on the switch fuse because they are not prepared to guarantee that their fuse will protect the tails under fault conditions.

 

[haptism]

Thanks SP, i read that the DNO base the 3m rule on an assessment of risk (ie its less likely to be drilled through if the length is shorter) and not on any mathematical equations. Im sure there is something in the regs relating to this. I looked at an older install earlier, the tails are about 4m without a SF. If I were to quote for a new CU, should I (need to) install a switch fuse on the tails ?

 

[Midwest]

Whats a metre between friends?

 

[telectrix]

Whats a metre between friends?

about 3 ft 3 inches.

 

[Midwest]

about 3 ft 3 inches.

Is that wot you tell her indoors

 

[telectrix]

i never brag. once had an accident with some farm machinery. left me with a 10" stump.

 

[Spoon]

3 meters is mentioned in the regs in sections 433.2.2 & 434.2.1.

 

[haptism]

Im thinking that as the (regs) are not retrospective, then if this install was completed before the 3m rule then it does not really need to be upgraded, but dont know when this reg (from IEE or DNO) was introduced. Im probably over thinking this but like to be sure, to be sure.

 

[Pete999]

Thanks SP, i read that the DNO base the 3m rule on an assessment of risk (ie its less likely to be drilled through if the length is shorter) and not on any mathematical equations. Im sure there is something in the regs relating to this. I looked at an older install earlier, the tails are about 4m without a SF. If I were to quote for a new CU, should I (need to) install a switch fuse on the tails ?

Deleted

 

[Midwest]

3 meters is mentioned in the regs in sections 433.2.2 & 434.2.1.

Isn't that applicable where a change occurs (e.g. csa, type of cable etc) in the installation?

 

[haptism]

433.2.2 concerns devices for protection against overload, however the next one (433.3.1 [iii]) concerns omission of devices for protection against overload, and says a device can be omitted "where the suppliers fuse affords protection to the part of the installation between the origin and the main distribution point of the installation where overload protection is provided" (The CU in this case). So that rules that out. Looks like it is the DNO rules regarding tail length

 

[SparkyChick]

Personally, I think I would install one if I were doing a board upgrade, the reason being they are typically after the meter which makes them the owners responsibility and since I'd be the one signing off, I'd want everything just so.

If the tails are already in place, and there is no isolator, then I'd get the switch fuse, install it at a suitable location (in the meter cupboard if you can) and have a kindly DNO engineer out to pull the fuse. If your DNO engineers are anything like WPD, have them pull the fuse, you then terminate the tails in the new shiny switch fuse, and they then re-energise. WPD charge per visit for pulling and re-inserting the fuse so if I have to have them out I line up an isolator before they get there so the client only gets one callout charge

 

[pirate]

3 metres over 3 meters
3 meters over 3 metres
3 meters over 3 meters

how many meters?
how many metres?

You guys just love to confuse an old pirate...
I'm used to fathoms!
(6', or not quite 2 metres...)

 

[haptism]

Think I will give the DNO a call

 

[kenny7askew]

if there is not much in it i wouldn't worry too much about it,i have a"slightly out of calibration tape measure" that comes in handy for things like that

 

[buzzlightyear]

Think I will give the DNO a call

computer says 3mtr max .

 

[Deleted member 26818]

433.2.2 concerns devices for protection against overload, however the next one (433.3.1 [iii]) concerns omission of devices for protection against overload, and says a device can be omitted "where the suppliers fuse affords protection to the part of the installation between the origin and the main distribution point of the installation where overload protection is provided" (The CU in this case). So that rules that out. Looks like it is the DNO rules regarding tail length

Only if the DNO agrees their fuse can be used.

 

[johnduffell]

I thought it was so the dno can guarantee the load is removed before pulling the fuse to isolate. If the trails go round the house then Henley blocked to an outbuilding or two they won't have a clue how to safely isolate.
I once calculated the clearance time of a 100a fuse on a 0.8 ohm efli (tn s) and off hand I've a feeling it was not quick enough, which would imply if it's bs7671 the 3m rule for omission of fault protection would apply.
Never mind the protection on the incoming supply, that's more than 3m long!

 

[Dave OCD]

Distribution rules and regs are very different , in street lighting it was common for a 16mm concentric service cable to be jointed straight off a 95mm or larger main. Quite legitimately though !