3 Meter rule?

[LastManOnline]

Should the 3 Meter electrical reg. be reconsidered.?? Surely its "un-electrical" to deem mechanical protection as a suitable substitute for proper electrical protection?

 

[IAmSparkytus!]

Which regulation are you referring to? Tails being 3m long?

 

[timhoward]

Should the 3 Meter electrical reg. be reconsidered.??

As above, do you mean the DNO 3m rule, or the stuff in reg 433.2.2 that let's you put the protective device in the run of the conductor in some circumstances, one of them being it's with 3m of the change of cross sectional area (and other additional requirements apply)?

If the latter I think it's safe enough and don't think we should start running 35 sq mm cables from a 3ph bus bar chamber to a fused spur 2m only to drop down to 2.5 or 4mm the other side! Not a lot can actually happen to cause a fault to the short length of cable in steel trunking as long as it's properly installed.

 

[Vortigern]

deem mechanical protection as a suitable substitute for proper electrical protection?

I see the two more as separate methods which can both be used for safety of Livestock and personnel. I don't think there is any suggestion one could/should substitute the other.

 

[Mikegh]

I assume you mean the tails ?

I would say there's more chance of short-circuiting the DNO Fuse at the main board than on the tails

I think I'm right in saying that UK tails are already SC/overload protected by the DNO Fuse ?

An MCB on the tails would only be there to provide discrimination between the DNO and the consumer

 

[LastManOnline]

As above, do you mean the DNO 3m rule,

Yes. And the reg in 434.3.(iv).Just a, personal view but I find it telling that the run of cable from the DNO, s cutout to the consumers main OC device is the only run of cable in a domestic installation whose length is not determined by the usual factors (volt drop, Fault loop Imp. etc). Its length is determined by the perceived risk of damage to the cable and the perceived consequences of unconventional fault protection (as I understand it). The 3 Meter length (some DNO, s stipulate 2 meters) is arbitrary not relying on a lot of science.
Would we approach any other circuit in the house like this?

(See "Efixx 3 Meter tails")

The "omission of fault protection" has undoubtly a role to play in electrical installations. My view is that a domestic installation should not be one of them

Just my thoughts.

 

[pc1966]

In domestic electrics it really only applies to the tails from DNO cut-out to the installation's OCPD (typically in the form of MCB in the CU), but more generally it also applies to connections from a busbar chamber to switch-fuses coming off that so you don't try and get 800A cable in to your 63A switch-fuse's terminals!

While sort-of arbitrary, 3m is a reasonable value for most systems where you have a short run that really has little risk of damage/short circuiting. You could argue for shorter (as over 1m or so distance and probably the cable is being routed oddly already) or for longer (as tails in trunking going up a floor is maybe OK) but as they get longer you are moving away from whatever installed Ze the DNO might have assumed for protection of their cables adiabatic limit, so you might be looking at fusing down from 100A to 80A or even less for that reason alone.

 

[Mikegh]

I must be getting my wires crossed on this

I'm assuming in relation to busbar it says you can tap off using smaller CSA cables up to 3M long


What does it say in relation to domestic tails ?

 

[pc1966]

I must be getting my wires crossed on this

I'm assuming in relation to busbar it says you can tap off using smaller CSA cables up to 3M long


What does it say in relation to domestic tails ?

Sort of.

The rule is really about cables that are not protected by a OCPD that is either appropriate for the cable or under the control of the installation designer. In the busbar case the risk and consequences of an internal fault are well controlled, for short meter tails (where the DNO fuse is probably OK) it is similar.

In fact the UK rules do allow the DNO fuse to be used if they agree to it. Realistically it is easier to add a switch-fuse.