Cable protection

[John Rumm]

A bit high tech for him I fear - he is more comfortable with a hammer! (which to be fair he does wield with amazing skill - the nearest equivalent to a human nail gun you are likely to find!)

[Graham Jones]

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Thanks John,

I've actually had a rethink about this and due to the thickness (or should that be thinness) of the stud wall I would really like to protect the cable from damage from the other side.

So I had a think about this SWA stuff and came up with:

1) Split the ring circuit under the floorboards with a junction box. 2) Run a short length of SWA from the junction box up the stud wall to the socket. This would be just one length of SWA, sort of like a spur from the junction box.

This I think solves the problem very nicely, I only need about 1 meter of SWA so it will cost very little.

As the maximum load on the socket is 13A, then a 1.5mm cable should suffice. Well according to tlc-direct cable calculator anyway.

There may be practical problems I don't know about, I'm assuming the shielding on the cable needs to be earthed, whether this is possible in the above scenario I don't know.

Graham

[ARWadsworth]

No. It would have to be 2.5mm. Don't forget, you have a 32A MCB for protection not a 13A fuse.

It would be very difficult to use SWA in the way you are suggesting. The glands would need to be accesible for inspection and testing.

How thick (or thin) is the stud wall? Why not use normal 2.5 T&E and if you are worried about damage from behind then install a vertical baton up to back box and clip the cable to that so that the cable is just behind the plasterboard on the socket side of the wall. The cable will then almost certainly be >50mm from the surface on the other side.

Cheers

[Graham Jones]

I like you idea about the baton, this will almost certainly ensure that the cable is >50mm from the other side.

With regards to the cable size then I must admit I don't fully understand. I understand that a 2.5mm ring circuit needs protecting by a 32A MCB as any greater load than this will cause the cable to overheat. So the 32A MCB protects the cable.

But surely the maximum load on the spur from the junction box can only be 13A, or are you saying that we need to allow for people plugging in more than 13A of load?

Graham

[ARWadsworth]

Well it would be 26A if a double socket was fully loaded! I am not sure how long the socket would last though.

I most cases a 1.5mm would work and never cause a problem (Christ knows how many times I have seen it done). But would it be safe in the event of a short circuit when there is a brief load of over 200amps? Installing 1.5mm would be a big departure from the regs and would certainly fail an electrical inspection.

Cheers

[John Rumm]

The shielding of the cable must be earthed (not much point in having it if not!), and in normal circumstances would actually be the circuit earthing conductor (i.e. you only use two core SWA usually for these applications - the shield acting as the earth). So termination would be difficult in way you describe.

See here:

all in all, not a way I would go...

[John Rumm]

I don't have my 17th edition OSG to hand, but the 16th edition certainly permitted a reduction to 1.5mm^2 for a fused spur (pg 152, appn 8). Its one of those situations where the overload protection is delegated to the destination protective device (i.e. the 13A fuse) and the fault protection remains at the circuit origin. The 1mm^2 CPC of the 1.5mm^2 T&E ought to be adequately protected by a B32 breaker (although probably not a 30A BS3036 fuse)

[ARWadsworth]

I have to disagree. A fused spur is the 13A protected side of a fused connection unit.

The 1.5mm is only allowed on the 13A side of the fused connection unit. It is only a fused spur from that point onwards.

It is bad terminology to blame. There is a difference between a fused spur (the cables after the 13A fuse) and a fused spur connection unit (what we call a fused spur).

Cheers

[John Rumm]

Quite possibly, I can't tell for sure from the wording exactly what was intended[1]. The bit I am thinking of is:

"Conductor size The minimum size of conductor cross-sectional area in the circuit and in non-fused spurs is given in Table 8A. However, the actual size of cable is determined by the current-carrying capacity for the particular method of installation, after applying appropriate correction factors from Appendix 6. The current-carrying capacity so calculated shall be not less than:

20 A for circuit A1, 30 A or 32 A for circuit A2 (i.e. the rating of the overcurrent protective device), 20 A for circuit A3 (i.e. the rating of the overcurrent protective device).

The conductor size for a fused spur is determined from the total current demand served by that spur, which is limited to a maximum of 13 A. When a fused spur serves socket-outlets the minimum conductor size is:

1.5 mm2 for cables with thermosetting or thermoplastic insulated cables, copper conductors, 1 mm2 for mineral insulated cables, copper conductors. The conductor size for circuits protected by BS 3036 fuses is determined by applying the 0.725 factor of Regulation 433-2-03; that is, the current-carrying capacity must be at least 27 A for circuits A1 and A3, and 41 A for circuit A2." [1] Its a bit of a moot point since I don't even bother keeping 1.5mm^2 cable usually! However the basic point stands, that if there is a 13A fused FCU on the end of it, then the cable will be protected from overload in most cases even if using 1.5mm^2 T&E (who's "clipped direct" current carrying capacity is 20A), and it will also be adequately fault protected with a B32 breaker (requiring a CPC CSA of a bit over 0.52mm^2 assuming max EFLI of 1.2 ohms)

(for the avoidance of doubt - not suggesting anyone does this, just highlighting that its unlikely to burst into flames in cases where it has been done!)

[ARWadsworth]

I agree that the wording could be better.

I can see how that sentence could be misread. How about "The conductor size of a fused spur...."

And on the next page. "A fused spur is connected to the circuit through a fused connection unit."

So the definition of a fused spur is "the bit after the fused connection unit". That would make the supply to the fused connection unit an non-fused spur (so table 8A applies and 2.5mm T&E is needed).

I take your point about the maximum load and the 1.5 T&E protection.

So you have never met an alarm installer then:-) 1mm T&E into ANY supply.

Cheers

[geoff]

Central heating fitter, is he ?

[chris French]

In my parents old house - putting up a shelf or cabinet or something. I plugged an electric drill into a socket and then proceeded to drill a hole in the wall above the socket......

No, I have no idea what possessed me either.

[Andy Burns]

When you're unblocking a sink and you've just removed the "U" bend, where do you empty it?

[Owain]

I thought it would be nice to listen to some music while I soldered the gubbins inside an old valve radio ...

Owain

[Owain]

Into a washing up bowl.

Which then gets emptied, oh, um, yes.

Owain

[jgharston]

I once removed a kitchen unit to get access to the CU to find a wallplug right /through/ one of the 2.5T+E cables running up from the CU.

JGH

[Huge]

*grin*

I have done both of these things.

[Ronald Raygun]

Oh um yes WHAT? You don't need to empty the bowl until the job's done. Come to think of it, you don't even need a bowl. Just empty the U bend into the sink. With the plug in place.

Then clean out the U bend in said sink, with plenty of water. Just remember to re-connect the bend before pulling the plug!