5A fuse replacement in fuse box - does not quite fit...

[tristan]

Hi,

I recently had the 5A lighting fuse go in our household fusebox. This fusebox takes household plug style fuses in the holders.

Except the sizes seem different. When I tried putting in a spare 5A fuse, it was too long, and therefore did not fit. Thankfully, a neighbour had a spare.

I want to order some of my own, but am trying to find the right ones! I've measured the required 5A one at a length of 22mm.

Are these the ones I need? ...

Maplin don't list the physical dimensions.

Thanks in advance,

Tris.

[Christian McArdle]

fused consumer unit, it could take plug top fuses instead, though. It depends on the design of the holder.

Christian.

[Christian McArdle]

I should say that you should use the BS1361 HRC fuse in preference even if both types fit. I don't know if BS1362 (standard plug fuses) have sufficient breaking capacity.

Christian.

[Andy Wade]

The rated breaking capacities are 16.5 kA for BS 1361 consumer unit fuses and 6 kA for BS 1362 plug fuses.

In principle you shouldn't use the BS 1362 fuse in a consumer unit, but since only the 5 amp types are interchangeable, the chance of the 6 kA breaking capacity being exceeded in practice is very very small. I'd have no hesitation of using a 5 A plug fuse as an temporary replacement, until a round tuit allows a BS 1361 fuse to be fitted.

[Christian McArdle]

6kA is normally enough. Most MCBs are 6kA anyway, with some 10kA. A quick check with a loop tester should indicate compatibility, if you happen to have one lying about...

Christian.

[Andy Wade]

Agreed. Although the fault level (PSSC) on a house service can be as high as 16 kA, it's rarely anywhere near that.

Yes, but the complete MCB CU system will have been type tested to BS EN

60439-3 at 16 kA and given a conditional rating - conditional, that is, on the presence of the supplier's 100 A (BS 1361 Type 2) fuse to provide backup protection. IOW even when the MCBs' breaking capacities are exceeded you have an assurance that the supplier's fuse will clear the fault without the CU blowing itself apart (at 16 kA the magnetic forces are huge) or catching fire. You can't necessarily assume that this will be the case with an old Wylex board fitted with the wrong type of fuse.

Of course I do :-). The lowest impedance domestic mains supply I've ever encountered was about 0.05 ohm (phase-neutral loop) so c. 5 kA fault level.

[Christian McArdle]

Mine is certainly much lower, despite living in an urban area. I can't remember the figure, but it is less than 2kA. Not that you are allowed to rely on it, in case of upgrading in the supply somewhere.

Christian.

[David Hansen]

On Sat, 19 Aug 2006 12:44:02 +0100 someone who may be Andy Wade wrote this:-

At the risk of being thought to be a fan of "mine's bigger than yours" discussions, the highest I have found in a house is 12,000A. The house concerned is perhaps 10m from the transformer which undoubtedly feeds the house. That is fed by I guess less than 100m of 11kV cable, which is fed by less than 2km of 33kV overhead, which comes from a large compound into which a number of pylon runs of

132kV size congregate.

[Owain]

So it wouldn't cost that much to upgrade to oxygen-free cable for maximum audiophile pleasure?

Owain

[Andy Wade]

Mine's very similar at about 0.16 ohm, or 1.5 kA. These seem to be very typical figures.

You are allowed to rely on the attenuation of the PSCC due to the impedance of your service cable. The idea is to assume that the fault level on the main distributor cable in the street is 16 kA at 0.55 PF at all points (since in principle they could drop a new transformer in at any point) and then calculate the reduction due to adding the impedance of the length of service cable between the street cable and your supply intake.

16 kA at 0.55 PF corresponds to a source impedance of 8.25 + j12.53 milliohm, to which add about 1.4 + j0.06 milliohm per metre of service cable (for the usual 25 mm^2 Cu or 35 mm^2 Al cable). So a 10 m long service will give a PSCC of 9.3 kA at 0.86 PF, an so on. (These numbers are for single-phase supplies, of course.)

[<me9]

Gold plated of course.