table lamp-replacment mains cable

[stem]

A small appliance develops an internal fault, and starts to draw more current than it would normally, sufficient to blow a 3A fuse, (but not a 13A) before any damage is done.

But, if the appliance is fitted with a 13A fuse and 13A rated cable, then the fuse won't blow, the cable won't be damaged, but the appliance catches fire because of the heat generated by the excess current flowing through its internal components not designed for such.

Central heating systems are also fitted with 3A fuses in their FCU, yet the cables used to wire them could be capable of carrying 13A. However the programmers, PCB's etc., could all be damaged by too much current. For example relay contacts often weld together. A 3A fuse stops the current flow before any damage is done.

 

[GaryMo]

Back to your 8w lamp wired in 2.5mm2 flex and protected by a 13A BS 1362 fuse.

BS 1363-1:1995 states:

Rewirable plugs are intended for use with flexible cords complying with
BS 6500:2000 having conductor cross-sectional areas from 0.5 mm2 to 1.5 mm2 inclusive.

So, no, 2.5mm2 conductors can't be used.

Also, table 2 states thats rated current and maximum fuse ratings in normal use are as follows:

0.5mm2 - rated at 3A, fused at 3A
0.75mm2 - rated at 6A, fused at 13A
1.0mm2 - rated at 10A, fused at 13A
1.25mm2 - rated at 13A, fused at 13A
1.5mm2 - rated at 13A, fused at 13A

So, a flex with a CSA between 0.75mm2 and 1.5mm2 could be used in conjunction with a BS1362 13A fuse for your 8w lamp.

Your example of a central heating system is somewhat different.
BS7671:2008 states that electrical equipment should be installed in accordance with the manufacturer of the equipment.
Most, if not all? manufacturers of domestic boilers state that a fuse with a maximum rating of 3A should be fitted to the FCU which supplies it. So no arguments with that one.

 

[stem]

manufacturers of domestic boilers state that a fuse with a maximum rating of 3A should be fitted to the FCU which supplies it. So no arguments with that one.

Good so we agree then. The fuse is to protect the appliance, even if the cable to it can cope with a higher current.

 

[GaryMo]

No, we don't agree.

In the case of plug tops, the fuse is selected depending on the CSA of the cable, not the rating of the appliance.

 

[stem]

So, if someone came on here and said. "I have a portable appliance rated at 100w, what size fuse should I put in the plug"

I wonder what the answers would be?

A) 3A

or

B) Go and check the cable size and then we'll tell you.

 

[GaryMo]

So, if someone came on here and said. "I have a portable appliance rated at 100w, what size fuse should I put in the plug"

I wonder what the answers would be?

A) 3A

or

B) Go and check the cable size and then we'll tell you.

The answer would be in BS 1363-1:1995 and going by that British Standard it would be B.

I accept that most, me included, would fit a 3A fuse but that's not what I'm getting at here.

The plug top fuse is placed to protect the flexible cord. It's sized with the CSA of the flexible cord in mind.
Have you got access to BS 1363-1:1995? If not, are you doubting what I've read in there?

If an appliance requires protection then it'll have an internal fuse for that purpose.

If you can agree to that then we'll leave it there.

 

[600rob]

Hi guys took your advice and it worked fine

Many thanks for the help.

Cheers
rob