A thought about fuses

[rewireIT]

Howcomes we use eg a 3a fuse from the FCU to feed a boiler that has 1.5mm wiring? Wouldn't a 13a still be ok and if not, why not?

TIA

 

[Rockingit]

Whilst the cabling is adequately protected at 13A, the vast majority of manufacturers have always specified a 3A fuse to protect the boiler. Also - whilst it might be 1.5mm conductor size at the start, consider the rest of the system downstream of that such as 2 ports, timers etc all of which can often have much reduced cable sizes.

 

[davesparks]

Howcomes we use eg a 3a fuse from the FCU to feed a boiler that has 1.5mm wiring? Wouldn't a 13a still be ok and if not, why not?

TIA

The process for selecting the size of fuse required is to assess the design current, which for a small/domestix boiler will be less than an Amp, then to select the nearest standard size fuse, which would be 3A.

We also have to take account of manufacturers instructions which will state that a 3A fuse is required for a gas boiler and usually a 5A fuse for an oil boiler.

Boiler wiring will also usually be 0.75mm as anything bigger is not required.

 

[Dustydazzler]

MI will usually state 3amp or 5amp fused for most boilers , so a bit or 1mm flex would do the job.
Using anything larger is just a waste of money really

 

[rewireIT]

Thanks for the responses guys; learning all the time

 

[DPG]

This is exactly the sort of post that Lucien would have given a really thorough and well thought out reply to.

 

[pc1966]

MI will usually state 3amp or 5amp fused for most boilers , so a bit or 1mm flex would do the job.
Using anything larger is just a waste of money really

This is something not seen much today but the UK system with 3A/5A fuses could support 0.5mm flex quite safely, where as the EU/USA/world system of unfused plugs means the flex has to cope with the I2t of something like 16A OCPD at the distribution board.

Of course now the copper saving cost is dwarfed by the desire to make products for multiple markets, so you find nothing below 1mm/0.75mm made as a result, even for small loads, as the UK market is small compared to all of those combined.

 

[Rockingit]

This is something not seen much today but the UK system with 3A/5A fuses could support 0.5mm flex quite safely, where as the EU/USA/world system of unfused plugs means the flex has to cope with the I2t of something like 16A OCPD at the distribution board.

Of course now the copper saving cost is dwarfed by the desire to make products for multiple markets, so you find nothing below 1mm/0.75mm made as a result, even for small loads, as the UK market is small compared to all of those combined.

Also easy to forget that most of the decent DB manufacturers also make 3A MCB's

 

[Dustydazzler]

This is something not seen much today but the UK system with 3A/5A fuses could support 0.5mm flex quite safely, where as the EU/USA/world system of unfused plugs means the flex has to cope with the I2t of something like 16A OCPD at the distribution board.

Of course now the copper saving cost is dwarfed by the desire to make products for multiple markets, so you find nothing below 1mm/0.75mm made as a result, even for small loads, as the UK market is small compared to all of those combined.

I come a lot of appliances these days come supplied with 1.25mm flex , presumably as this is the smallest ( cheapest ) sized flex for 16amp appliances around the globe

 

[pc1966]

Also easy to forget that most of the decent DB manufacturers also make 3A MCB's

True, the the fault I2t of such an MCB is usually way higher than a 3A fuse, except in the region just after the magnetic trip.

E.g. typical Hager 3A MCB has about 70 A2s of let-through at 100A fault current, 3A BS1362 fuse at that point is worst-case specification is around 15 A2s (slope implying in the constant I2t sort of region) and probably less, so almost 5 times less.

Higher up at 2kA fault the MCB let-through is around 3 kA2s but fuse still offering about 15 A2s and now the fuse is around 200 times less.

 

[LastManOnline]

Whilst the cabling is adequately protected at 13A, the vast majority of manufacturers have always specified a 3A fuse to protect the boiler.

As far as I am aware manufacturer,s only seem to get away with this in Ireland and the UK only.In other countries manufacturers are expected to take care of this issue themselves by fusing the appliance internally

 

[pc1966]

As far as I am aware manufacturer,s only seem to get away with this in Ireland and the UK only.In other countries manufacturers are expected to take care of this issue themselves by fusing the appliance internally

<chough> Bathroom fans?

 

[Dustydazzler]

<chough> Bathroom fans?

you can now buy a fused fan switch

 

[pc1966]

you can now buy a fused fan switch

One issues is if both the permanent and switched lives both need fused. Really someone should slap down on this and say that anything falling in to those categories has to be designed as safe with a 6A MCB.

 

[nicebutdim]

One issues is if both the permanent and switched lives both need fused. Really someone should slap down on this and say that anything falling in to those categories has to be designed as safe with a 6A MCB.

I've fitted maybe two dozen fans in the last couple of years and all were safe on a 6A circuit.