Immersion fuse or MCB?

[nicebutdim]

Did you actually read all of that?

Just the later highlights - enough to jog my memory.

 

[Steve-S]

A 3kw immersion heater shouldn't really be on a 13amp fuse.

All within regs to have an immersion of a 13A fuse. Millions are.

 

[Steve-S]

Suggesting that 24.7A could 'fry' 2.5mm seems a tad over-dramatic.

Your example of a 3kW immersion off a ring final is also an unlikely scenario. Not only does BS7671 suggest all fixed loads over 2kW be given their own circuit, but historically immersion heaters have been fed from a dedicated circuit.


If there's an actual question you'd like answered, perhaps you could ask specifically? At present it appears as though this thread is suggesting an MCB offers better protection than a fuse in circumstances which should never present themselves in reality.

A 2.5mm cable can be fried using a 13A fuse. Least is around 19A when in insulation. MCBs are superior of course.

The key to the thread is one-shot.

 

[nicebutdim]

A 2.5mm cable can be fried using a 13A fuse. Least is around 19A when in insulation. MCBs are superior of course.

The key to the thread is one-shot.

I'll respond this last time and be done, now that we at least know the reason behind your initial post.

You're taking a very limited set of circumstances and less than reasonably applying a worst case scenario to them. Would that cable be operating beyond regulatory parameters if enclosed in a thermally insulated wall? Yes it would, although that's not to suggest it would 'fry' and nor does it suggest this worst case is representative to all (or even a majority of situations).

I'll also add that, while such loads are indeed to be found supplied from ring final circuits, most were fitted long ago and no credible electrician will have considered hanging a 3kW immersion off a ring for quite a number of years. I can't think of a single instance where I've encountered a 3kW immersion supplied from a ring and that includes installations dating back to the 1950s, wired in rubber cables. Maybe standards have traditionally been higher over here (I doubt that), but every immersion I've seen has had its own circuit - even when distribution was limited to a total of four circuits for an entire property.

 

[brianmoooore]

I go back to a time when a 4W Wylex was a fairly standard installation, and the circuits would be 30A cooker, 30A 'ring' sockets, 15A immersion (seen some weird and wonderful spellings of that word inside the cover), and 5A lights. The water heater was always on a dedicated circuit.
Any FCU or plug/socket fitted with a 13A fuse that is asked to carry a 3kW load fr long periods of time will show signs of thermal degradation after a year or so.

 

[Steve-S]

13A MCBs are superior in many ways to a fuse. Is there one available that cannot be immediately switched back on by the user after tripping?
Using a fuse it is best to use 10A rather than 12A to prevent a cable frying? A 10A fuse can pass 19A before blowing, so a 2.5mm cable will not fry. The fuse may get hot but decent makes of FCU have enough ventilation around the fuse to dissipate heat preventing scorching.

 

[Steve-S]

I'll respond this last time and be done, now that we at least know the reason behind your initial post.

You're taking a very limited set of circumstances and less than reasonably applying a worst case scenario to them. Would that cable be operating beyond regulatory parameters if enclosed in a thermally insulated wall? Yes it would, although that's not to suggest it would 'fry' and nor does it suggest this worst case is representative to all (or even a majority of situations).

I'll also add that, while such loads are indeed to be found supplied from ring final circuits, most were fitted long ago and no credible electrician will have considered hanging a 3kW immersion off a ring for quite a number of years. I can't think of a single instance where I've encountered a 3kW immersion supplied from a ring and that includes installations dating back to the 1950s, wired in rubber cables. Maybe standards have traditionally been higher over here (I doubt that), but every immersion I've seen has had its own circuit - even when distribution was limited to a total of four circuits for an entire property.

I have come across many.

 

[brianmoooore]

I have come across many.

Many date from about 30? years ago, when the government offered free insulation top ups for the lofts of pensioners. Many of those pensioners lived in bungalows which already had 4" - 6" of loft insulation, with all the power wiring lying on top of it.
Now that insulation has another 6" on top of it, with the wiring cooking nicely in the middle of it.

 

[westward10]

13A MCBs are superior in many ways to a fuse. Is there one available that cannot be immediately switched back on by the user after tripping?
Using a fuse it is best to use 10A rather than 12A to prevent a cable frying? A 10A fuse can pass 19A before blowing, so a 2.5mm cable will not fry. The fuse may get hot but decent makes of FCU have enough ventilation around the fuse to dissipate heat preventing scorching.

So you think it is safe for the end user to reset the 13A mcb because it has tripped through a high fault situation which is likely to be the cause?

 

[Steve-S]

So you think it is safe for the end user to reset the 13A mcb because it has tripped through a high fault situation which is likely to be the cause?

Read what I wrote again. It was clear.

 

[westward10]

Read what I wrote again. It was clear.

I did you say the mcb trips I presume through a fault and it can be turned back on by the user even if the fault is still there resulting in a flash, bang from the mcb.

 

[Steve-S]

I did you say the mcb trips I presume through a fault and it can be turned back on by the user even if the fault is still there resulting in a flash, bang from the mcb.

Very clear what I am on about which no one has grasped after all these posts.

1. A fuse has the advantage of being one-shot. A disadvantage is that under regs a cable can still fry using a fuse. A 13A fuse can run to near 25A until blowing, so it can draw a constant say 24.5A. A 2.5mm cable can be rated to 19A, which means it may fry. A 1.5mm cable serving say an immersion on a dedicated radial is rated 14A to 19.5A. And all within regs.

2. An MCB gives greater protection than a fuse, but it is not one-shot.

So what is a solution?
a) Downsizing a 13A fuse to 10A which will blow at 19A. But the fuse will run hot causing scorching of say an FCU. The scorching can be eliminated by good ventilation of the fuse? 13A plugs are, or were, ventilated, with a gap on the bottom cable entry and a hole at the top (large lug locating hole).
b) Raising the cable size to 6mm? (which may be not be practical).
c) Fit an MCB that cannot be instantly reset by the user, if they are available?

 

[westward10]

An immersion heater is a fixed resistive load and is not likely to overload and any fault whether this be between conductors or to earth will result in a high current fault which will be cleared by the protective device.

 

[Steve-S]

An immersion heater is a fixed resistive load and is not likely to overload and any fault whether this be between conductors or to earth will result in a high current fault which will be cleared by the protective device.

I did say it could be any other appliance.

 

[westward10]

#1 refers to a 3kw immersion heater.