Immersion fuse or MCB?

[Steve-S]

An immersion heater 3kW, say off a final ring. A number of points.
Is is best have a 13A double pole FCU switch or a double pole 13A type C MCB?
A fuse is one-shot. If it blows the danger is eliminated until seen to. The MCB in a local enclosure doubles as switch but can be switched back on by the user.

Gradual current increase on a 13A fuse can cook a 2.5mm cable before a fuse blows. This can be a dangerous yet all legal. This is not such a problem with an MCB which will trip quicker.

Any views on this welcome.

 

[nicebutdim]

Need to get my scale rule to figure out the trip time for a 13A Type C MCB.

 

[westward10]

If the mcb trips it is likely the immersion needs replacing so it is not a sensible solution for the user to try and switch it back on if there is a high current fault failure, it will flash and bang.

 

[littlespark]

3kW off an rfc? Doesn’t leave much for the rest of the ring.

Put it on its own radial, 16A breaker with an isolating switch or controller

 

[nicebutdim]

I'm guessing this is a case of looking at options for an existing situation, but it's difficult to be certain from information provided in the opening post.

 

[Steve-S]

The main point is preventing a cable from cooking. This may also apply to other appliances.

 

[davesparks]

The main point is preventing a cable from cooking. This may also apply to other appliances.

This is achieved by selecting the correct size protection for the load and then the correct size cable from that.

 

[Steve-S]

BS 1362 specifies the fusing current as 1.9 times the rated current. If the current exceeds the fusing current, the fuse must blow within 30 minutes. So a 13A fuse must blow within 30 minutes when carrying a current that exceeds 24.7A. 24.7A can fry a cable, especially when it is derrated say in an insulated wall.

MCBs are better, but a 13A fuse is a one-shot. MCBs are better for protection but can be switched back on again by the user as they are not one-shot.

 

[Steve-S]

I am using an immersion as an e.g., as it can be on for long periods with an ability to cook a cable while all within regs, while plug-in appliances are by design only drawing maximum current for short periods.

 

[nicebutdim]

BS 1362 specifies the fusing current as 1.9 times the rated current. If the current exceeds the fusing current, the fuse must blow within 30 minutes. So a 13A fuse must blow within 30 minutes when carrying a current that exceeds 24.7A. 24.7A can fry a cable, especially when it is derrated say in an insulated wall.

MCBs are better, but a 13A fuse is a one-shot. MCBs are better for protection but can be switched back on again by the user as they are not one-shot.

I am using an immersion as an e.g., as it can be on for long periods with an ability to cook a cable while all within regs, while plug-in appliances are by design only drawing maximum current for short periods.

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.

 

[mainline]

BS 1362 specifies the fusing current as 1.9 times the rated current. If the current exceeds the fusing current, the fuse must blow within 30 minutes. So a 13A fuse must blow within 30 minutes when carrying a current that exceeds 24.7A. 24.7A can fry a cable, especially when it is derrated say in an insulated wall.

MCBs are better, but a 13A fuse is a one-shot. MCBs are better for protection but can be switched back on again by the user as they are not one-shot.

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

 

[westward10]

Is this a real situation or are you speculating?

 

[nicebutdim]

Is this a real situation or are you speculating?

"I am using an immersion as an e.g"

 

[nicebutdim]

Just read through this thread again and on that basis I'll no longer be contributing to the discussion.

 

[westward10]

Just read through this thread again and on that basis I'll no longer be contributing to the discussion.

Did you actually read all of that?

 

[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.