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?