Immersion fuse or MCB?

[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.

 

[UNG]

I did say it could be any other appliance.

I think you have been here before trolling with your ridiculous scenarios and circular arguments which resulted in members getting bans
Therefore this will be my only response on the subject while adding you to the 101 list

 

[Steve-S]

#1 refers to a 3kw immersion heater.

Later clarified. But this digression does not answer the prime points

 

[Steve-S]

I think you have been here before trolling with your ridiculous scenarios and circular arguments which resulted in members getting bans
Therefore this will be my only response on the subject while adding you to the 101 list

I see nothing circular. Link the link below:

Immersion fuse or MCB? | Page 2 | on Free Electrical Advice - Electricians Forums

Find Immersion fuse or MCB? Advice and Help. How-to Immersion fuse or MCB? in the UK Electrical Forum advice boards on ElectriciansForums.net | Free Electrical Advice - Electricians Forums: Electricians' Talk Forums. Immersion fuse or MCB? on ElectriciansForums.net Electrical Advice Forum.

www.electriciansforums.net

 

[westward10]

Later clarified. But this digression does not answer the prime points

I answered your prime points from #1.

 

[Steve-S]

I answered your prime points from #1.

The points are in the link below. If you cannot offer a solution then fine.

Immersion fuse or MCB? | Page 2 | on Free Electrical Advice - Electricians Forums

Find Immersion fuse or MCB? Advice and Help. How-to Immersion fuse or MCB? in the UK Electrical Forum advice boards on ElectriciansForums.net | Free Electrical Advice - Electricians Forums: Electricians' Talk Forums. Immersion fuse or MCB? on ElectriciansForums.net Electrical Advice Forum.

www.electriciansforums.net

 

[westward10]

The points are in the link below. If you cannot offer a solution then fine.

Immersion fuse or MCB? | Page 2 | on Free Electrical Advice - Electricians Forums

Find Immersion fuse or MCB? Advice and Help. How-to Immersion fuse or MCB? in the UK Electrical Forum advice boards on ElectriciansForums.net | Free Electrical Advice - Electricians Forums: Electricians' Talk Forums. Immersion fuse or MCB? on ElectriciansForums.net Electrical Advice Forum.

www.electriciansforums.net

Yes that refers to an immersion heater.

 

[Steve-S]

Best you you say "I do not know", or not get involved.
I am waiting for some responses.

 

[mainline]

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

No decent electrician these days would install a 13 amp fcu on a 3kw immersion.

I would replace with 20 amp switch and 16amp mcb/rcbo.

 

[westward10]

Best you you say "I do not know", or not get involved.
I am waiting for some responses.

I have responded your posts refer to an immersion heater.

 

[Steve-S]

No decent electrician these days would install a 13 amp fcu on a 3kw immersion.

I would replace with 20 amp switch and 16amp mcb/rcbo.

A 20A switch and 16A breaker in the CU on a 2.5mm cable is not fine on a dedicated circuit as it still could cook a 2.5mm cable rated at 19A. 13A would be safer as it can pass current constantly over 13A meaning less risk of cooking a 2.5mm cable. But the regs say a 13A appliance can be on an FCU and off a ring. All legal. Many are off final rings and on FCUs.

The point of this thread is how to stop a cable cooking, while the installation is still within regs, without ripping a house apart. A simple, quick, and cheap solution. One which combined the advantages of an MCB and fuse.

 

[mainline]

A 20A switch and 16A breaker in the CU on a 2.5mm cable is fine on a dedicated circuit. But the regs say a 13A appliance can be on an FCU and off a ring. All legal. Many are off final rings and on FCUs.

The point of this thread is how to stop a cable cooking, while the installation is still within regs, without ripping a house apart. A simple, quick, and cheap solution. One which combined the advantages of an MCB and fuse.

An appliance would have a plug and fused accordingly so would be downstream fused.
A 3kw immersion heater isn’t going to “cook” the cable as you put it as already said it’s a resistive load and a short would trip the 32 amp rings ocpd even without the downstream fuse.

 

[Steve-S]

An appliance would have a plug and fused accordingly so would be downstream fused.
A 3kw immersion heater isn’t going to “cook” the cable as you put it as already said it’s a resistive load and a short would trip the 32 amp rings ocpd even without the downstream fuse.

The fixed hard wired appliance may be any other rather than an immersion. Drop the immersion as it confuses many here. A fuse can constantly pass more than 13A over the rating of a derrated 2.5mm cable, which can be 19A - cooking time.

 

[davesparks]

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.

The current carrying capacities in the regs book do not represent the point at which a cable suddenly goes from working safely to being 'fried' as you put it.

The current carrying capacities in the regs book have already taken in to account the behaviour of fuses and MCBs and allowed for this.

As long as you follow the cable size calculation process in the regs correctly the resultant cable size will be suitable.

 

[davesparks]

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.

Unless it's is buried in a lot of insulation in a very high ambient temperature a 13A fuse will protect a 2.5mm cable perfectly well.

And MCBs are not superior when compared to fuses, they are just more convenient for resetting.

 

[Steve-S]

A 16A MCB will draw over 20A for 2.75 hours cooking a 2.5mm cable rated at 19A. So best have a 13A MCB for a 13A appliance giving better protection of the cable on a one appliance dedicated circuit.

 

[davesparks]

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

You have misunderstood cable sizing and current ratings.

The current ratings in the regs book, and the cable sizes arrived at from the calculations in the regs, are for the cable operating normally at that current constantly for 24hrs per day. There is already an allowance made in this for the short overload that can occur under fault conditions.

Yes it takes a while for a 13A fuse to operate with a current of 24.5A, but when it does the cable will be unharmed.

If you follow the regulations correctly then your installation will be safe.


The regulations allow overload protection to be omitted for loads which are not subject to overload faults, such as purely resistive loads.

 

[davesparks]

View attachment 117604
A 16A MCB will draw over 20A for 2.75 hours cooking a 2.5mm cable rated at 19A. So best have a 13A MCB for a 13A appliance giving better protection of the cable on a one appliance dedicated circuit.

Could you define exactly what you mean by 'cooking a cable' in this? If you mean it will get warm then yes it will, but if you mean it will be damaged in some way then no it won't.

If after using the ratings and calculations in the regs a 2.5mm is rated at 19A then this will be absolutely fine protected by that 16A MCB for the time it takes it to operate at 20A.

The current ratings in the regs do not represent the point at which a cable suddenly goes from working normally to being damaged.

 

[DPG]

This reminds me of the threads John-SJW used to create. Where he just kept enticing people to reply to them even though he was just spouting the same (and often wrong) stuff the whole time.

I think it must class as entertainment?

 

[Steve-S]

Could you define exactly what you mean by 'cooking a cable' in this? If you mean it will get warm then yes it will, but if you mean it will be damaged in some way then no it won't.

If after using the ratings and calculations in the regs a 2.5mm is rated at 19A then this will be absolutely fine protected by that 16A MCB for the time it takes it to operate at 20A.

The current ratings in the regs do not represent the point at which a cable suddenly goes from working normally to being damaged.

If a cable is constantly warm the insulation will deteriorate far quicker than always staying cool.

 

[Steve-S]

This reminds me of the threads John-SJW used to create. Where he just kept enticing people to reply to them even though he was just spouting the same (and often wrong) stuff the whole time.

I think it must class as entertainment?

Enticing? Enticing answers. I asked two questions, with no one really fully addressing them. Lots of peripheral stuff, interesting though it is. One unanswered is that I assume there an MCB that is not instantly re-settable by the user.