Immersion fuse or MCB?

[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.

 

[DPG]

Enticing? Enticing answers. I asked two questions, with no one really 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.

A non-resettable MCB is not really a thing which is in big demand though for standard domestic circuits.

And how would you decide which circuits in a household consumer unit had such an MCB? How do you decide which circuits need to be reset by calling out an electrician, or whatever other method of resetting is envisaged.

 

[littlespark]

It’s been explained already that cable sizes are determined on the design load of the circuit, but you also take other things into consideration, such as length of cable, wiring method, cable grouping and temperature.

Any cable taking anything close to its current carrying capacity is likely for heating, and very likely a purely resistive load.
It may heat up for a time, then cycle on and off with a thermostat for the remainder of the time it’s turned on for.

This non-existent immersion, say, could be turned on 24 hours a day… but it won’t take its full 3kW for all that time, constantly.

We can also pick and choose the tripping curve of circuit breakers. A choice between how quickly and how far over its rated current it should trip off.


It’s all in the regs book… and already been taken into account.

 

[nicebutdim]

Can't believe this has been allowed to run for another two pages.

Must have been a quiet day for a lot of you?

 

[Steve-S]

A non-resettable MCB is not really a thing which is in big demand though for standard domestic circuits.

And how would you decide which circuits in a household consumer units had such an MCB?

A double pole MCB could be used in a small local enclosure isolating and protecting an appliance. Far better, but it is not one-shot and instantly re-settable.

 

[DPG]

A double pole MCB could be used in a small local enclosure isolating and protecting an appliance. Far better, but it is not one-shot and instantly re-settable.

Not really answered my point there though have you. Pot calling kettle (kettle on 13A fuse)

 

[Steve-S]

It’s been explained already that cable sizes are determined on the design load of the circuit, but you also take other things into consideration, such as length of cable, wiring method, cable grouping and temperature.

Any cable taking anything close to its current carrying capacity is likely for heating, and very likely a purely resistive load.
It may heat up for a time, then cycle on and off with a thermostat for the remainder of the time it’s turned on for.

This non-existent immersion, say, could be turned on 24 hours a day… but it won’t take its full 3kW for all that time, constantly.

We can also pick and choose the tripping curve of circuit breakers. A choice between how quickly and how far over its rated current it should trip off.


It’s all in the regs book… and already been taken into account.

It’s been explained already that cable sizes are determined on the design load of the circuit, but you also take other things into consideration, such as length of cable, wiring method, cable grouping and temperature.

Any cable taking anything close to its current carrying capacity is likely for heating, and very likely a purely resistive load.
It may heat up for a time, then cycle on and off with a thermostat for the remainder of the time it’s turned on for.

This non-existent immersion, say, could be turned on 24 hours a day… but it won’t take its full 3kW for all that time, constantly.

We can also pick and choose the tripping curve of circuit breakers. A choice between how quickly and how far over its rated current it should trip off.


It’s all in the regs book… and already been taken into account.

I have seen a 13A FCU on a 3kW immersion heating a 400 litre cylinder. It was hardly ever off. It was on a Schneider FCU and obviously a quality fuse as there were no scorch marks on the FCU. Good FCUs allow air movement over the fuse for cooling.

 

[nicebutdim]

I have seen a 13A FCU on a 3kW immersion heating a 400 litre cylinder. It was hardly ever off. It was on a Schneider FCU and obviously a quality fuse as there were no scorch marks on the FCU. Good FCUs allow air movement over the fuse for cooling.

So now fuses are better if they're provided with sufficient air circulation? You had the answer all along and just needed some prompting to remember.

Another conundrum resolved and this thread can fade into history.

 

[littlespark]

I have seen a 13A FCU on a 3kW immersion heating a 400 litre cylinder. It was hardly ever off. It was on a Schneider FCU and obviously a quality fuse as there were no scorch marks on the FCU. Good FCUs allow air movement over the fuse for cooling.

badly designed system for a start.

Next youll be telling us its bare metal, no insulation.

A standard domestic tank, even the old ones with the yellow polystyrene coating might heat up from cold for 2 to 3 hours to get to temperature... then cut in and out for the remaining time.... as long as no hot water has been used, and theres no fresh cold water entering the tank to dilute the hot.



Its basic electrical principles.... The cable can carry more amps than the fuse, the fuse can carry more amps than the load is supposed to be.

 

[mainline]

A double pole MCB could be used in a small local enclosure isolating and protecting an appliance. Far better, but it is not one-shot and instantly re-settable.

Double pole mcb in its own enclosure jezzzzz

The protection is for the cable, not the appliance.

That's my one shot, I'm done with this nonsense.

 

[James]

Wow, this whole thread is a mass of rubbish about what is better, a fuse or an mcb.

They both have advantages and disadvantages depending upon the load, cable, installation methods and expected use.

In general an mcb (of the right trip characteristics) is better for most applications but there are still plenty of applications that a fuse would be a preferable protective device.

as professional electricians, I cant understand why there is an argument about what is better, they both have there uses and if you choose wrongly then there is a higher likelihood of bad things happening.

get out your books, look at the trip characteristics, expected current, fault current and all the other factors.
make a calculation and come up with the answer.
most of the time it can be done with either one or the other but sometimes one is better than the other.
it cant be decided on a theoretical if this then.... that.

each circuit we install or alter should be given consideration to protective devices and if required, calculation.