Immersion fuse or MCB?

[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.

 

[mainline]

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?

Entertainment as in court jester

 

[davesparks]

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.

How is it far better?

 

[Steve-S]

Double pole mcb in its own enclosure jezzzzz

Yes. Few know you can have a 16A load off a final ring. Needs to be fixed wiring, overcurrent protection and a switch. A double pole MCB covers it all off the ring and being the isolator for the 16A appliance.

But a 13A MCB can be used as well for 13A appliances instead of an FCU. Which gives superior dp protection.

 

[davesparks]

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

Standard cables used in the UK are designed to operate with a conductor temperature of 70 degrees C.
Current ratings given in the regulations are based on that conductor operating temperature and have tolerance already built in to them to ensure this is not exceeded when installed in accordance with the regulations.

 

[Steve-S]

How is it far better?

As James wrote: "In general an mcb (of the right trip characteristics) is better for most applications". dp even better.

 

[davesparks]

Yes. Few know you can have a 16A load off a final ring.

Can you support that with evidence?
This was certainly true under previous versions of the regulations but as far as I can remember it has not been permitted for a number of years.

. Wich gives superior dp protection.

DP protection is not needed in standard UK domestic wiring fed from the public supply. It is not in any way superior in that situation.

 

[davesparks]

As James wrote: "In general an mcb (of the right trip characteristics) is better for most applications". dp even better.

I'll ask again, how is it better?

How is DP better?

 

[Steve-S]

So now fuses are better if they're provided with sufficient air circulation? You had the answer all along

Nope.

 

[Steve-S]

How is it far better?

You need to ask that?

 

[James]

Yes. Few know you can have a 16A load off a final ring. Needs to be fixed wiring, overcurrent protection and a switch. A double pole MCB covers it all off the ring and being the isolator for the 16A appliance.

But a 13A MCB can be used as well for 13A appliances instead of an FCU. Which gives superior dp protection.

I would have to check that but thought that if not prohibited it is certainly discouraged.

I don't understand why you are pushing a solution to a problem that rarely exists and more often than not, a better solution is available.

 

[DPG]

You need to ask that?

Just answer the question. You seem to struggle with that.

 

[James]

I'll ask again, how is it better?

How is DP better?

I prefer an mcb in MOST applications as you have a better control over the trip characteristics.

please don't take that as one is better than the other, they both have there uses and should be chosen correctly depending on that.

 

[davesparks]

I prefer an mcb in MOST applications as you have a better control over the trip characteristics.

please don't take that as one is better than the other, they both have there uses and should be chosen correctly depending on that.

I know you know the answers, I'm trying to get the OPs answer to try to understand what their point is

 

[davesparks]

You need to ask that?

Yes because I don't see any reason, for the type of circuit being discussed here,why DP protection is any better than SP protection.

I am trying to understand your point but cannot do that without you actually explaining it in a way that makes sense. If you cannot offer a clear, technical explanation of your side of the discussion then there's no point having the discussion.

 

[mainline]

Can you support that with evidence?
This was certainly true under previous versions of the regulations but as far as I can remember it has not been permitted for a number of years.

It's likely a 16A MCB fed from a ring could still comply with all of BS 7671's requirements.

DP protection is not needed in standard UK domestic wiring fed from the public supply. It is not in any way superior in that situation.

DP is superior in many ways as it gives safe isolation, guards against polarity reversal and also isolates neutral to earth faults.