Fuse v MCB

[AndyPRK]

A stupid question.

Of the same rating, What will trip/blow first ?

A fuse or a type C MCB (or type D for that matter).

Do all (working) MCB's trip sooner than a fuse blows ?

Why did they change the standard of the ratings ? e.g
Why did 5A >>> 6A
30A >>> 32A
etc

 

[RF Lighting]

It depends on the nature of the fault, but for a dead short on a supply with a high short circuit capacity, there will not be much in it, but the rewireable would probably disconnect fastest.

An MCB of the same rating on the same circuit would probably still trip too, but it is limited to how fast the mechanical parts can move, where as the fuse wire will melt as fast as the current flow will allow. (We are talking milliseconds difference here)

On an sustained overload of say 30A on a 15A fuse / 16A MCB, the MCB would trip in about 230 seconds, but the rewireable fuse would take about 300 seconds

A breif surge such as a motor starting would generally not operate a BS 3036 fuse, but it may well trip a type B MCB.

I beleive MCB ratings were slightly changed from fuse ratings to keep the actual operating characteristics of both devices as close as possible

 

[DaveSinner]

rewireables generally perish as well, so the older they are the less strength/ heat resisitance they have generally. but the above post basically answers your question

 

[Adam_151]

Thats a hard question to answer, becuaseit depends on what the situation is and what type of fuse

If we are talking sustained overload, then a circuit breaker will certainly react faster than a semi-enclosed fuse, but then so will a cartridge fuse... I suspect the breaker will be faster than the cartridge fuse, but I don't think the standards require any difference... both will trip between 115% and 145% overload in the conventional time, but the breaker is 'probably' more sensitive.

If we are talking fault currents, then thats another can of worms, if you don't hit the Zs for the magnetic part then your circuit doesn't acheive 0.4 or 5 sec disconnection time... the thermal part will disconnect it sometime, but we can't really say when (but the manufacturer just might), with a fuse depending on what its feeding, if we are allowed 5s disconnection time, then we have a higher max zs to work to. So again the breaker is faster if Zs is in spec, but if its not, if you decided you only need 5s, for a fuse it might still comply, but for a breaker you're stuffed (the magnetic part trips in 0.1 seconds when it does, but if it doesn't, it doesn't!)

Now if we are talking about a fault current of large magnatude, then the fuse will probably be faster... the amount of energy needed to clear a fuse is more or less constant, and the bigger the fault, the faster it blows, but for a breaker, the contacts move at a more or less constant speed, regardless of fault current, meaning the let through energy rises in proportion to the square of the fault level

Ensuring total discrimination at all fault levels beween fuses and breakers isn't an easy task

As to the ratings, They changed just before BSEN60898 took over from BS3871, so that would put it as being the early '90s

 

[AndyPRK]

Thanks for the replies.
Not so much of a stupid question than I thought then.

The reply below, was my concern.

If we are talking fault currents, then thats another can of worms, if you don't hit the Zs for the magnetic part then your circuit doesn't acheive 0.4 or 5 sec disconnection time... the thermal part will disconnect it sometime, but we can't really say when (but the manufacturer just might), with a fuse depending on what its feeding, if we are allowed 5s disconnection time, then we have a higher max zs to work to. So again the breaker is faster if Zs is in spec, but if its not, if you decided you only need 5s, for a fuse it might still comply, but for a breaker you're stuffed (the magnetic part trips in 0.1 seconds when it does, but if it doesn't, it doesn't!)

So if some cowboy was to change a CU with fuses to a CU with MCB's without any testing of Zs. Would they make it less safe.
Assuming correct ratings were used.

I guess this happens quite a lot when fuses are changed with plug in MCB's

 

[RF Lighting]

It's possible, but it is unusual in a domestic situation to find Zs values too high for a type B replacement of an existing circuit.

Testing is a must though, as neither device will work if the earth has got lost somewhere along the circuit

 

[ericmark]

There are some places where fuses are the only option. Semi-conductors being one where only a fuse can clear quick enough to save the semi-conductor so items like Induction Hobs will be protected by a fuse.

However saying that there are many types of fuse and all have different uses. There are fast blow and slow blow and different voltages and what is called "Let Through" amps this is the value needed for next protection down the line.

When we needed to charge radio batteries from the supply to a tower crane we had to use a fuse because the Ze was far too low to use a MCB the PFC was over the 10,000A rating of a MCB and only other way would be using a Moulded Breaker something like this

In domestic the let through amps of the fuse in the head means there is no problem using MCB's although not sure on RCBO's as these are often 4700A and I am not sure of let through amps of the fuse in the incomer.

The re-wirable fuse can't be used in Domestic consumer units as they have to be under the control of trained personnel but for firms employing Electricians they can still use them. Most have been replaced because of asbestosis used in them rather than any electrical problem. But the old consumer units with either cartridge or rewirable fuses rarely went open circuit due to a bulb failing but the MCB does seem to be more prone to open when a bulb blows.

The original MCB

was a pure thermal device and for a near short circuit they were rather slow. The more modern MCB combines both thermal and magnet devices and the magnetic device is 5, 10, or 20 times the rating (B, C, or D) of the thermal part so unlike the fuse there is a sharp cut off point when the magnet part will not operate and there is a big time delay before the thermal part will work.

This is why the earth loop impedance is so important. If you look at a 32A fuse where a Zs of 1.04 ohms is required to open it in the prescribed time if you went over to say 1.5 ohms it would still open but just take a little longer. However with a MCB needing 1.44 ohms to open at 1.5 ohms it could fail to work the magnetic part and revert to thermal part and opening time could be many times the permitted time as a result.

With a MCCB (Moulded case circuit breaker) often the user can fine tune the current settings this is only permitted where the user is trained so they are not allowed for domestic use. Motor control circuit breakers

also have adjustable settings so are not permitted with domestic.