Over-current protection & the fusing factor.

[david911cockburn]

Hi All,

Why is it that when calculating the size of over-cuurent protective devices we must use a fusing factor, twenty years after we all stopped using fuses?

 

[holmslaw]

..

 

[david911cockburn]

OK,
I'll re-phrase the question:
Why are we using a fusing factor when calculating the size of circuit breakers and RCBO's?

 

[bernardgreen]

Instead of being rude why not provide a LOGICAL and reasoned argument in answer to the question.

Even the most irrational questions deserve a reply explaning why they are irrational and explained in a way that a lay person can understand.

Being agressive and rude can be interpreted as meaning you have no logical answer to the question. That is a pity if your answer would have been logical and correct had you given it instead of being rude or otherwise trying to make the question seem irrelavent and not worthy of an answer.

 

[holmslaw]

..

 

[westie101]

MCBs?, RCDs? the DNO's don't use them, all we use at LV is fuses so fusing factors are very important to us!

Once you get away from domestic supplies you may/will have to grade with our fuses so these factors are essential. (other wise we start charging if our fuses operate before your protection does)

 

[JohnW2]

Why is it that when calculating the size of over-cuurent protective devices we must use a fusing factor, twenty years after we all stopped using fuses?

Are you just worried about the words "fusing factor" (which are obviously there for historical reasons)? Whether you are using fuses or MCBs, some measure of the I2/In ratio (i.e. the amount of overcurrent needed to operate the device within an hour) is obviously important, whatever you choose to call it.

Kind Regards, John

 

[mikhailfaradayski]

lol, Bernard, John and cocky all posting in the same thread.

Whatta y'reckon? 30-50 pages?

I'll put my 50p on 38 pages

BTW, wheres sheddy at the moment? Strange co-incidence?

 

[Spark123]

What do you think a fusing factor is Mr Cockburn?
Why do you think you need to use fusing factors when calculating the required protective devices size?

 

[flameport]

Why is it that when calculating the size of over-cuurent protective devices we must use a fusing factor, twenty years after we all stopped using fuses?

Unfortunately, any answer to that would be beyond your comprehension.

 

[Stoday]

The adverse comments on holmslaw's apt post suggest that his subtle and amusing joke went completely over their heads.

C'mon can't you spot a wind-up by a troll?

 

[bernardgreen]

The adverse comments on holmslaw's apt post suggest that his subtle and amusing joke went completely over their heads.

C'mon can't you spot a wind-up by a troll?

How's your dick to-day, do you need some more ointment?

Sorry, I didn't realised the target owned a Richard

 

[david911cockburn]

Hi All,
The 're-phrased' question was/is why are we using a fusing factor when calculating the size of circuit breakers and RCBO?

Therefore to anybody using fuses and suppling rotating machinery or any machinery with a high starting current, this question obviously is not for you. But this doesn't mean that you won't be interested in what is happening out there, or that your input will not be valuable.

It has become common practice recently to protect 4mm T&E with a 32amp circuit breaker or RCBO.
4mm T&E has a maximum current carrying capacity of 36amps and yet a 32amp circuit breaker/RCBO doesn't begin to operate (I2 value) until the load reaches 46amps!

 

[holmslaw]

..

 

[JohnW2]

4mm T&E has a maximum current carrying capacity of 36amps and yet a 32amp circuit breaker/RCBO doesn't begin to operate (I2 value) until the load reaches 46amps!

The situation is actually potentially 'worse' than that with a 2.5mm² ring final protected by a 32A device, since a load near one end of the ring can result in a high proportion of the current travelling down one leg of the ring.

You are not alone. I asked esentially the same question a few weeks back, since it also seemed a bit crazy to me. What I hadn't realised (and you clearly don't realise, either) is that the tabulated 'current carrying capacities' of cables include a built in margin such that they are deemed to be able to safely carry 1.45 times (i.e. the I2/In for a Type B MCB) the tabulated ('rated') current for at least an hour. . If protection is by a fuse which has an I2/In appreciably greater than 1.45, then the cable has to be 'de-rated' accordingly [hence 'fusing factor'!]

Kind Regards, John