Over-current protection & the fusing factor.

[david911cockburn]

123,

Do you not agree that 13amps is the maximum current rating of a 13amp plug?

I will assume that you do, therefore why would you assume that the nominal rating of a fuse carrier is anything more than its' maximum current rating?

Have you compared a 32amp BS 3871 circuit breaker to 20amp fuse wire?

 

[bernardgreen]

I can see the point that Mr Cockburn is making. 30 amp fuse wire can carry far more than 30 amp for a significant period of time and it is not impossible that the cable could be compromised by that "excessive" current.

If the maximum design load is 20 amps and the cable's rated (*) current is 30 amps then fitting 20 amp wire in the fuse holder would make sense.

(*) what is the parameter that determines "rated" current for a cable ?

Obviously the most common rating is based on how much heat the cable can disipate as shown by current rating determined by method of installation.

But a cold cable could take a current 50 or more times the rated current for a short duration as the thermal mass of the copper will absorb the heat generated.

The fuse / MCB reaction time has to be such that the overload it passes before blowing / operating has to have a time and current that is less than the thermal mass of the cable can absorb before getting to hot. A cable that is already warm from carrying its rated current will have less abilitiy to absorb into its thermal mass the energy from short duration over loads.

So maybe there is a point in using fuse wire based more on the maximum expected load than on the maximum "rated" current of the cable.

 

[david911cockburn]

Bernard,

As you know the advice right from the beginning has always been that "cabling should not be subjected to small overloads of a long duration", or the cable will be prematurely damaged.
30amp fuse wire will allow for small overloads of a long duration to occur in a domestic 2.5mm T&E ring main!
It is the complicated nature of fusing arrangements like this, that presumably prompted the invention of the MCB for domestic use (as opposed to the adjustable circuit breakers that were previously available); and as I have said "compare a BS 3871 32amp MCB to 20amp fuse wire".

 

[JohnW2]

I can see the point that Mr Cockburn is making. 30 amp fuse wire can carry far more than 30 amp for a significant period of time and it is not impossible that the cable could be compromised by that "excessive" current.

We've been through all this before. The tabulated values of current carrying capacity which are deemed to be safe take into account the fact that the current may be up to 1.45 times the rated value for short periods - and are therefore applicable to circuits protected by Type B MCBs (and some types of fuses). For other types of fuses, one has to apply a rating factor to the tabulated CCCs - e.g. a factor of 0.725 for a BS3036 fuse ... see section 4 of Appendix 4 of the regs.

Kind Regards, John.

 

[david911cockburn]

No John,
1.45/1.5 is a fusing factor applicable if you have a starting current to consider, when your over-current protection is a fuse.

When over-current protection is provided by a circuit breaker, you simply use a B/C/D Type breaker as appropriate.

Again this is simple really.

mattie,

Somebody has closed the other thread down, therefore regarding the misuse of the word 'radial';
Please look up 'radial circuit', 'radial feeder' and 'stub/stub end feeder' in an electrical engineering dictionary.

 

[JohnW2]

No John, 1.45/1.5 is a fusing factor applicable if you have a starting current to consider, when your over-current protection is a fuse.

Utter rubbish. 1.45 is I2/In for a Type B MCB. Goodness knows why you think 'starting current' has got anything to do with it.

Regards, John.

 

[david911cockburn]

John,

Starting current can exceed full load current by quite a considerable amount, which can rupture a fuse that is chosen to accomadate full load current.

 

[JohnW2]

John, Starting current can exceed full load current by quite a considerable amount, which can rupture a fuse that is chosen to accomadate full load current.

No argument with that - but it's nothing specifically to do with the 1.45 factor I'm talking about. A Type B MCB will allow a current of potentially up to 1.45In to flow before operating, regardless of whether that is a 'starting current' or a constant (over)load. Whatever, the point I was making to Bernard (not you) was that tabulated values of safe current carrying capacity take that into account.

Regards, John.

 

[westie101]

Starting current can exceed full load current by quite a considerable amount, which can rupture a fuse that is chosen to accommodate full load current.

So for a direct online starting motor it is commonly accepted (or has in the industry for the 38 years I've been in it) that the initial starting current is 7 times the normal running current.

So Mr Dyson makes some vacuum cleaners with 1.3kW motors which (not taking into account power factor) have a running current of around 5 amps on 230V.

So the starting current is 35A.

So that is rather higher rated than the flex, the 13A plug and socket the fuse in said plug. And if on a radial circuit probably more than the continuous ratings of any of the circuit.

So why does nothing operate due to the overcurrent?

 

[bernardgreen]

I haven't ( and am not going to ) measured the starting current on our Dyson but I believe the Dyson motors are designed to be soft start and therefore do not pull a very heavy starting current.

That said the lights on the same consumer unit do noticably "blink" when the Dyson starts so the start up must be significant. Perhaps the start up is designed to be ultra fast and be completed before the average domestic MCB can operate.

 

[securespark]

I'll tell you what Ricey, you summarize EEBADS for me and if I think its' any good I'll put it at the end of my Green book.

Dave does seem rather stuck in his 16th Ed., doesn't he?

Mind you, with "ring mains", what Ed. is he stuck in there?

The chap I did my apprenticeship with called them ring mains and he started under the 13th....


You never stop learning, and anyone who thinks they have tends to fall flat on their face and make a fool of themselves.

In case you don't have a copy of the 17th, it's "ADS" and "ring final circuit" these days.

Oh, and I'm sorry to inform you that the IET have taken supplementary under the kitchen sink out of the regs now. Mind you, if you're stuck in the 16th, you'd know that. It was taken out in 1991.

 

[Spark123]

123,

Do you not agree that 13amps is the maximum current rating of a 13amp plug?

No, it is the maximum design current for continuous use. As others have said, 13A may be exceeded for short periods.

I will assume that you do,

therefore why would you assume that the nominal rating of a fuse carrier is anything more than its' maximum current rating?

As long as the fuse carrier is rated at least that of the fuse what does it matter?

Have you compared a 32amp BS 3871 circuit breaker to 20amp fuse wire?

Yes, one is a lump of plastic and one is a piece of wire, they look completely different.
As for the regs, I don't know whick edition you are looking at but BS3871 breakers haven't been in there for ages.
As long as I know I need to apply a correction factor to the selection of a cable where the ocpd is a BS3036 as it says in the appendix of BS7671 and I don't for an ocpd which has a fusing factor of 1.45 then why even bother comparing them.

 

[matt1e]

mattie,

Somebody has closed the other thread down, therefore regarding the misuse of the word 'radial';
Please look up 'radial circuit', 'radial feeder' and 'stub/stub end feeder' in an electrical engineering dictionary.

Why? I think you have me mixed up with someone else mate

Matt

 

[david911cockburn]

OK,
Have you compared the time/current characteristics of a 32 BS 3871 MCB and a piece of 20amp fuse wire?

A Dyson, along with many other machines can start at very high currents if they need to. But this current will very quickly drop, it is only the point at which the fuse wire or circuit breaker becomes vulnerable to over load that we are concerned with.

Matt,
So you have now looked them up have you?

 

[david911cockburn]

Why would you claim that local supplementary bonding around a metal sink wasn't required in 1991?