Over-current protection & the fusing factor.

[david911cockburn]

Hi JohnW,

Who told you that?
Because I'm afraid it not the truth!
The use of a fusing factor will simply allow you to absorb a current spike on start up, when over-current protection is to be supplied by a fuse.
If you intend to use a circuit breaker/RCBO to protect a machine with a high starting current, you simply use a C or D Type circuit breaker/RCBO of the same nominal current value.
Simple!

We are talking about over-current protection for general purpose socket outlets, which will be supplying constant steady loads for the next 25 years or so. A faulty appliance drawing 45amps will start to overheat 4mm T&E in 100 seconds (approx), but a 32amp circuit breaker/RCBO will allow this to carry on indefinately. The result will be that the cable will stretch, therefore its' CSA will reduce and its' current carrying capacity will be reduced even further. Use the same faulty appliance again and the cable will be less able to carry the load, become hotter and the problem will begin to spiral, eventually causing fire, accusations of negligence and at worst prosecution for manslaughter!

So exactly who told you this rubbish about current carrying capacity tables being wrong by a factor of 1.45, because if you look back over the history of the regulations it is obvious that this is untrue!

 

[JohnW2]

So exactly who told you this rubbish about current carrying capacity tables being wrong by a factor of 1.45, because if you look back over the history of the regulations it is obvious that this is untrue!

No-one said the tables were 'wrong'. As I understand it, they are correct 'continuous' ratings, but a short-term overloading, corresponding to the I2 and associated disconnection time for a Type B MCB, is considered/deemed to be acceptable. On reflection, that seems to be reasonable; it would be unnecessary, and a nuisance, to have a requirement for devices to disconnect circuits immediately the continuous current rating of the cable was exceeded, even if only slightly and briefly,

I have explained the situation as it was explained to me. I will leave others to argue with you about whether or not it is rubbish.

Kind Regards, John

 

[david911cockburn]

Hi again JohnW,
Your not far from the truth, if you look at your time current characteristics graphs you can see what the cables are capable of coping with.
As long as your current carrying capacity is greater than the I2 value of your over-current protective device, your cable will not be over-heated when suppling steady loads.
Eg. a 25amp circuit breaker will cope quite happily with 3 x 3Kw loads for up to an hour and a 2.5mm ring final circuit, although above its' tabulated current carrying capacity will not be overstressed during this period.
But a fusing factor can only safely be factored in when supplying loads with a starting current that will be high for a matter of seconds, which is a period of time in which the cable will not have the time for its' circuit conductor temperature to be increased to above its' predicted full load circuit conductor temperature.

PS.
Since this was pointed out to the 'powers that be' their reaction has been, the planned removal of the 'time current characteristics' graphs from the Regs!
Are you now beginning to see what type of people we are dealing with here?

 

[ricicle]

TROLL

 

[david911cockburn]

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.

 

[ricicle]

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.

Why would I want to - it doesn't exist. And I wouldn't lower myself to your level anyway.

Now go away Troll, you have had your laughs

 

[ban-all-sheds]

Hi again JohnW,
Your not far from the truth, if you look at your time current characteristics graphs you can see what the cables are capable of coping with.

No - you'll see the operating characteristics of the protective device.

As long as your current carrying capacity is greater than the I2 value of your over-current protective device, your cable will not be over-heated when suppling steady loads.

Indeed not.

Or we could require that the ccc is greater than I2 x 2, that also would ensure that the cable will not overheat when supplying steady loads.

But the regulations don't require that, nor do they require Iz to be greater than I2, they require 1.45 x Iz to be ≥ I2.

But a fusing factor can only safely be factored in when supplying loads with a starting current that will be high for a matter of seconds, which is a period of time in which the cable will not have the time for its' circuit conductor temperature to be increased to above its' predicted full load circuit conductor temperature.

That is not what the regulations say, and they have not said it for several decades.

Are you suggesting that they have been wrong all of that time?

Since this was pointed out to the 'powers that be' their reaction has been, the planned removal of the 'time current characteristics' graphs from the Regs!

Was it?

Who pointed it out?

Who were "the powers that be"?

What was their exact reply?

Some kind of proof of all of this would be very useful.

And is the plan to remove the graphs from the regulations nothing to do with the fact that they available from the makers of the protective devices and/or in other standards publications and it would make more sense to defer the publication and change control to the people responsible for those standards?

Are you now beginning to see what type of people we are dealing with here?

If you are talking about Technical Committee JPEL/64 we're dealing with people with a great deal of experience and expertise.


Some of those people are active on the IET Wiring Regulations forum - why don't you take your argument there and see what replies you get?

 

[Spark123]

Hi JohnW,

Who told you that?
Because I'm afraid it not the truth!

Actually it is very close to the truth. The only bit that I would say isn't is the mention of fusing factor in the last sentance of the paragraph, I believe it should be correction factor.

The use of a fusing factor will simply allow you to absorb a current spike on start up, when over-current protection is to be supplied by a fuse.
If you intend to use a circuit breaker/RCBO to protect a machine with a high starting current, you simply use a C or D Type circuit breaker/RCBO of the same nominal current value.

Really?? I suggest you look up what a fusing factor is - even try google.

Simple!

 

[stillp]

Since the poster purporting to be Mr Cockburn is either a troll or a dangerous deluded fool (or even both) why are we continuing to give him the oxygen of publicicty by responding to his misguided nonsense?

 

[JohnW2]

Hi JohnW,
Who told you that? Because I'm afraid it not the truth!

Actually it is very close to the truth. The only bit that I would say isn't is the mention of fusing factor in the last sentance of the paragraph, I believe it should be correction factor.

Indeed so - and that is why, although I mentioned 'fusing factor' (in deference to the thread title), I put it in quotes. I gather that it's probably not been officially called that since 14th edition, but the phrase is still extensively used (e.g. by BAS, when he was explaining all this to me - see his 02 FEb 2:30am post in here) and I think we all understand what it means.

Kind Regards, John.

 

[david911cockburn]

Hi All,
Using a 'fusing factor' is just common sense.

Can you not see for yourselves that using a cable with a 36amp maximum current carrying capacity and 'protecting' it with an over-current protective device that doesn't begin to operate until the load reaches 46amps is potentially dangerous?

There have over the years been may attempts to get this right, in recent years it has been with the use of 'correction factors', which worked very well with British Standard over-current protective devices (as long as you only used your 'fusing factor' when it was actually needed, that is supplying machinery with a high starting current). Unfortunately BS EN over-current protive devices have a higher I2 value than their BS predecessors and the IET/ESC and others, not understanding their principles, have failed to react to the change in the same way that they did not react to the change when copper water pipe changed to plastic.

 

[ban-all-sheds]

Hi All,
Using a 'fusing factor' is just common sense.

Something you are devoid of.

Can you not see for yourselves that using a cable with a 36amp maximum current carrying capacity and 'protecting' it with an over-current protective device that doesn't begin to operate until the load reaches 46amps is potentially dangerous?

I suppose it might be.

Unless the "36A" rating assumes the cable is protected by a device with a given characteristic?

i.e. a tabulated value of 36A means that the cable is safe to be used with a device with an I2 of no more than 52.2A?

Either that or the Wiring Regulations have been fundamentally wrong and unsafe for decades. Are you suggesting that they have been wrong and unsafe all of that time?

You'll find some of the people to blame on the IET forum - please take your complaint there, and see what they say.

Let us know when you do, as I'm sure people here would benefit from reading the responses.

the IET/ESC and others, not understanding their principles, have failed to react to the change in the same way that they did not react to the change when copper water pipe changed to plastic.

I'm sure that they will value your input on both of these matters.

Please go to http://www.theiet.org/forums/forum/categories.cfm?catid=205 and express your concerns, and let them know that they are wrong - you'll be doing millions of people a great service by putting an end to dangerous installation design.

Really, if you think about it, it's your absolute duty to go to the IET, who can do something about it, rather then come to a DIY site that can't.

 

[david911cockburn]

Hi Sheds,

I don't wish to sound paranoid but the IET have a history of 'editing' what I have to say in order to make it un-readable, but I will give it some thought.

 

[bernardgreen]

Either that or the Wiring Regulations have been fundamentally wrong and unsafe for decades. Are you suggesting that they have been wrong and unsafe all of that time?

My opinion is that there were and maybe still are situations where the wiring reqs were overtaken by changes in the way electricity is used in the home and that until the regulations were ammended to take account of those changes the regulations were AT TIMES not going to ensure safety. At times they might, maybe through mis-interpretation of badly worded paragraphs, have increased the dangers.

The example that springs to mind is TT supplies with voltage operated earth leakage breakers that operated if the CPC rose to 50 volts above true ground. These worked well provided that the CPC was INSULATED from true ground. Even if the CPC was connected to true ground the breaker would still limit the voltage on the CPC to a "safe" 50 volts.

But if the CPC was connected to true ground by a 2 ohm path then to reach 50 volts and trip the breaker there would need to be 25 amps flowing through that 2 ohm path to true ground. That level of current and power ( 750 watts ) would present a significant hazard of heat and fire or melting of the CPC which would disconnect the breakers CPC connection to the fault preventing the breaker from sensing the fault.

The low impedance connection of CPC to true ground first appeared with immersion heaters and metallic water supply systems. Then gas boilers with electrical controls. That was the end of the era when safe use of voltage operated earth leakage breakers could be ensured. So they had to be declared illegal for domestic use.

The question is how long was it before some one realised that fitting an immersion heater would dis-able the main safety device ? And how was that discovered ?

 

[ban-all-sheds]

Surely immersion heaters pre-date VOELCBs?

But in any event - what you're describing is something a lot more complex than the rated ccc for a cable.....