Something has come up for discussion from the wiki.
First a little explanation as to why fuses & mcbs dont trip on all fault currents.
During high current faults, a certain amount of i squared times t is needed for a fuse to trip, and for mcbs there are published curves for the characteristic. There are plenty of cases where a brief dead short didnt blow a fuse for the simple reason that for the given i squared, t wasnt long enough for a blow. Most of us have seen this, and probably wondered why it didnt blow.
There are also low current faults that dont blow the protective device because the fault current is below the trip current threshold. This typically applies to faults consisting of charred plastic. Typically these grumble on for a while before either burning themselves open circuit, going high enough current to trip the device, or catching fire. (Even with very low power consumption they can get red hot, because of the small concentration of power and the time involved.)
There are 3 main causes of wiring fires:
high current parallel faults, ie dead shorts. These are what fuses & mcbs are designed to protect against, and they do so quite well.
low current parallel faults - fuses and mcbs have NO action agaisnt these slow smouldering situations, so fire is more likely
series faults, where current is limited by the load. Again fuses & mcbs have no protective action against these, so fires are more likely. On the upside, the end user gets to notice some of these faults due to intermittent drop in performance of the appliance.
Finally, the current in some parallel smoulderers is erratic, resulting in repeated trips at unpredictable times. MCBs are less safe than fuses with these because they can very easily be, and thus sometimes are, repeatedly reset, each time re-energising a fault that may cause a fire.
If we find some common ground here, hopefully this can migrate in som form back into the wiki.
NT
PS thinking of perhaps turning this into a 'fault current' article
Just for context, there is a section of the article on MCBs that seeks to compare MCBs with fuses (which as someone pointed out the other day is now linked to from wikipeadia):
Original wording:
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other day I tweaked this to remove some questionable bits regarding breaking capacity claims for fuses, and also dropped a couple of statements that I thought were either equally applicable to both and hence not really highlighting differences, or were ok but out of place in that particular section. The revised version:
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First a little explanation as to why fuses& mcbs dont trip on all > fault currents.
As I commented on the wiki talk page, one needs to take great care with terms like "fault current" that have a specific meaning and usage within the wiring regs. I understand what you are are saying, but qualifications like "high" and "low" don't really have a formal meaning in this context.
Its true that a very short duration fault may not trip the device. However I don't see this as a problem in itself - quite the reverse.
One can argue (and indeed it is mentioned IIRC) that a disadvantage of MCBs is the tendency to nuisance trip on things like bulb failures because of their increased sensitivity to brief shorts.
No offence intended, but this is being a bit sloppy with terminology again. "Fault current" (or PSC and PFC) should really only be used to imply short circuit faults between line and neutral or line and earth.
One of the points of using standardised circuit designs from the OSG, or going through a proper design exercise for a circuit is to make sure that should a fault occur (in the regs use of the word fault), that the protective device will clear it in adequate time to prevent cable damage.
Indeed, these are circuit faults, that do not usually result in either a fault current or an overload current being seen by the protective device. Most of this is addressed in the article:
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There are 3 main causes of wiring fires:
Not sure what you mean by parallel fault in this context....
But yes, agreed this is a fundamental reason for having circuit protection.
Quite possibly, the MCB article and the cable sizing one describe in some detail the overload characteristics of the devices, and how a general purpose circuit needs to be designed so that its cables are protected from the "allowable" overloads when necessary.
However neither fuses or MCBs posses clairvoyance as a trip mechanism, and so can't react to loose screws etc causing local hot spots. Not to say that this issue is ignored, but it is managed in other ways. For example the insistence that all junctions, and accessories etc have backboxes or an enclosure of some sort is one of the primary ways that local hot points are contained and kept out of contact of combustible materials. Cables are manufactured with flame retardants to ensure that the PVC will not sustain a flame etc, and guidelines are in place to use of alternative cable types like MICC when particular fire resisting properties are required. RCDs are also quite helpful here. In the rare cases that cable damage from a local hot spot migrates out the enclosure and begins to damage exposed insulation, they will often react to the fall in insulation resistance.
IME local high resistance faults, usually get noticed by damage to the accessory. The most common example being a shower isolator switch that simply stops working or jams. Socket fronts discolouring and getting very hot to the touch in use etc. Quite often in these cases the first few inches of cable either side of the termination is damaged with the inner insulation discoloured and a bit crispy etc. Bad cases usually eventually burn themselves open circuit in the end.
By the same argument, fuses can be replaced etc. So its not really fair to criticise a MCB for its ease of use. You could equally say that the type of person who would keep resetting a MCB that trips for no apparent reason is also the type likely to wrap tinfoil round a fuse - and so in that sense the MCB is safer since there is no way for the end user to change its trip characteristics.
I think many of these points are already covered, although as I commented on the talk:mcb page, there would be no harm in having an intro section on the fuse and mcb articles that dispel some of the common misunderstandings. I am sure there are plenty of folks who assume (quite logically) that if it says 13A on the side, then it will blow when you try to draw 14A etc.
In the regs meaning of the term, I thing that has been done to death in quite a few articles. In other meanings, it needs a different title IMHO ;-)
Actually, none of these are the main causes of wiring fires. Connections deteriorating and increasing resistance is the main cause. Can be caused by poor workmanship, or poor quality wiring accessory, or a wiring accessory which should have been replaced years ago.
Called a "Fault Current".
The power dissipated at the scene tends to quickly turn them into shorts, or blow them into open circuits.
Are you referring to "Overload Current"?
It would help if you used the real names for the conditions. A "Fault Current" happens when you get a short circuit or similar, such that the current flowing is mainly limited by the supply impedance or earth fault loop impedance. An "Overload Current" happens when the circuit is passing more current than it's safely capable of doing, but the current is still limited by the load and the circuit is otherwise not faulty.
Sorry, but this is rubbish. Fuses were less safe because people either fitted higher current wire, or in the case of a cartridge fuse where they were unlikely to have a spare, refitted it covered in aluminium foil.
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