The regs make a clear distinction between fault current and overload current. In situations where overload is not expected (e.g. a single fixed piece of equipment) the rating of the protective device can be above the rated current-carrying capacity of the cable, as long as the prospective fault current will cause it to break the circuit.
But something makes me uneasy about this: are the two scenarios really separate? Can you get a fault current that manifests as an overload - who is to say that the fault makes a low-resistance connection between L and N or L and E?
I'm not suggesting the regs are wrong, but I would like to understand why they are right.
In faults. You can also get a fault current which is less than the overload in which the equipment may not be safe. I've often wondered how you might plan for this, but looking back in time I seem to recall that some tvs, rather than simmer and burst into flames monitored themselves and if there was such a fault they shoved a short across the power supply to blow the fuse. Crowbar protection I seem to remember it beingcalled. OK until the fault detector went wrong and merely shorted everything out when you switched it on!
Yes, but it's no different really to whether the cable is protected at overload or fault levels.
If you have a L-N fault drawing 10A through a cable it will be dissipating 2.4kW of heat at that point. The 2.4kW of heat at the fault point is going to result in damage causing either an open circuit (current stops flowing) or a short-circuit fault (protective device operates) hopefully quickly enough that the heating effects on the rest of the cable are within the permitted levels. The fault protection therefore provides adequate protection against overload.
If the 10A were due to non-fault overload (too many appliances) it would usually be for a considerable time, so the protective device would have to prevent that (and the circuit has to be designed so that overload which is not cleared by the protective device should not occur.) Short-term overloads have little effect on cable heating.
The wiring regs use the slightly odd definition of 'fault current' as a dea d short. It is indeed possible to get faults within appliances that result in them running at above rated current. In fact this is fairly common with immersion heaters, and when they aren't RCDed it continues for months, some times years. But it doesn't normally result in a fire, electrical cable is rated for very long term reliability, not the currant at which hell breaks loose.
Makes sense. I suppose it's unlikely the heat generated at the fault location is enough to start a wider fire. Self-exstinguishing plastic and all that jazz.
Exactly. They explicitly assume negligible impedance, hence the PFC calculation.
That's actually the situation that prompted the question. Replaced an immersion heater that was still working but intermittently blew the fuse. Pulled it out and the element was split down the middle and, presumably, had been heating the water by passing current through it. Not uncommon.
When I had a look at the fuse, it was pretty charred and looked like it had blown many times. Not unreasonable given that immersion heaters are basically a consumable item.
But it got me thinking, what if the fuse had been rated much higher owing to it being a fixed piece of equipment? Or would immersion heaters not be exempt from overload protection for this very reason?
Indeed - its important not to confuse the regs use of the term fault (meaning L to N or L to E short circuit basically) with that of the common language usage of "a fault"
This is only something you do in certain restrictive circumstances. As an installer you would need to use knowledge of the load to ascertain if its a safe circumstance.
For example you might have two radials wired in 2.5mm^2 cable, each feeding a fixed 13A load via an FCU on the end - say a pair of immersion heaters. These both originate from a 32A MCB. The overload protection (such that is needed) is taken care of by the fuses in the FCUs. However there is in effect no overload protection for the cables themselves. However there is also no realistic way you could put either into a sustained overload. So all the MCB at the origin needs to is provide fault protection for the cables (i.e. nail through cable type of protection!)
Now if you were to omit the FCUs and just use flex connectors, this would be less satisfactory since there are failure modes (rare admittedly) of an immersion that can yield long duration over-currents.
Yes, but you are confusing regs "faults" with common english ones...
If the load has a realistic and likely possibility of overloading the cable, then this would not be a candidate for omitting overload protection at the origin.
So something like an electric shower that draws a nominal 42A, and the CU fitted has a range of MCBs that include only 40A and 50A devices. Fitting a 50A device to protect the circuit wired in 6mm^2 would IMO be acceptable, since showers don't typically fail by drawing 70A for long periods.
By being very particular about the language they use, and how terms are defined.
Immersion heaters should be on a 15A fuse or 16A mcb, which is only slightly above their run current, providing protection in this situation.
Even if one were wired directly to a 30A ring circuit its unlikely a fire would result, though as you point out the protection would be poorer. The charring you saw was in the fusebox, indicating fusebox deterioration rather than a problem with the cable.
In fact thinking about that - it does not really apply on 2.5mm^2 T&E anyway unless there are other derating factors at work (which you may have in an airing cupboard)
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