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As I said before, I find that a lot.
I've had countless appliances over the years going faulty and tripping the B32 breaker. Yet, when checking the 13A 1362, it's intact!
Saturday Night Discrimination Quiz/Guess
- Thread starter JohnW2
- Start date
As I said before, I find that a lot.
I've had countless appliances over the years going faulty and tripping the B32 breaker. Yet, when checking the 13A 1362, it's intact!
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Its also worth remembering that not all breakers are made equal, the tollerance is quite wide for the magnetic trip.
As long as a type C does not trip the mag at 5x In, but does at 10x it meets requirements for type C
I've seen a piece of industrial machinery which was alright for many years on a type 2 20A, throw a C 20A out when starting following a board change, it had to go back on a type D. As the range for type 2 is 4xIn to 7In compared to 5 -10 for C, Youd expect the type 2 to be more sensive, but the type 2 must have been towards the least senstive part of its range, and the type C towards the more sensitive part!
I've also seen a fault on emergency lighting circuit wired in 1.5 or 1mm twin and earth blow its self clear when the cable was crushed rather than trip the type 4 federal stablok... although it was a rather long circuit and I wouldn't put any money on the zs being in spec!
As long as a type C does not trip the mag at 5x In, but does at 10x it meets requirements for type C
I've seen a piece of industrial machinery which was alright for many years on a type 2 20A, throw a C 20A out when starting following a board change, it had to go back on a type D. As the range for type 2 is 4xIn to 7In compared to 5 -10 for C, Youd expect the type 2 to be more sensive, but the type 2 must have been towards the least senstive part of its range, and the type C towards the more sensitive part!
I've also seen a fault on emergency lighting circuit wired in 1.5 or 1mm twin and earth blow its self clear when the cable was crushed rather than trip the type 4 federal stablok... although it was a rather long circuit and I wouldn't put any money on the zs being in spec!
So have I, which is why I said that none of it particularly surprised me - as perhaps would have been more apparent if you had quoted what I said in full (rather than the slightly edited version above) (and the reason I put quote marks around 'surprising'), namely:
I was merely saying that the 'nearest to surprising' was that, of the devices which failed to operate when the B32 did, it was the one with greatest In ratio (compared to B32) which failed to operate.
Kind Regards, John
Very true and when (as in the case under consideration) we are interested in discrimination between a fuse and an MCB, we also need to remember that the fuse is entirely thermal.
With a magnetic trip, increasing the current beyond the threshold for tripping presumably has little effect on speed of disconnection (hence those vertical curves). With a fuse, however, speed of disconnection presumably carries on increasing as current increases. I would therefore expect that (in a case like we're discussing) one would be more likely to see the discrimination one 'expected' (on the basis of respective In figures) with ('sustained') very high fault currents than with more modest fault currents.
Furthermore, the particular situation we've been discussing is (or, at least could be) fairly unusual in that the fault is (or may be) non-persistent. Even without any protective devices, the fault current may only have lasted for a few milliseconds (as the metal blade sliced through the cable) - I would think very much favouring operation of a magnetic trip (almost regardless of its In, within reason) rather than a fuse.
Kind Regards, John
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Doesn't the fact that the 32A tripped indicate that it must be 'faulty' and over-sensitive rather than it being surprising that the other devices 'failed' to operate?
I don't quite understand why so many thought the 32A would trip before the 16A.
The fact that the 10A and 16A survived is, surely, how it should be.
My comments regarding the 30A causing the 5A fuse to blow 'instantly' (whilst not being enough to operate the MCB and mcb part of RCBO) was because, perhaps misguidedly, of the cable being cut which may not immediately result in a full short circuit.
I don't quite understand why so many thought the 32A would trip before the 16A.
The fact that the 10A and 16A survived is, surely, how it should be.
My comments regarding the 30A causing the 5A fuse to blow 'instantly' (whilst not being enough to operate the MCB and mcb part of RCBO) was because, perhaps misguidedly, of the cable being cut which may not immediately result in a full short circuit.
I don't really understand. Until some other device operated (or the fault self-terminated), there would be a very high current (>>32A) flowing through the MCB - so why do you think the MCB would be 'wrong' to operate? As below, are you perhaps (incorrectly) assuming that the fault current will rise gradually, rather than be very high (>>32A) from the outset?
Nor do I. In the face of a very high fault current, I would expect it to be fairly random as to which operated first - but I can't think of any reason for suspecting that it would be more likely for the 32A one to trip first.
In a perfect world, only the device with the lowest In would operate - but that obviously is not 'how it happens'. Such discrimination would mean that devices would be required only to trip if the fault current persisted for longer than the disconnection time (for same current) of any device with a lower In. There is no such requirement, and nor would it be practical/realistic to have such a requirement.
It sounds as if you were invoking something analagous to the 'classic misconception' about RCDs - i.e. the belief that a 30 mA RCD limits L-E fault current (e.g. through a person) to 30 mA. As you know, that would only be the case if there were a gradually increasing current - whereas, in reality, the 'full current' (determined only by Ohm's Law) will flow 'immediately' (and continue until the device operates). In present context, you seem to be assuming a gradually increasing current due to the L-N fault, such that device with the lowest In will operate before the current even rises to high enough to operate any of the others. That obviously is not how it is. An L-N fault will result in an 'immediate' fault current (usually of a few hundred amps) flowing through all of the protective devices, not a gradually increasing fault current. Which device operates first will be entirely dependent upon the operating characteristics of those devices (when 'a few hundreds of amps' is flowing through all of them).
Also note my recent comment that the sort of fault we've been discussing is somewhat atypical, in that it quite possibly persisted for only milliseconds before 'self-clearing'. That has the potential to alter everything.
Kind Regards, John
K
kai
Most importantly - at least a device operated quickly in order to clear a potentially dangerous fault within a few milliseconds - perhaps saving someone's life! 
As I said, it's very likely that the actual 'fault' probably cleared itself within a few milliseconds. What's perhaps interesting is that a good old-fashioned 5A fuse operated, as it would have done decades ago - thereby potentially saving life without the assistance of any 'new-fangled technology'!Most importantly - at least a device operated quickly in order to clear a potentially dangerous fault within a few milliseconds - perhaps saving someone's life!
Had no OPD operated, I agree that there would have been a persisting threat to life had someone picked up the live cut end of the cable. In that situation, had they touched both L and N conductors, then no protective device could have saved them, but if they touched L and were in continuity with earth then, although the devices which actually operated (OPDs) would have been of no use to them, hopefully the RCD and/or RCBO would have limited the duration of their shock, hence potentially saving their lives.
Kind Regards, John
I may be thinking that but not from wrongly applying the theory, more from what actually happened.
The fault current could have been limited by high resistance contact between a rusty blade and conductors.
Because the 10A fuse did not blow 'instantly' (but the 5A did) can we not therefore assume the current did not exceed 60A?
That is obviously possible, but we'll never know. What we do know is that the 32A MCB did operate in response to some current which flowed through it for the very brief time taken for the 5A fuse to blow ('instantly', as you put it). The most likely explanation is that the current during that brief period was, indeed, very high. Your theory seems to be that it could have been a much lower (<32A) current that caused the 5A fuse to blow but should not have (but did) cause the 32A MCB to operate. Whilst impossible to disprove, I would have thought that sort of malfunction of an MCB you are postulating (an MCB tripping at below its In) would be extremely rare - in fact, not something I have personally ever heard of happening.
I'm not sure that we can. As I've been saying, one has to take into account the duration for which the current flows, as well as its magnitude, and that has the capacity to complicate this situation, particularly if the fault itself persisted for only a very brief duration (fuse blowing is essentially a matter of I²t). Also remember that the MCB may have operated (correctly, in response to a high fault current) before the 5A fuse actually blew, and that the final stages of the 'thermal death' of the fuse occurred after current flow had ceased.
Kind Regards, John
was there any additional
other load current on that 32 amp mcb at the time from other circuits on the CU
other load current on that 32 amp mcb at the time from other circuits on the CU
No, there wasn't any other load at all. That MCB fed just the garage CU which, in turn, fed only the socket we're talking about (with just the hedge cutter pluigged in) and some lights (which, FWIW, were not on).
In any event, as I've said, whilst I accept the other possibilities that EFLI has postulated, I think it most likely that the brief fault current was very high (very many times the In of the B32 MCB), in which case it should not have made any significant difference even if the MCB were 'fully loaded' at the time.
Kind Regards, John
ok thanks john, realised after i posted you had said it was a MINI CU, and i didnt think .
its just we do a lot of pub work and we often get asked why the mcbs trip yet often the plug or FCU fuses dont blow, and on investigation the circuit mcbs are already loaded nearly at there max rating when the faults or surges develop,
so hopefully its fair to say an mcb already loaded at about 30 amps would trip faster than a slightly loaded mcb of the same rating and possibly faster operating than some fuse ratings, which in my case a majority of times are found well overrated at 13 amp
its just we do a lot of pub work and we often get asked why the mcbs trip yet often the plug or FCU fuses dont blow, and on investigation the circuit mcbs are already loaded nearly at there max rating when the faults or surges develop,
so hopefully its fair to say an mcb already loaded at about 30 amps would trip faster than a slightly loaded mcb of the same rating and possibly faster operating than some fuse ratings, which in my case a majority of times are found well overrated at 13 amp
That obviously would make total sense in terms of overload (not uncommon in pubs, I might suspect!), but I would personally doubt that it was much of a factor in relation to 'true' faults ...
As above, I would personally suspect that, in the presence of a 'true L-N fault' (very low/negligible impedance), it would make minimal, if any, difference to disconnection time. Even with a fairly modest true fault current (say 300A), operation would be so fast with a previously slightly loaded (or even totally unloaded) MCB that I doubt that it would have scope to operate appreciably faster if there were already 30A flowing through it before the fault arose. However, I may be wrong.
Of course, if (per EFLI's postulation) the fault current was much lower (more like an 'overload'), then what you suggest would certainly be true. Indeed, an additional 'fault current' of just 30A would result in the operation (eventually) of a B32 MCB that was already loaded with ~30A, but would definitely not result in operation of the MCB if it was unloaded prior to the fault. However, I would essentially regard that as an 'overload' situation, not 'fault' one.
Kind Regards, John.
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