Ever thought about ... ?

HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.

Interesting. Do you have numbers for BS 3036 fuses? (I (or rather Google) have failed to locate any so far).


Given that the permitted Zs values for a rewireable are if anything a little higher than for either a BS88-3 (e.g. according to table 41.2) I would have though BS 3036 ones would have if anything slightly quicker disconnection times for the same current than an BS 88-3 and therefore (broadly) a lower I²t.

 

Thanks, Andy, for the comment a couple of days ago with BS3036 I²t inferred from Fig 3A2 of BS7671. And Mike for the link to melting resistors: it's a good approach for keeping thecurrent low so it's easy to break, and I hadn't seen this component before. Over the weekend I had some login trouble with the IET site so I didn't get a chance to reply earlier.  I agree (Dave) that values of even 2 kA would be unusually high among all the installed population of sockets, and that faults in the electronic parts themselves could be made self-handling.  How many designers actually do consider this, even for a high current circuit with a slow protection device, and what do product standards require? Some wall warts I've seen are well sealed internally.  Others do have capacity for flying leads from pins to PCB to become loose and touch. I've seen cases of loose leads, probably after dropping. I'm not confident what worst case would happen for arcing duration with a strong source. Teenage memories of shorting a BS1363 plug, putting some strands of copper wire across its fuseholder, plugging it into a cooker socket (BS3036, 30A) and turning on with a stick, are that the arc in the plug blew itself out without blowing the fuse, though it left plenty of sign of its presence. That makes me more confident than I'd otherwise have been about the wall warts, but not that it would always be ok with internal arcing. 


Anyway: the following is about the semi-enclosed fuses, which I and Graham seem to have assumed would do worse than many other devices. Is that justified...?


I did, finally, find the text of BS3036. In it I see no requirement on let-through, nor on the detail of the time/current curve. It requires a fusing factor not greater than 2. And that the manufacturer shall provide a time/current plot over a range at least including 5% above the minimum fusing current and the 0.5 s fusing current. It's a far less tight specification than is found in standards for modern cartridge fuses, which typically require a t/I curve to fit between specified points, and total and pre-arcing let-through to be within some limits. Of course, a manufacturer could provide further specifications for semi-enclosed fuses, such as let-through versus prospective current, but I haven't found any example.  


Fig 3A2 in BS7671 mentions 'maximum' disconnection times. The times given actually look rather low even for just the melting time. BS7671 Table 533.1 shows 0.85 mm diameter tinned copper for a 30 A fuse, whereas I need to come down to about 0.80 mm copper if calculating adibatic melting of the wire in 0.1 s with 450 A. Perhaps the tinning is expected to be thicker than the few-micron sizes I saw in references about other tinned copper. 

The time that BS3036 wants used in manufacturers' time/current plots is the pre-arcing time. An appendix in BS3036 gives an example oscillogram from operation on 'moderate prospective current', in which arcing continues for 2 cycles with little reduction in current. So it seems the arcing period shouldn't be considered negligible in its effect on duration and let-through when one comes into the hundreds of amps range (which is the range I assume the plot means: low enough to have several cycles for melting, but high enough to have significant arcing). This fits with what I'd expected for semi-enclosed fuse operation in short circuits. Yes - melting times of fuses tend to a constant let-through at high currents. And good quality HRC fuses don't add much arcing let-through at prospective currents so low (by their standards) as 1 kA. But a couple of cycles of arcing of a semi-enclosed fuse at 1 kA could mean another 40 000 A²s just from the time of arcing. I'd risk a guess that the arcing period would not get shorter for higher current. If we move towards the 6 kA case mentioned earlier, the BS3036 fuses would be out of their depth, and presumably the let-through could be the value for the upstream cutout. In contrast, a good cartridge fuse or modern MCB will typically cut the current in a half cycle or less. This is partly why I'd (still) expect the semi-enclosed fuse to have high let-through at currents such as 1 kA. Also, because its element doesn't have the carefully formed restrictions of a cartridge fuse, I'd expect it to be a good deal slower at melting when coming up to the 1 kA level. Unfortunately, there don't seem to be specifications for BS3036 fuses at these higher currents that give times <0.1 s, and even the BS7671 value for 0.1 s may be rather low and not include arcing time. 


As you'll see, this is mainly curiosity about how BS3036 fuses really behave, as an aside to the wall warts.  Having become used to diazed fuses, which are about 5 for a pound, always rated for prospective 50 kA, and with a nice low fusing factor like 1.45, I'm a bit suspicious of the venerable semi-enclosed things. 


I don't remember having met a 45 A BS3036, but I see they existed, so perhaps there's somewhere a BS1363 socket on a cooker controller with such a beast behind it.  

HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.

Interesting. Do you have numbers for BS 3036 fuses? (I (or rather Google) have failed to locate any so far).


Given that the permitted Zs values for a rewireable are if anything a little higher than for either a BS88-3 (e.g. according to table 41.2) I would have though BS 3036 ones would have if anything slightly quicker disconnection times for the same current than an BS 88-3 and therefore (broadly) a lower I²t.

 

Thanks, Andy, for the comment a couple of days ago with BS3036 I²t inferred from Fig 3A2 of BS7671. And Mike for the link to melting resistors: it's a good approach for keeping thecurrent low so it's easy to break, and I hadn't seen this component before. Over the weekend I had some login trouble with the IET site so I didn't get a chance to reply earlier.  I agree (Dave) that values of even 2 kA would be unusually high among all the installed population of sockets, and that faults in the electronic parts themselves could be made self-handling.  How many designers actually do consider this, even for a high current circuit with a slow protection device, and what do product standards require? Some wall warts I've seen are well sealed internally.  Others do have capacity for flying leads from pins to PCB to become loose and touch. I've seen cases of loose leads, probably after dropping. I'm not confident what worst case would happen for arcing duration with a strong source. Teenage memories of shorting a BS1363 plug, putting some strands of copper wire across its fuseholder, plugging it into a cooker socket (BS3036, 30A) and turning on with a stick, are that the arc in the plug blew itself out without blowing the fuse, though it left plenty of sign of its presence. That makes me more confident than I'd otherwise have been about the wall warts, but not that it would always be ok with internal arcing. 


Anyway: the following is about the semi-enclosed fuses, which I and Graham seem to have assumed would do worse than many other devices. Is that justified...?


I did, finally, find the text of BS3036. In it I see no requirement on let-through, nor on the detail of the time/current curve. It requires a fusing factor not greater than 2. And that the manufacturer shall provide a time/current plot over a range at least including 5% above the minimum fusing current and the 0.5 s fusing current. It's a far less tight specification than is found in standards for modern cartridge fuses, which typically require a t/I curve to fit between specified points, and total and pre-arcing let-through to be within some limits. Of course, a manufacturer could provide further specifications for semi-enclosed fuses, such as let-through versus prospective current, but I haven't found any example.  


Fig 3A2 in BS7671 mentions 'maximum' disconnection times. The times given actually look rather low even for just the melting time. BS7671 Table 533.1 shows 0.85 mm diameter tinned copper for a 30 A fuse, whereas I need to come down to about 0.80 mm copper if calculating adibatic melting of the wire in 0.1 s with 450 A. Perhaps the tinning is expected to be thicker than the few-micron sizes I saw in references about other tinned copper. 

The time that BS3036 wants used in manufacturers' time/current plots is the pre-arcing time. An appendix in BS3036 gives an example oscillogram from operation on 'moderate prospective current', in which arcing continues for 2 cycles with little reduction in current. So it seems the arcing period shouldn't be considered negligible in its effect on duration and let-through when one comes into the hundreds of amps range (which is the range I assume the plot means: low enough to have several cycles for melting, but high enough to have significant arcing). This fits with what I'd expected for semi-enclosed fuse operation in short circuits. Yes - melting times of fuses tend to a constant let-through at high currents. And good quality HRC fuses don't add much arcing let-through at prospective currents so low (by their standards) as 1 kA. But a couple of cycles of arcing of a semi-enclosed fuse at 1 kA could mean another 40 000 A²s just from the time of arcing. I'd risk a guess that the arcing period would not get shorter for higher current. If we move towards the 6 kA case mentioned earlier, the BS3036 fuses would be out of their depth, and presumably the let-through could be the value for the upstream cutout. In contrast, a good cartridge fuse or modern MCB will typically cut the current in a half cycle or less. This is partly why I'd (still) expect the semi-enclosed fuse to have high let-through at currents such as 1 kA. Also, because its element doesn't have the carefully formed restrictions of a cartridge fuse, I'd expect it to be a good deal slower at melting when coming up to the 1 kA level. Unfortunately, there don't seem to be specifications for BS3036 fuses at these higher currents that give times <0.1 s, and even the BS7671 value for 0.1 s may be rather low and not include arcing time. 


As you'll see, this is mainly curiosity about how BS3036 fuses really behave, as an aside to the wall warts.  Having become used to diazed fuses, which are about 5 for a pound, always rated for prospective 50 kA, and with a nice low fusing factor like 1.45, I'm a bit suspicious of the venerable semi-enclosed things. 


I don't remember having met a 45 A BS3036, but I see they existed, so perhaps there's somewhere a BS1363 socket on a cooker controller with such a beast behind it.