Sorry but brain not working this evening and can't get an adiabatic equation to give me a reasonable answer
PFC=12.2Ka
OCPD BS88-2 E 100A might be 80A but disconnection should be 0.1 at those fault currents
k=115 ie copper sleeved conductor
I'm getting over 30mmCSA main earth which doesn't seem correct
Thanks
Once things get really fast the 'time to blow' curves are not really much uses, as the fault cannot be reliied upon to come on at the zero crossing and present a nice sinusoidal half cycle, so the total energy in joules per ohm in the fault path, or I2t becomes more helpful, as that is an integral of the thing that will do the damage to the fuse link and anything else in the loop in an I2R = watts
I2R* t = joules = total heating energy sort of way. (only approx, R changes as it heats up...)
Then for non easy waveforms, to estimate the damage done we tend to look for the bullrush plots. Each stalk is a fuse rating, and the bottom of the bullrush 'head' can be seen as the point of no return - if you get this much energy in , it has started to melt the element, and will open circuit soon after, while the top of the 'head' represents the maximum amount of energy that will be let-through - and that is voltage dependent, as it is the extra time for the arc to be stretched to extinction as the metal burns back from the initial point of breaking.
(if you want to cascade fuses and always want the lower rated one to blow first - think sub mains an so on, the top of the bullrush representing the lower valued fuse needs to be below the bottom of the head of the bigger fuse that is the 'death or glory' back up)
Note that the rule of thumb of a ratio of 2.5 to 3 in current rating between cascaded fuses to get reliable discrimination suits the lower rated fuses better, and a 3 :1 current rating is a 9:1 let through energy if nothing else changes - but it does, because bigger fuses are made quite differently inside, more of a foil ribbon or a perforated lace than a fuse 'wire' as such.
Mike
Once things get really fast the 'time to blow' curves are not really much uses, as the fault cannot be reliied upon to come on at the zero crossing and present a nice sinusoidal half cycle, so the total energy in joules per ohm in the fault path, or I2t becomes more helpful, as that is an integral of the thing that will do the damage to the fuse link and anything else in the loop in an I2R = watts
I2R* t = joules = total heating energy sort of way. (only approx, R changes as it heats up...)
Then for non easy waveforms, to estimate the damage done we tend to look for the bullrush plots. Each stalk is a fuse rating, and the bottom of the bullrush 'head' can be seen as the point of no return - if you get this much energy in , it has started to melt the element, and will open circuit soon after, while the top of the 'head' represents the maximum amount of energy that will be let-through - and that is voltage dependent, as it is the extra time for the arc to be stretched to extinction as the metal burns back from the initial point of breaking.
(if you want to cascade fuses and always want the lower rated one to blow first - think sub mains an so on, the top of the bullrush representing the lower valued fuse needs to be below the bottom of the head of the bigger fuse that is the 'death or glory' back up)
Note that the rule of thumb of a ratio of 2.5 to 3 in current rating between cascaded fuses to get reliable discrimination suits the lower rated fuses better, and a 3 :1 current rating is a 9:1 let through energy if nothing else changes - but it does, because bigger fuses are made quite differently inside, more of a foil ribbon or a perforated lace than a fuse 'wire' as such.
Mike
