davidbeach said:
What waross said. Trip characteristics and interrupting ratings are independent of each other in any breaker I'm familiar with. If a 10kA breaker is adequate and I install a 22kA (in the same family) instead I've only spent a bit more money, but I haven't changed the trip characteristics at all.
I know
A 10ka, 22, 65ka all have the same tripping characteristic. But you said the same thing as I did, a 22ka breaker costs more than a 10ka breaker. So if you have a large number of consumer units where the typical fault current is say 5ka it makes sense for manufactures to stock a cheaper 6ka in addition to 10ka instead of only 10ka or say only 16ka and cover everything. Yes its less logistics, but more cost to the customer.
The arc fault breakers add electronics to detect the signature of the arcing fault, but the thermal and magnetic tripping characteristics remain essentially the same.
Correct, however (AIC aside) the magnetic trip component has gone down over the years in single pole 120 volt residential breakers, but you can only go do down so far, hence the electronics found in AFCIs. In the early days of AFCI development the idea was to simply have a standard thermal magnetic breaker with a known magnetic trip level of 75amps. 75amps was chosen as being the lowest anticipated level of fault current in a dwelling (500amps available fault current at the panel plus a long circuit length, say 150 feet). While such a breaker would provide parallel arc fault protection, it would nuisance trip on motor and tungsten inrush. Thus, a solution was created where instead to relying on a coil at 75amps, electronics would monitor the current waveform starting at 75amps. Thus, discrimination could be achieved. A motor starting with a 120 amp inrush would not trip because the waveform is mostly sinusoidal, but if someone drove a nail into NM, the electronics would see the sporadic/distorted sine wave and trip mimicking the advantage of a low magnetic trip breaker.
Of course 15 years latter AFCI were required to also look for series arc faults which has nothing to do with parallel arc faults (whole other ball game), but it gives you the the idea behind the history.
Similar philosophy in the IEC, however its up to the electrician to select the right pickup and then make sure the system can pass the required current to trip the breaker magnetically.
Sounds like another one of those areas where both sides of the ANSI/IEC divide find what's done on the other side to be completely weird. Somehow the same physics results in very different engineering.
Well, also keep in mind whats at play. 230 volts has more hazards and is less forging than 120 volts, so it does play a role in the design.
