Yes, the practical difference between most B vs C designs is the different settings of the electromagnetic trigger mechanism. But many have also differences in the thermal trigger (C has extra thermal mass attached to it). The D is so much slower vs B, it always has the extra mass on the thermal trigger (and even higher threshold for the electromagnet). But what matters is the overall current vs time envelope. The upper limits are imposed to what the protected thing can handle, the lower by what the load needs during normal operation to not trip it. The area between is the space for tolerances and mismatch between ideal characteristics vs how real mechanisms perform. Yes, better breakers have "the area between" smaller (usually because of tighter manufacturing tolerances and faster contact mechanism, leaving more time space before the trigger has to trip), so allow higher load current spikes so suffer less from nuisance tripping, yet stay safe for the protected things.
Short vs long cables is not the only limitation. The thing is (the philosophy what the standadr here is based on) you can attempt to draw any current with any time profileyou wish yet still the breaker should not allow the wire to overheat. Really no limitation, anything from open load and short circuit. If it would overheat the wire, the breaker should disconnect it in time to protect the wire. So includes if you draw current for a time just a hair below where the breaker would trip (that includes currents many times the breaker nominal current rating) and then have a hard short circuit (so practically a current corresponding to the the rated current breaking capacity flowing over the time the mechanism needs to really break it after it got triggered), the wires still shall stay within safe temperature limits. So even theshorter wire may overheat when you draw current limited by something else than the long cable length. It uses to be the case when e.g. motors or transformers are failing: As the insulation breakdown progresses (when the motor itself is practically already burned - assume the equipment is made so it by itself is still safe in that condition), the current increases gradually (even when we are talking about time frime like a a second or so) and ends up as a hard short.
The B characteristics was designed exactly to match the profile the wires can tolerate (with the margin for a hard short circuit at the end of the load current profile), so practically allows you (within the limits of the window for the attainable tolerances) the highest currents the wire can safely handle for the given wire size (16A for 2.5mm^2 in a wall or conduit). C breaker (the same nominal current) may allow higher inrush, but it leaves no margin for e.g. a short circuit just at the end of the inrush limit, the B restricts the inrush current so the wire still has enough temperature margin for the extra heat surge the short circuit event imposes.
|
|
|
Logged
|