AndyS said:
Andy asks:
I was looking at a switch that specified it's contacts rated
at 100A in a 12 volt system, and at 50A in a 24 volt system.
So, can anyone tell me why the current ratings are different
for the voltage applications ?
Andy
Switch ratings have more than one limiting property.
When the switch is closed, the contacts are heated by the
RMS current passing through them, and the current limit for
this operation depends on the contact material, whether the
contact point just meets or wipes at closure, the contact
pressure, the thermal conductivity to the surroundings that
heat sinks the contact point, etc.
As the contacts open, an arc is drawn that cumulatively
degrades the contact surfaces. So the load current and
voltage limits depend on both the off state voltage, the
load current, whether the current is AC or DC (AC has two
moments per cycle when the current goes through zero,
letting the arc go out, easier), whether the load is
resistive or inductive, how fast the contacts separate, how
far the contacts separate, the expected cycle life of the
contacts, etc.
As the contacts close, they approach, bounce, possibly wipe,
and increase their contact pressure, just as the load
current through them is rising. The load current limit here
depends on whether the load is inductive or resistive
(resistive load currents rise almost instantaneously) and if
resistive, if they are incandescent (low cold resistance
produces large inrush current), how many bounce cycles,
whether or not the contacts wipe (to smear the arc oxide
damage out and freshen the metallic surface), etc.
Different cycle conditions (closing, carrying, opening)
produce different kinds and quantity of damage, so either
different life expectancy for a given rating (voltage,
current, AC or DC and load character), or different ratings
for the same life expectancy.
Banging two pieces of metal while they carry current or
block voltage, with predictable results, is an art form.
