# Switch Contact Rating Question

Discussion in 'Electronic Design' started by AndyS, Apr 7, 2007.

1. ### AndySGuest

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

2. ### Guest

If you look at the power handling ability (W) it is the same! In
effect the contacts are rated for a certain maximum wattage.
It is difficult to think of it in those terms but that is the way it
is.

Rolf

3. ### Phil AllisonGuest

** Cos it is a **switching ** current rating !!

Means the ability of the switch to BREAK a current flow of so many amps in
a circuit with so many volts available after that break opens.

Its all about how DC current arcs behave - the more the volts, the harder
it is break the circuit successfully.

Too many DC amps at too many volts with too small an opening gap = a *
continuous DC arc * that heats and melts the metal surfaces to bits.

....... Phil

4. ### Phil AllisonGuest

<

** BOLLOCKS !!

** No it is not - you know nothing, bullshitting ass.

........ Phil

5. ### Spehro PefhanyGuest

The ratings are also different, in general, for AC vs. DC and for
different types of load (resistive, tungsten, motor).

Some types of current and/or load simply cause more contact wear due
to arcing (or the switch may even have difficulty in breaking the arc
at all), or the type of load may have a large surge current on 'make'
that tends to weld the contacts.

Best regards,
Spehro Pefhany

6. ### John PopelishGuest

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
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.

7. ### martin griffithGuest

more stuff here
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf

martin

8. ### Homer J SimpsonGuest

That's a good question. Doubling the voltage shouldn't halve the current
rating. Why not ask the makers?

9. ### Phil AllisonGuest

"Homer J Simpleton"

** See the Derating Curves on page 2.

http://www.automationdirect.com/static/specs/qlrelays.pdf

10 amp ( switching capacity) rated relay contacts drop to 1.5 at 50 volts DC
and nothing at 100 volts DC - even with purely resistive loads.

........ Phil