# Question on switch rating

Discussion in 'Electronic Design' started by Mike Miller, Jan 7, 2014.

1. ### Mike MillerGuest

All,

I have a rocker switch that I am looking to purchase.

I need all the contacts to be rated for at least 10A and 24 VDC.

Now when I look at the datasheet for the part it gives several different current ratings and different AC voltages not DC.

From datasheet:
Power Rating: 5A/250VAC, 10A/125VAC (UL); 7A/250VAC, 10A/125VAC (CSA); 6A/250VAC~µT85 (D, S, N, FI, VDE)

Based on my 24VDC and 10A requirements does the UL spec cover this? What isthe DC equivalent to 125VAC? 125VAC * 0.707 = 88.375 VDC? What is this µT85? The 6A raises a flag in my mind.

-Mike

Mike Miller, Jan 7, 2014

2. ### Maynard A. Philbrook Jr.Guest

The problem isn't so much the handling current, it's the arc that
starts when the contacts open and close, mostly open.

The spacing and acceleration of contact separating matters to reduce
plasma burn that destroys contact surfaces. With DC, there is no nulling
of ignition like you have with AC at the base line (0 volts), that gives
it time to put the flame out..

Plasma is generate which is a very conductive gas and since you have DC
it continues to arc as the contacts pull away, until a couple of things
happen, heat raising and there for pulling fresh air that helps remove
the plasma and spacing of the contacts the weakens the current in the
burn and thus cools it down..

Most switches derate their voltage or current to reduce the problem.

4 time multiplier sounds good.

Jamie

Maynard A. Philbrook Jr., Jan 8, 2014

3. ### Phil AllisonGuest

"Mike Miller"

I have a rocker switch that I am looking to purchase.

I need all the contacts to be rated for at least 10A and 24 VDC.

** Really ??

How did you come to that conclusion ?

Everything depends on the nature of the load being switched.

What is it ?

Incandescent lamps, a motor, DC to AC inverter ?

Don't be coy.

Withholding crucial info like this PREVENTS people helping.

..... Phil

Phil Allison, Jan 8, 2014
4. ### Guest

This says that the switch is rated by UL for the currents listed at
125 V AC and at 250 V AC. If the whole device using this switch was
going to have a UL listing, and the currents involved are provably
lower than those limits, then you wouldn't have to prove much else
Similar thing, except for CSA (Canadian standards). They sometimes let
you have a little more than UL does.
This is the rating for some European standards, including Demko
(Denmark), Semko (Sweden), Nemko (Norway), Fimko (Finland), and VDE
(Germany/EU). Some of these standards bodies have since become part of
other companies.

The micro symbol means "microdisconnection", but I'm not exactly sure
what *that* means - something about the contact gap, probably.

T85 probably means an 85 C temperature rating.

http://www.cherrycorp.com/english/switches/pdf/switch_cat.pdf (page 20)
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_safetymarks
http://en.wikipedia.org/wiki/Conformance_mark
No. Switching DC is not the same as switching AC. When you turn the
switch *off*, an arc forms at the contacts. You have to get rid of this
arc before the switch is well and truly turned off. With AC, the arc is
trying to put itself out 120 or 100 times a second, so it's pretty easy
to get rid of the arc. DC usually never tries to put itself out, so
it's harder to get rid of the arc. For DC, the contacts are usually
designed to get further apart faster, and may be made out of a different
material.
Doesn't work that way, at least for switch and relay contacts. You need
to buy a switch that specifically has a DC rating from the manufacturer.

You also get to think about what *kind* of load you are switching.
Switching 240 watts of electric heater is different than switching 240
watts of incandescent lamps; both of those are different than switching
240 watts of electric motor.

Matt Roberds

, Jan 10, 2014