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Current rating: 3A @ 125 VAC, 1A @ 250 VAC

What can I read to understand this ratings better? What exactly does it mean?

They're used to rate this Momentary N.O. Raised Push Button Switch.

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revised 2012-07-15

Motor and solenoids loads create more arc and can reduce **life expectancy of the switch , so contacts are rated closer to load characteristics that generate arcs. The VA may be a reactive load but the arc is a real power drop in the switch which rises during turn-off before quenched to off state.

Simple Voltage rating depends on gaps. Current ratings depends on material, pressure , contact resistance and lifetime testing. But complex loads, switches depend on a Safe Operating points that tend towards the VA product of the load.

Since high current, it is safe to say not gold plated nor used with small signal DC. If so use cap discharge circuit to "wet" contacts on closure, eg low ESR cap with high R dc bias across contacts.

Lower VA rating at higher voltage is normal.

  • 3A @ 125 Vac, so VA = 375
  • 1A @ 250 Vac, while current rating drops to 1/3, VA=250 is 2/3 of above

There is a phoneme of a plasma condition during arc where the air ionizes and has a low resistance and the power dissipated in the arc depends on the motor or inductive load and voltage and time until the voltage drops near the next zero crossing. The duration of this arc can generate substantial heat and reduce life on contacts until quench voltage is reached. This is similar to all negative resistance switches including SCR's and transistors hence there is a sage operating curve. But for simple mechanical switches they use simple specs to match common loads for safe long life.

If you need anything more specific for an answer , more design details are needed for you application in another question as I hope this is sufficient.

2nd edit

THere are many design factors that affect ratings and certifications of the switch besides gap; Materials such as steel, silver, palladium, gold, nickel , beryllium copper, tungsten, are common ingredients that significantly affect price and specs. Secondly ruggedness and safety where the design must be reliable and fail safely for the user's sake according to CE/UL safety tests. These all affect the ratings.

YOu may choose to use it for low voltage DC applications where safety is not an issue for the rating, but this is an agency required rating for safety and reliability. Just be mindful about sufficient wetting current is typically 10% of rating so apply a surge to de-oxidize the contacts when possible by design.

Thirdly , there are contact bounce characteristics in all mechanical switches which creates arccing and depending on the back EMF of a motor, now generating power during no load conditions then receiving power when contact closes, you need to reduce current rating of the switch when voltage is increased to reduce this transient power dissipation in the switch.

Trying to keep this answer brief is difficult, so my final comment is to consider bounce suppression snubber filters to reduce EMI radiation and also suppress arcing in switch, somewhat. enter image description here

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  • \$\begingroup\$ "Voltage rating depends on gaps." That doesn't explain the 125 V vs. 250 V. On the contrary. \$\endgroup\$ – stevenvh Jul 15 '12 at 11:45
  • \$\begingroup\$ "opening statement was the reason... depends more on VA rating of load characteristics.. I added some more details in an effort to remain brief. \$\endgroup\$ – Sunnyskyguy EE75 Jul 15 '12 at 14:51
  • \$\begingroup\$ What I mean is that the gap is a mechanical given for a given switch, so where do these two different voltages for the same switch come from? \$\endgroup\$ – stevenvh Jul 15 '12 at 14:58
  • \$\begingroup\$ THe voltage comes fro the external circuit. I this case an AC line standard voltage \$\endgroup\$ – Sunnyskyguy EE75 Jul 15 '12 at 15:06
  • \$\begingroup\$ Sorry, I don't seem to make myself clear. The gap determines a 125 V rating, yet that same gap also determines a 250 V rating. I think this is what OP's question boils to. \$\endgroup\$ – stevenvh Jul 15 '12 at 15:13
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I got this link which explains about an experiment done on the current ratings of a roller and bar micro switches made by MICRO(switch) Corporation. Here they specifies about the change in V AC and current and even have a chart regarding the same. Here they explains the total power is not the constrain regarding the ratings. Several loads which have various inductance is also a constrain regarding the switch rating. Hope this link helps you to clear the doubt.

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