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If Contact Rating of switch is 5A 240V AC & 0.25A 240 DC, Then is this switch can be used for contact rating of 220V (+10, -15% voltage variation) AC & 127V (+10, -15% voltage variation)

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    \$\begingroup\$ Would Electrical Engineering be a better home for this question? \$\endgroup\$ – Qmechanic Apr 8 '15 at 10:19
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    \$\begingroup\$ It's simply how much current a switch can interrupt. Arcs across a switch can weld it shut as it tries to break the circuit and staying within the contact rating simply makes sure that this accidental "welding" won't happen. AC ratings are much greater than DC because the driving voltage repeatedly returns to nought: this greatly helps shutdown because the cyclic return to low voltage quenches any contact through an ionized arc. Once an arc is quenched, it takes a high voltage to re-establish it. \$\endgroup\$ – WetSavannaAnimal aka Rod Vance Apr 8 '15 at 10:40
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    \$\begingroup\$ @Qmechanic I do believe it's off topic here \$\endgroup\$ – David Z Apr 8 '15 at 10:59
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The contact rating is the voltage/current that the switch can repeatedly connect and interrupt without degrading it's design life-cycle operation count.

Take the ratings as the maximum safe operational ratings of a switch. Yes, a 240VAC 5A rated switch will operate just fine on 127 VAC, but up to 5 Amps.

The DC rating is almost always lower because of the ability of switch to quench the arc when opening a DC circuit.

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The max current that won't over-heat the contacts; the max voltage that won't burn them by arcing during break.

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