If an NC contact is rated 3 A @ 220 vac, what does it mean…?
It means that, in the closed position it is rated to carry a current of 3 amps and the open position, it is rated for a voltage of 220V (or 220V to ground even when closed).
excavating (for a mind)
hmm. i cant understand. can you explain it ?
It can safely carry three amps without overheating or melting. It can interrupt three amps without excessive arcing, and it can safely withstand 220 volts without arcing, shorting to ground/neutral or burning up.
what does arcing mean?
Or perhaps this.
[QUOTE=gantzm3]
what does arcing mean?
[/QUOTE]
It means you probably shouldn’t be messing with 220.
Seriously. If you don’t know amps and volts, and what arcing is, you probably shouldn’t be touching it.
To be more specific, the switch can interrupt up to 3 amps at 220 volts, both at the same time, without arcing or damage; the current rating can be different at other voltages; for example, a switch might be rated at 15 amps at 125 VAC and 7.5 amps at 250 VAC; switches with a DC rating will usually have far lower ratings, such as 6 amps at 30 VDC and 16 amps at 125/250 VAC (both of these are off of actual switches); this video shows why this is so (continuous DC can cause sustained arcing even at low voltages; if the voltage is really low, like 12 volts, you might be able to switch the rated AC current but it isn’t advisable unless you look up the specs).
This, though the voltage is a bit higher than 220.
The same thing happens on a smaller scale inside small switches.
Explanation is an excellent idea.
Please explain why you are asking these questions. People on this board are happy to help others but they’d rather not do homework for you nor do they want to encourage you to work with things that you are unprepared for.
A little background about why you are interested in electrical components would give us some assurance.
Just to be clear: there can’t be 220 VAC across the switch contacts, and 3 A through the contacts, at the *exact *same time. An arc has a nonlinear I-V curve (and negative dynamic resistance) that depends on the gap width, contact material, and atmosphere. And the arc voltage is relatively low throughout most of the curve.
For the OP’s switch, the 220 VAC rating means the voltage across the open contacts should not exceed 220 VAC RMS, and the 3 A rating means the current through the closed contacts should not exceed 3 AAC RMS. Check out UL 20 for more info.
If it does not also give a DC spec, then assume the switch cannot be used in a DC circuit. Also, it is assumed the frequency of the AC circuit is not lower than 50 Hz. You should not, for example, use the switch in a 220 VAC / 3 A circuit with a frequency of 0.1 Hz.
I meant that the switch was rated to switch up to 3 amps at a maximum voltage of 220 volts, which is what I meant by “at the same time”; of course, the voltage across the switch will be zero (ignoring resistance) when on and 220 volts when off and some combination in between (with current still flowing) when switching off and arcing, which is when the ratings are most important. Also, some switches are rated at twice the current at a lower voltage (e.g. 15 amps 125 VAC, 7.5 amps 250 VAC); in this case, it is a arc power dissipation limitation (assuming a resistive load, the arc will have up to 1/4 the load power, when its effective resistance is the same as the load’s).