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Below is a specifications of this magnetic relay:

enter image description here

For AC and DC the max switching current is given same as 10A. On the other hand, max switching voltage for DC is drastically lower than its AC counterpart as 40V and 240V respectively.

Now here is my confusion. For instance, let's say we apply 1V DC across a 1 Ohm resistor and the power becomes 1 Watt. Now if we want to apply AC voltage across this 1 Ohm resistor and we want to obtain again 1W it means we should apply AC 1V rms voltage across the resistor. This means in this case max AC becomes sqrt(2) V. So for the same power dissipation the instantaneous AC voltage can be sqrt(2) times higher than max DC.

But in the relay example it is not that simple. Why is the difference in max AC and DC voltages not close to sqrt(2) but much greater?

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Because one of the key features of a relay is its ability to interrupt a current.

But when the contacts begin to separate, there is inevitably a small arc that forms, especially if there is any inductance in the circuit being switched. It is important that this arc be "quenched" (stopped) quickly, and this is more difficult at higher voltages.

With AC power, the voltage and the current drops to zero twice each cycle,1 which goes a long way toward stopping the arc. But with DC, there are no such interruptions, which allows the arc to last much longer. And if the contacts don't separate by enough distance, the arc might not get quenched at all.

Therefore, any given relay design will have a much higher voltage rating for AC than for DC.

See also:

... and probably many others.


1 Not necessarily at the same time, if you're switching a circuit that has any reactance.

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  • \$\begingroup\$ So I was focusing on power dissipation but the main reason here is the electric "arc". (?) \$\endgroup\$ – user16307 Oct 2 at 12:44
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    \$\begingroup\$ Yes. The relay itself dissipates essentially zero power in the contacts -- either the voltage across it is zero, or the current through it is zero. The only time it dissipates power (other than the coil power) is when it is switching, and opening a circuit is much more critical than closing one. \$\endgroup\$ – Dave Tweed Oct 2 at 12:55
  • \$\begingroup\$ Not sure 10W is essentially zero. \$\endgroup\$ – Spehro Pefhany Oct 2 at 13:02
  • \$\begingroup\$ @SpehroPefhany: Where are you getting 10W? The contact resistance is specified as 50 mOhm max, which would be at most 5W @ 10A. Which is negligible in relation to the AC load power. The problem is that most newbies confuse power dissipated in the load with power dissipated in the relay itself. \$\endgroup\$ – Dave Tweed Oct 2 at 13:09
  • \$\begingroup\$ 5W per contact x 2 contacts. \$\endgroup\$ – Spehro Pefhany Oct 2 at 13:10

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