Why does this magnetic relay max DC voltage rating is much less than its max AC voltage rating?

Question

Below is a specifications of this magnetic relay:

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?

Answer 1

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:

• Using an AC relay for DC switching, safe limits
• relay power ratings - AC vs DC
• Purchasing a Switch - AC vs DC Current Specifications?
• Exceeding Relay contact voltage

... and probably many others.

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