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Can I use a mercury contact switch for a control circuit that would normally be 240V 1.5A? If I use 480V instead? The switch has a rating for 240V. This is an electro-pneumatic switch and I can't find the differential I can use without buying a 600V switch which costs three times as much.

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Seriously? Don't you think that if the manufacturer could sell the 240 V switch for three times more if it really can withstand 600 V that they would do that? Or do you think you alone are entitled to different laws of physics than the rest of us? –  Olin Lathrop May 18 '14 at 21:11

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No, I wouldn't bother even trying - if it blows up and sprays you with mercury then it could be hazardous, especially if it forms a conducting mist (not sure if this is possible) and passes a few hundred volts to anything the mist hits.

If the manufacturer of the mercury switch was happy with rating it ia 480 VAC then it would be stupid of them to miss this marketing opportunity.

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The voltage rating on a switch is to prevent breakdown of insulating materials in the switch as well as protect against arc fault between contacts or conductive parts inside the switch. If the manufacturer rates the switch at 240V, it's likely that it is actually overbuilt somewhat, for margin-of-error reasons, but you cannot rely on that.

To use an analogy I've seen somewhere before: Engineers build a bridge which needs to support 10 tons. Obviously you need to build the bridge to withstand more, to handle additional stresses from wind and vehicle acceleration, etc. You wouldn't want the bridge to collapse at 10.1 tons, but nor should the bridge support 20 tons (safely) because it would be overbuilt and over budget. Depending on safety standards, the bridge might be tested to safely support more than 10 tons, but unless you have inside knowledge of the exact testing procedures done, you have to assume that 10 tons is the maximum load, period.

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A safety factor of 2 or more on a bridge would not be unheard of, depending on the bridge. Though an airplane structure with that much redundant strength probably wouldn't fly. –  Seth Battin May 18 '14 at 21:14

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