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I have a need for a switch that can handle 28Vdc at about ~1A.

The switches I would like to use have a rating of 24Vdc at 5A.

I would think that the limit of switch is the power, so if we take a switch at 24V and 5A, then it can handle 120W. My reasoning is that a switch can run at higher or lower voltage currents, so long as its less than the 120W (in this case). A 24V 5A switch can be used for a 28V 1A application.

Is the reasoning behind that correct ? Am I absolutely limited to 24V 5A spec'd by the datasheet ?

Example switch

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    \$\begingroup\$ See this old question about connectors. It applies also to switches. The mechanisms that lead to failure from over-voltage and over-current are different, and it isn't simply a power issue. \$\endgroup\$ – The Photon Dec 5 '14 at 17:14
  • \$\begingroup\$ Re the power argument : I would feel very uncomfortable running your example relay at 120V and 1A as that argument might imply. However I would expect 28V to be probably OK. As The Photon points out, overvoltage failures tend to involve arcs, so I would still be cautious if the load is inductive rather than e.g. a lamp. What are the consequences of its failing? Are you selling a million or trusting your life to it, or will it be mildly inconvenient if you have to fix it? \$\endgroup\$ – Brian Drummond Dec 5 '14 at 18:17
  • \$\begingroup\$ @BrianDrummond I understand. In this case, it's a one-off and worst case, at at most, I would need to remove the switch to replace it. There are safety measures inplace in case something were to go wrong. This particular voltage, it's not quite as easy to find a switch that fits the style we are looking for, so 24V was the closest we could find. The manufacturer never responded back to my emails which is why I came here to get other opinions. \$\endgroup\$ – efox29 Dec 7 '14 at 2:14
  • \$\begingroup\$ The other way of looking at it is: can you genuinely not find a 100VDC (or 120V DC) 1A (or 3 or 5A) switch of the right size? If you can, that would be my choice. \$\endgroup\$ – Brian Drummond Dec 7 '14 at 11:39
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The answer to this is closely related to the answer to an old question about connectors.

As in the connector question, the main thing you need to remember is that the current through the switch and the voltage between the switch and ground don't tell you the power consumed by the switch. Typically they tell you the power consumed by the load.

But with switches the failure mechanisms are slightly different than in connectors. While the current rating is typically related to \$I^2R\$ heating at the contact, the over-voltage failure meachanism is typically reduced durability due to arcing across the contacts during making and breaking, rather than breakdown in the material between the terminals.

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  • \$\begingroup\$ Very useful link. Can arcing occur at even such a low voltage ? Isn't the minimum breakdown voltage for air like 300V ? At 28V, could/would 4V be the difference between safe and reliable to distatrious ? \$\endgroup\$ – efox29 Dec 5 '14 at 17:55
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    \$\begingroup\$ The breakdown voltage of air is 300 V per mm. As the contacts of the switch open and close, they have to get closer together than 1 mm. As for wheter 4 V makes a difference, good design practice is to add margins rather than design outside the manufacturer's specs. \$\endgroup\$ – The Photon Dec 5 '14 at 18:38
  • \$\begingroup\$ I understand. Unfortunately there is not much I can do about the existing setup. We do have other measures in place in case something were to go wrong. Much appreciated! \$\endgroup\$ – efox29 Dec 7 '14 at 2:16
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If the 28V is in an aircraft electrical system (typically 28VDC in smaller aircraft), I would feel very uncomfortable with this despite being 99.98% sure it would be okay. I would rather find a 30V switch and sleep at night (part of the reason why aircraft stuff is so expensive).

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  • \$\begingroup\$ Yeah I would count any aircraft use as "trusting my life to it". \$\endgroup\$ – Brian Drummond Dec 7 '14 at 11:37

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