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I've a relay with Maximum contact voltage of 24VDC @ .5A. Switching capacity of 12W. So can I use the relay contacts for 110VDC and 5mA. As it is much lower than the Switching capacity.

If it can be used how is the de-rating determined? (When manufacturer does not provide any info)

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    \$\begingroup\$ You have to be careful. The device is rated for a maximum of 24V. Using it at 110V Volts will cause much larger arcing. \$\endgroup\$ – Sada93 May 28 '15 at 7:55
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The problem with DC over about 40V is that an arc can form much more easily, compared to AC where the 'zero-crossing' provides 100/120 opportunities per second (assuming 'mains') for the initial arc that can occur at the instant of disconnection to extinguish itself.

DC relays are always more expensive at a given current-capability compared to AC, because they need to separate the contacts further to achieve the same current interrupting capability.

Relay voltage ratings are also about insulation materials types, thicknesses, & distances between the contacts and the coil, so even though your circuit may have some mA-scale limited current capability, there's still potential for arc between contacts & coil, depending on how well or poorly the 24VDC-rated relay is designed & manufactured. I really would recommend going for a relay rated for what you're doing.

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    \$\begingroup\$ 50Hz mains provide 100 zero crossings per second. 60 Hz mains, 120. \$\endgroup\$ – EM Fields May 28 '15 at 9:06
  • \$\begingroup\$ due to constraints of finding another source this is the only relay present (for smoke sensor). Is there any way to extrapolate the safe region for the arc extinguishing (without datasheet)? \$\endgroup\$ – Karthik Shetty May 28 '15 at 9:25
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    \$\begingroup\$ The safe region is 24VDC. In addition to the correct points above by Techydude, relays and contactors for switching DC often have magnetic blowouts to quench the arc when the contacts open. The ratings are listed for a reason. Exceeding them by 458% might work a few times, but will result in a failed device, probably with the contacts welded shut. \$\endgroup\$ – R Drast May 28 '15 at 10:47

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