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How will the wear of a relay be affected if the current through the relay is brought to zero before the relay is operated?

I'm thinking about having a solid state relay in series with 3 mechanical relays. The 3 mechanical relays will all be connected to the output of the solid state relay, and channel the current to one load each. Thus there are three loads, and three mechanical relays. I can arrange to turn off the solid state relay before opening or closing any of the mechanical relays.

Can I assume that the lifetime of the mechanical relays will be equal to the speciefied mechanical lifetime of the relay, or will current flowing through the closed relays somehow cause aging in and of itself?

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  • \$\begingroup\$ Current flow = energy dissipation = work. So, theoretically, yes. It will shorten the lifetime. Practically - not sure how significant. \$\endgroup\$ – Eugene Sh. Jul 27 '15 at 16:05
  • \$\begingroup\$ If a circuit has a fault, then the contacts of the associated relay might weld together from the associated fault current passing through the relay. Apart from that, I don't think normal current flow will do anything to a relay's lifetime - it's what they're made to do. \$\endgroup\$ – Li-aung Yip Jul 27 '15 at 16:16
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Yes, you can most likely use the mechanical lifetime for this purpose. However I have to question your premise a bit.

The SSR (assuming it's an AC type, you don't say) will likely have an internal snubber so that there will be a small current surge at switching (depending on where in the AC cycle it switching) but it's unlikely it will reduce the lifetime by much.

Still might be worth testing if this is an important point. 10^6 operations at 1 per second will only take a couple weeks.

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    \$\begingroup\$ Good point, any parasitic capacitance in the solid state switch will lead to a bit of inrush current when operating the relay, even if the solid state relay is off. Hadn't though about that! \$\endgroup\$ – avl_sweden Jul 27 '15 at 19:26

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