I have trouble understanding the specification for inrush contact in relay contacts (more or less the issue in SMPS inrush time)

Background: I have some beefy power supplies (about 16A at full load) and I need to electromechanically switch them for safety reasons. So a relay/contactor is needed. The datasheet specify inrush as 60A at full load (no duration specified!). The power supply has active power control and active PFC but these are its specifications (we buy them as modules).

Since luckily we don't start them at full load I did some measurement of the input current with open output (took a lot of time to find the 'worst' wave point).

So I seen about 20-30A on the first half wave cycle but about 60-80A at switch on (single damped pulse, about 100µs).

The idea would be to either specify an adequate relay or suppress the pulse.

For suppression I know there are power NTCs but they heat up a lot and are somewhat hard to acquire (as in quantities below 1000k pieces). That could be a solution.

The other way would be to rate correctly a power relay. For example I've found some (relatively!) cheap dual contact (tungsten+AgSnO2) that are rated as:

165A peak for 20ms
800A peak for 200µs

These would fit perfectly the bill for the application, since they are actually designed for SMPS and incandescent lamp load.

However most of the other power relays simply say:

Rated current/Maximum peak current 16/30A

That would fit the 20ms transient but not the 200µs one, unless there is some standard/rule of thumb/whatever to derate correctly the contact (as in, maybe a 50A peak would plausibly handle an 80A short peak). In my experience AgSnO2 contacts are good for inrush (like CdO, even if toxic, is good for arcing)

After all these relay switch only about 5-10 times a day (probably less) so even if they survive only 5k cycles it would be acceptable.

Do you have any pointer or experience with that?

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    \$\begingroup\$ FWIW, short-term inrush (likely due to charging EMI filter capacitors) could be mitigated with a series (R||L) network, having a similar time constant (i.e. 4.7Ω || 250µH or thereabouts). Downside: the inductor must be rated for full peak current, which will be pretty large; an adequately rated relay will be more effective. \$\endgroup\$ Commented Sep 6, 2022 at 8:45
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    \$\begingroup\$ Tought about that but a 16A choke is quite a beast :D \$\endgroup\$ Commented Sep 6, 2022 at 8:48
  • \$\begingroup\$ @LorenzoMarcantonio Good question, +1. 16 A chokes are 'heavy iron.' But that's what we used to use, back when (and where it mattered.) Things were bigger back then, though. (Vacuum tube days.) \$\endgroup\$
    – jonk
    Commented Sep 6, 2022 at 22:38

1 Answer 1


Look for relays with "TV" ratings such as TV5 or TV8. They are specifically rated for such inrush loads. For example this one. Life should be 25,000 operations with the rated inrush. Details are in (probably paid) UL publications, but here is Omron's intepretation of the ratings:

UL paid a great deal of attention to televisions ca. 1970 since they were at that time a significant cause of fatal house fires, and that is where the "TV" comes from.

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  • \$\begingroup\$ UL publications are always paid, but the preview usually is good enough to read it;P I'll look into the TV-15 G4A, it seems interesting, thanks \$\endgroup\$ Commented Sep 6, 2022 at 8:59

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