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So many data sheets focus mainly on the parameters when contact is switching, such as the "maximum switching voltage", "maximum switching current". But what's the maximum current the contact can afford when the relay is already closed?

update:

I've noticed some manufacture give this parameter, such as, in Omron's data sheet for G8P

enter image description here

The "rated carry current" should be the current the contact can afford when closed, right? And you see, here it's just equal to the max. switching current. But some data sheet won't give this parameter directly, so can I assume that current shouldn't exceed the max. switching current?

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  • \$\begingroup\$ That is generally the current rating of the relay itself, e.g. the SRD-05VDC can handle 7A at 240VAC or 10A at 28VDC. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 7 '18 at 3:09
  • \$\begingroup\$ The maximum current rating of the relay when it is in contact is usually mentioned at top of the relay itself. click this link: i1.adis.ws/s/maplin/N30AW_set/STEM+%26+Components/…*,*&w=283&h=283 \$\endgroup\$ – CNA Mar 7 '18 at 3:35
  • \$\begingroup\$ when the contacts are closed the voltage across them is close to zero, so it should be safe to use at-least the highest current listed \$\endgroup\$ – Jasen Mar 7 '18 at 7:16
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This is typically referred to as "rated contact current" (or just "rated current"), and should be listed in any (reputable) relay datasheet.

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  • \$\begingroup\$ Yes, but some not so reputable datasheet doesn't give it :). \$\endgroup\$ – diverger Mar 7 '18 at 4:03
  • \$\begingroup\$ Many relay datasheets give a maximum current rating based on the maximum during disconnection, not the maximum when the connection is kept closed. \$\endgroup\$ – The Photon Mar 7 '18 at 4:38
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This does not answer your direct question , but it explains why.

The reason switching current needs a bigger relay is that most switching loads are not pure fixed resistance and energy is stored in reactive loads. The energy storage creates a surge when cap loads are switched On and inductive loads are switched Off.

There is far more energy when the motor inertia acts as a generator to dissipate more energy in the contact gaps when they open. This arc energy is what heats silver alloy contacts and degrades their life from 10 million mechanical operations to say 1k operations for the rated Hp motor which is far less than the resistive rated current.

DC motors are rated the lowest relative to the max relay current, because there is no zero crossing for arc currents to quench (stop)

The VI product ,DC current and inductive loads all reduce the Max current useful for Relay contacts. Not shown are DC inductive motors which reduce the current rating as much at least 60% or down to 40% of rated current even if not stated by other supplier specs.

enter image description here

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