I am confused about inrush current for a relay. For example, if I had 12A as the inrush current due to a motor, if the contact rating of a relay is only 6A, does a new relay need to be chosen?

As in how do you know what the inrush current rating of relays is? Is there a thumb rule, does it typically not matter due to it only being for a few milliseconds?

Example for this relay: RV8H-L-D24, Datasheet: http://www.farnell.com/datasheets/1769354.pdf How do you know what the inrush current rating is of that relay for 24Vdc?

  • 2
    \$\begingroup\$ For that particular RV8H relay, in the contacts rating plot, I see a maximum current for inductive loads of only 2 A. As this relay does not specify any allowable peak currents you should not assume anything. Even a few milliseconds can be enough to melt contacts together when they're used outside their rating. For a 12 A load, get a relay that can handle that. \$\endgroup\$ – Bimpelrekkie May 21 '19 at 14:22

How do you know what the inrush current rating is of that relay for 24Vdc?

It doesn't have one - its contacts are rated for a maximum of 6 amps RMS or 6 amps DC: -

enter image description here

Additionally, when driving an inductive load (such as a motor) it has a rating of only 2 amps.

  • \$\begingroup\$ Thanks, @Andy aka, is there any particular reason why it would have 2A max for an inductive load (as in typical properties)? \$\endgroup\$ – Student May 21 '19 at 14:28
  • \$\begingroup\$ The inductive load scenario creates a back emf when the contact opens and this degrades the contact surface due to the spark that forms. \$\endgroup\$ – Andy aka May 21 '19 at 14:31
  • \$\begingroup\$ Thanks, would a flyback diode prevent this for dc inductive loads? \$\endgroup\$ – Student May 21 '19 at 14:35
  • \$\begingroup\$ Yes it would but the data sheet still doesn't help you understand by how effective this will be. My inclination is to either choose a different relay that is more suited for DC applications or use a MOSFET. Do you need to use a relay that is suited for hazardous gas class 1 situations or would a regular relay (with a better spec) meet your needs? \$\endgroup\$ – Andy aka May 21 '19 at 14:38
  • \$\begingroup\$ Suited for hazardous gas class 1 situations (however if you have any ideas for general I will look into that), however, the initial question was more just to understand the concept. The actual application is to use a relay to switch on a Cooler (causes the temperature to go to -40degC). This cooler has an RMS current at a maximum power of 2.46Arms (and I was just going off an estimate of 10x standard current to get inrush current so roughly 30Arms inrush current) \$\endgroup\$ – Student May 21 '19 at 14:44

Ratings are presented in various ways. For the data sheet linked in the question, use the curves shown below to determine the allowable switching current for DC. For a 24-volt inductive load, the maximum current is something less than 0.2 amps.

enter image description here

The Contact Ratings curves seem to show the continuous current carrying current. for resistive and inductive loads.

The Maximum Switching Power AC curve shows the maximum allowable switching current for AC voltages.

It is possible that the Switching Power curves are intended only for disconnecting current and the Contact Ratings curves are intended to cover connecting current as well as steady-state current.

  • \$\begingroup\$ thanks, however, what is the difference between the contact ratings graph and Maximum Switching Power DC graph? \$\endgroup\$ – Student May 21 '19 at 14:31
  • \$\begingroup\$ Sorry, so what is maximum switching Power DC? \$\endgroup\$ – Student May 21 '19 at 14:40
  • \$\begingroup\$ So what's the difference between carrying current then as I thought that was the maximum? \$\endgroup\$ – Student May 21 '19 at 14:46
  • \$\begingroup\$ Carrying current (contact rating graph) and the maximum current (max switching power graph) \$\endgroup\$ – Student May 21 '19 at 14:50
  • \$\begingroup\$ Revised question to cover comments \$\endgroup\$ – Charles Cowie May 21 '19 at 16:24

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