When reading Omron's datasheet of G6E, I find the words as below:

enter image description here

I can't understand well the words

"The difference in polarity applied to the contacts will affect the endurance of the Relay due to the amount of contact movement".

So, why? And I only see this in G6E's data sheet, not ever in others, then is this only applied to G6E, or common to others?

  • \$\begingroup\$ Good question!! \$\endgroup\$
    – Andy aka
    Jul 7 '17 at 12:27
  • \$\begingroup\$ I wonder if the load current mag field generated might aid in the direction shown and un-aid in the opposite direction? \$\endgroup\$
    – Andy aka
    Jul 7 '17 at 12:33
  • \$\begingroup\$ If I had to guess, I'd say that there are magnetic suppressors to help quench the arc on opening. For DC then, polarity matters. \$\endgroup\$
    – R Drast
    Jul 7 '17 at 13:08

When a relay is operated under DC conditions, there is material transfer between the contacts when they close and when they open. Without the presence of an arc in the contacts, the material transfer is always from the anode to the cathode. This can form uneven surfaces (asperity) on the contacts that resemble spikes and craters when examined under magnification. But this is normal for any relay. This is sometimes called "metal bridge transfer".

When arcing occurs on the contacts, the material transfer is in the opposite direction - from the cathode to the anode. And most relay applications involve some degree of arcing. This can result in more material being transferred than under non-arcing conditions.

Since the ability of a relay to extinguish an arc is based on the distance between the contact points for a given DC load, the faster that this distance is traveled by the moveable contacts, the sooner the arc is extinguished. And the acceleration of the armature is dependent upon the mass of the armature. It is therefore a reasonable conclusion that the recommendation ensures the net material transfer results in the stationary contacts gaining material, thereby reducing the mass of the armature contacts in order to achieve an increasingly faster contact switching time.

In any case, notice that the app note is a technique to obtain a longer than specified life from the relay. It is always nice to see such insightful tidbits.

  • \$\begingroup\$ Thanks, I think the material migration caused by "Thompson effect" when switching may be one reason. I'll wait for some more voices. \$\endgroup\$
    – diverger
    Jul 7 '17 at 14:51
  • \$\begingroup\$ Yes. Can you provide more infos on this? I've never saw docs talking about the differences on relay wiring. \$\endgroup\$
    – diverger
    Jul 7 '17 at 14:57
  • 1
    \$\begingroup\$ When an arc develops between the relay contacts, there is a temperature gradient across the arc column due to the Thomson (aka Lord Kelvin) effect. As a result, the cathode becomes hotter than the anode. This raises the spot temperature on the cathode to the point of freeing material in molecular and atomic form which is deposited on the cooler anode. The welding industry has perfected this mechanism. \$\endgroup\$
    – Glenn W9IQ
    Jul 7 '17 at 15:22

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