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Yet another relay question. In this document the moving and fixed part of a relay contact are named anode and cathode. This suggests that there's a preferred current flow through the contact in DC applications. Is this taken in consideration if both contact parts have different contact materials? For instance, if contacts are tungsten and silver-nickel for anode and cathode resp. (just an example, not necessarily realistic), are you supposed to have the anode more positive than the cathode?

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    \$\begingroup\$ +1 for a good question. I never gave it much attention, and I think few people do, but I can imagine that for a DC application there could be a preferred polarisation if the contact has different materials for anode vs cathode. (But then I switch mostly AC.) \$\endgroup\$ – stevenvh Jul 11 '11 at 8:57
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With high current contactors (big relays) the stationary contact is usually the most positive terminal. The load gets connected to the armature (movable) contact. Very high current contactors, or large DC circuit breakers ~4KA-20KA will have a "Blow out coil" or a permanent magnet of sorts which will direct the arc discharge upwards usually through an "Arc chute" to dissipate the arc into the atmosphere. If the polarity of such a contactor were reversed, the arc would deflect down and into the closing coil likely burning it up.

EDIT: I stated that the stationary contact is usually the most positive, but this is not always the case. One needs to follow the manufacture's guidelines. I have seen 2 pole contactors with both a positive and negative arc chute, showing that the stationary contact can be either polarity. However the polarity is still defined, and must not be reversed. AC contactors also use arc chutes, and often (not always) the line side is the stationary contact.

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  • \$\begingroup\$ Do you have any information I could access to read more about this? \$\endgroup\$ – Kortuk Jul 11 '11 at 13:22
  • \$\begingroup\$ @Kortuk - Unfortunately no, I can not quote a source directly. My statement is based on my many years as a DC Substation engineer. My comments are common knowledge within this field. Much of what I learned came from documents from GE, Westinghouse, and a few others who were (and still are) big players in the industry. However these documents were written in the 40's and 50's (some even older!) way before the internet. I am since retired and no longer have access to those documents, otherwise I would offer to scan them. I did see a lot of hits on Google however, searching for "DC contactors". \$\endgroup\$ – SteveR Jul 11 '11 at 14:50
  • \$\begingroup\$ I just meant if you had anymore to read for someone like myself whom is forever curious it is nice. Your answer made logical sense and I have already upvoted you. Thank you for sharing your experience. Glad to hear we have a few power guys. I do not go higher then 13kV at work. The 300V switches have polarity also though. \$\endgroup\$ – Kortuk Jul 11 '11 at 15:34
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    \$\begingroup\$ +1 Interesting. I always thought it was heat which caused the upward motion of the arc. \$\endgroup\$ – starblue Jul 11 '11 at 19:39

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