Contact protection

Contact protection refers to methods designed to limit the wear and degradation that occur during the normal use of contacts within an electromechanical switch, relay or contactor.

Contact wear

From left to right:
1. Pristine contacts from a relay
2. The nearly destroyed contacts from a relay operated under power for nearly 100,000 cycles

Every time the contacts of an electromechanical switch, relay or contactor are opened or closed, there is a certain amount of wear. The electric arc occurs between the contact points (electrodes) both during the transition from closed to open (break) or from open to closed (make).

For a very brief time, only a small section of the contact is touching (constriction), and all of the current must go through this part of the contact. The degradation caused by the high current density in the constriction during the contact make may result in a micro-weld akin to spot welding.

If the current being switched is large, part of the contact is degraded or destroyed. Because there is a very small gap in the contacts for a brief time when the contacts are broken, an electric arc is also generated across the gap if the voltage is high enough. This arc caused during the contact break (break arc) is akin to arc welding, as the break arc is typically more energetic and more destructive.[1]

Contact life is terminated when the contacts stick or weld, or when excessive material is lost from one or both contacts and a good electrical make is not possible. These conditions are the result of cumulative material transfer during successive switching operations, and of material loss due to splattering.[2]

Protection

The degradation of the contacts can be limited by including various contact protection methods.

One method is to add electronic components such as: capacitors, snubbers, diodes, Zener diodes, transient voltage suppressors, varistors, in-rush current limiters PTC resistors, NTC resistors, voltage-dependent resistors.[3] However, this is the least effective method as these do neither significantly influence the creation nor suppress the arc between the contacts of the electromechanical power switches, relays and contactors.[4]

A slightly more effective method is to make the contacts themselves larger, i.e., a contactor.[5]

A similar method to increasing contact size is to make the contacts out of more durable metals or metal alloys such as tungsten.[2]

The most effective methods are to employ arc suppression circuitry including arc suppressors, solid state relays, hybrid power relays, mercury displacement relays and hybrid power contactors.[6][7][8][9]

See also

References

  1. ↑ Ragnar Holm (1958). Electric Contacts Handbook (3rd ed.). Springer-Verlag, Berlin / Göttingen / Heidelberg. pp. 331–342.
  2. 1 2 "Relay Contact Life" (PDF). Retrieved February 10, 2012.
  3. ↑ Tyco P&B, Relay Contact Life, Tyco Electronics Corporation – P&B, Winston-Salem, NC, Application Note 13C3236, pgs. 1-3
  4. ↑ "The Snubber Myth". Retrieved February 10, 2012.
  5. ↑ Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-013932-6 page 7-124
  6. ↑ "Current solutions". Retrieved February 10, 2012.
  7. ↑ The National Association of Relay Manufacturers, Engineers’ Relay Handbook, NARM, 8th Edition, 1980, Chapter 13
  8. ↑ Reference Patent Application Publication # 20080266742 assigned to Watlow Electric Manufacturing Company
  9. ↑ "Arc Suppression". Retrieved December 6, 2013.
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