Wetting current

In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact.^[1] It is typically far below the contact's nominal maximum current rating.^[2]

A thin film of oxidation, or an otherwise passivated layer, tends to form in most environments, particularly those with high humidity, and, along with surface roughness, contributes to the contact resistance at an interface.^[3] Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.^[4][5]

Capacitor discharge solution

In some low voltage applications, where switching current is below the manufacturer's wetting current specification, a capacitor discharge method may be employed by placing a small capacitor across the switch contacts to boost the current through contact surface upon contact closure.^[4][6]

Sealing current

A related term sealing current (aka wetting current or fritting current) is widely used in the telecommunication industry describing a small constant DC current (typically 1-20 mA) in copper wire loops in order to avoid contact oxidation of contacts and splices. It is defined in ITU-T G.992.3 for "all digital mode ADSL" as a current flowing from the ATU-C (ADSL Linecard) via the phone lines to the ATU-R (CPE).^[7][8] Carbon brushes develop high resistance glaze when they're used without current flow for an extended period. A special circuit is utilized for turbines and generators to introduce current through the brushes into the shaft to prevent this contact fritting.^[9]

Contact cleaner

Contact cleaner can be applied to the contact surfaces to inhibit the formation of resistive surface films and/or to ameliorate existing films.^[10]

See also

• Chemistry:Anodic protection
• Physics:Cathodic protection – Corrosion prevention technique
• Engineering:Coherer – Early radio wave detector
• Physics:Contact protection
• Chemistry:Electromigration – Movement of ions in an electrical field
• Engineering:Electrowetting

References

Further reading

• Ney Contact Manual - Electrical Contacts for Low Energy Uses (reprint of 1st ed.). Deringer-Ney, originally JM Ney Co.. 2014. http://www.deringerney.com/techpapers/abstract/?DocumentId=pnk8SYvTdl5PBEYgylFWFjQO5IfWmXnrKiYltdIYKjM%3d.  (NB. Free download after registration.)
• Electrical Contacts: Principles and Applications. Electrical engineering and electronics. 105 (2 ed.). CRC Press, Taylor & Francis, Inc.. 2014-02-12. ISBN 978-1-43988130-9. 
• Williamson, J. B. P., ed (2013-06-29). Electric Contacts: Theory and Application (reprint of 4th revised ed.). Springer Science & Business Media. ISBN 978-3-540-03875-7.  (NB. A rewrite of the earlier "Electric Contacts Handbook".)
• Electric Contacts Handbook (3rd completely rewritten ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag. 1958. ISBN 978-3-662-23790-8.  [2] (NB. A rewrite and translation of the earlier "Die technische Physik der elektrischen Kontakte" (1941) in German language, which is available as reprint under ISBN:978-3-662-42222-9.)
• (in de) Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren (3 ed.). Berlin / Heidelberg / New York / Tokyo: Springer-Verlag. 2016. ISBN 978-3-642-45426-4. 
• "Overcome Wetting Voltage and Current Limitations With an External Wetting Power Supply". Schweitzer Engineering Laboratories, Inc. (SEL). 2016-01-11. https://cdn.selinc.com/assets/Literature/Publications/Application%20Notes/AN2016-01_20160111.pdf?v=20160406-152853. 
• The Test, Usage and Maintenance of Power Switching Subsystems. North Reading, MA, USA: Teradyne, Inc. / IEEE. 2011. ISBN 978-1-4244-9363-0. http://www.teradyne.com/File%20Library/Defense-Aero/Technical%20Papers/2011/Paton_The-Test--Usage-and-Maintenance-of-Power-Switching-Subsystems_2011.pdf. Retrieved 2018-05-21. 
• Fretting corrosion in power contacts: Electrical and thermal analysis, 2014, https://hal.archives-ouvertes.fr/hal-01104802/file/Holm%202014%20fin%20(1).pdf, retrieved 2018-05-21 

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