Wetting current

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In electrical engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance.[1] The film of oxidation occurs often in areas with high humidity. 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.

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 snubber capacitor across the switch contacts to boost the current through contact surface upon contact closure.[2]

A related term sealing current (aka wetting current or fritt current) is widely used in the telecommunication industry describing a small constant DC current (typically 1-20mA) 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).[citation needed]

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References[edit]

  1. ^ Gregory K. McMillan (ed) Process/Industrial Instruments and Controls Handbook (5th Edition) (McGraw Hill, 1999) ISBN 0-07-012582-1 page 7.26
  2. ^ The Case of the Intermittent Relay, Design News, Stewart, Anthony, 2011