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. It is typically far below the contact's nominal maximum current rating.
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. 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.
Capacitor discharge solutionEdit
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.
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 1-20 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). 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.
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[…] Wynn Quon: The Bellcore Layer 1 specs talk about "sealing current". This is a low current (1-20mA) DC signal applied to tip and ring. […] It is supposed to reduce oxidation at line splices and it provides a troubleshooting aid in the field. […] Brad Bennett: I was personally the researcher that did the sealing current work while at Bellcore […] sealing current does effectively keep a copper loop intact (through a process called electromigration […] it does work on copper loops which have splices […] for direct copper loops (CO to customer sites) it is ALWAYS suppose to be applied (and is built into the line cards). For other technologies (e.g. BRITE cards), which synthesize ISDN from 3 DS0 circuits at a subscriber loop cabinet (SLC), I am […] not sure it is a requirement. […] other interesting upshots of […] this work, which I am not certain have ever made it to the public. For example there are certain metal pairs [for] which you definitely do NOT want to use sealing current […] or […] run any continuous DC current (e.g. copper and precious metals). Such contacts (splices) are materially designed to fail if current continually flows in the wrong direction. […] on […] copper wires, sealing current helps to maintain good electrical connections […]
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