In
electrical and electronics engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, wetting current is the minimum
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, and, along with surface roughness, contributes to the
contact resistance
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
at an interface.
Providing a sufficient amount of wetting current is a crucial step in designing
systems
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
that use delicate switches with small
contact pressure
Contact may refer to:
Interaction Physical interaction
* Contact (geology), a common geological feature
* Contact lens or contact, a lens placed on the eye
* Contact sport, a sport in which players make contact with other players or objects
* ...
as sensor inputs. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.
Capacitor discharge solution
In some low voltage applications, where switching current is below the manufacturer's wetting current specification, a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
discharge method may be employed by placing a small capacitor across the switch contacts to boost the current through contact surface upon contact closure.
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
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
G.992.3 for "all digital mode ADSL" as a current flowing from the
ATU-C (
ADSL
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ...
Linecard
A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network.
A line card typically interfaces the twisted pair cable of a plain ...
) 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 contact fritting.
Contact cleaner
Contact cleaner
Contact cleaner, also known as switch cleaner, is a term for a chemical, or a mixture of chemicals, intended to remove or prevent the build-up of oxides or other unwanted substances on the conductive surfaces of connectors, switches and other el ...
can be applied to the contact surfaces to inhibit the formation of resistive surface films and/or to ameliorate existing films.
See also
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References
Further reading
* (NB. Free download after registration.)
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* (NB. A rewrite of the earlier "''Electric Contacts Handbook''".)
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(NB. A rewrite and translation of the earlier "''Die technische Physik der elektrischen Kontakte''" (1941) in German language, which is available as reprint under .)
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Electrical engineering
Electrical parameters
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