Contact protection methods are designed to mitigate the wear and degradation occurring during the normal use of contacts within an electromechanical
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
,
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
or
contactor
:''In semiconductor testing, contactors can also be referred to as the specialized socket that connects the device under test.''
:''In process industries, a contactor is a vessel where two streams interact, for example, air and liquid. See Gas- ...
and thus avoid an excessive increase in
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 ...
or switch failure.
Contact wear
![Contacts - new and used](https://upload.wikimedia.org/wikipedia/commons/6/6c/Contacts_-_new_and_used.jpg)
Every time the contacts of an
electromechanical
In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
switch, relay or contactor are opened or closed, there is a certain amount of contact wear. The sources of the wear are high
current densities in microscopic areas, and the
electric arc
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
. Contact wear includes material transfer between contacts, loss of contact material due to splattering and evaporation, and oxidation or
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
of the contacts due to high temperatures and atmospheric influences.
While a pair of contacts is closed, only a small part of the contacts are in intimate contact due to
asperities
In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (from the Latin ''asper''—"rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, a ...
and low-conductivity films. Because of the constriction of the current to a very small area, the
current density
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional ar ...
frequently becomes so high that it melts a microscopic portion of the contact. During the close-to-open (break) transition, a microscopic molten bridge forms and eventually ruptures asymmetrically, transferring contact material between contacts and increasing the surface roughness. This can also occur during the open-to-close (make) transition due to
contact bounce
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
.
The
electric arc
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
occurs between the contact points (electrodes) both during the transition from closed to open (break) and from open to closed (make) when the contact gap is small and the voltage is high enough. Heating due to arcing and high current density can melt the contact surface temporarily. If some of the melting material solidifies while the contacts are closed, the contact may stick closed due to a micro-weld, similar to
spot welding
A spot welder
Spot welding (or resistance spot welding) is a type of electric resistance welding used to weld various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance ...
.
The arc caused during the contact break (break arc) is similar to
arc welding
Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding powe ...
, as the break arc is typically more energetic and more destructive. The arc can cause material transfer between contacts. The arc may also be hot enough to evaporate metal from the contact surface.
The high temperatures can also cause the contact metals to more rapidly oxidize and corrode.
Contacts reach end of life for one of two reasons. Either the contacts fail to break because they are stuck (welded) closed, or the contacts fail to make (high resistance) because of contact corrosion or because excessive material is lost from one or both contacts. These conditions are the result of cumulative material transfer during successive switching operations, and of material loss due to evaporation and splattering.
There are additional mechanisms for stuck closed failures, such as mechanical interlocking of rough contact surfaces due to contact wear.
Protection
The degradation of the contacts can be limited by including various contact protection methods.
One method is to add electronic components such as:
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 ...
s,
snubber
A snubber is a device used to suppress ("snub") a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid movement in mechanical systems.
Ele ...
s,
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
s,
Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great varie ...
s,
transient voltage suppressors (TVS),
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s,
varistor
A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non- ohmic current–voltage characteristic that is similar to that ...
s or in-rush current limiters (
PTC and
NTC resistors). However, this is the least effective method as these neither significantly influence the creation of nor suppress the arc between the contacts of electromechanical power switches, relays and contactors.
A slightly more effective method is to make the contacts themselves larger, i.e., a contactor.
A similar method to increasing contact size is to make the contacts out of more durable metals or metal alloys such as tungsten.
The most effective methods are to employ
arc suppression Arc suppression is the reduction of sparks formed when current-carrying contacts are separated. The spark is a luminous discharge of highly energized electrons and ions, and is an electric arc.
Uses
There are several possible areas of use of arc s ...
circuitry including arc suppressors, solid state relays, hybrid power relays, mercury displacement relays and hybrid power contactors.
See also
*
Arc suppression Arc suppression is the reduction of sparks formed when current-carrying contacts are separated. The spark is a luminous discharge of highly energized electrons and ions, and is an electric arc.
Uses
There are several possible areas of use of arc s ...
*
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 ...
*
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. It is typically far below the contact's nominal maximum curre ...
*
Wetting voltage
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 curre ...
References
{{Switches
Switches
Power engineering
Electric arcs