Editing Relay (section)

=== Selection considerations ===
[[file:Uy-multi1-hy.jpg|thumb|Several 30-contact relays in "Connector" circuits in mid-20th century [[Number One Crossbar Switching System|1XB switch]] and [[Number Five Crossbar Switching System|5XB switch]] telephone exchanges; cover removed on one.]]

Selection of an appropriate relay for a particular application requires evaluation of many different factors:

* Number and type of contacts&nbsp;— normally open, normally closed, (double-throw)
* Contact sequence&nbsp;— "make before break" or "break before make". For example, the old style telephone exchanges required make-before-break so that the connection did not get dropped while dialing the number.
* Contact current rating&nbsp;— small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
* Contact voltage rating&nbsp;— typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000&nbsp;V
* Operating lifetime, useful life&nbsp;— the number of times the relay can be expected to operate reliably. There is both a mechanical life and a contact life. The contact life is affected by the type of load switched. Breaking load current causes [[#Undesired arcing|undesired arcing]] between the contacts, eventually leading to contacts that weld shut or contacts that fail due to erosion by the arc.<ref name="electronic-components.com.au">{{cite web | title = Arc Suppression to Protect Relays From Destructive Arc Energy | url = http://www.electronic-components.com.au/products-services/arc-suppression/ | access-date = December 6, 2013}}</ref>
* Coil voltage&nbsp;— machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils, 
* Coil current&nbsp;— Minimum current required for reliable operation and minimum holding current, as well as effects of power dissipation on coil temperature at various [[duty cycle]]s. "Sensitive" relays operate on a few milliamperes.
* Package/enclosure&nbsp;— open, touch-safe, double-voltage for isolation between circuits, [[Electrical equipment in hazardous areas|explosion proof]], outdoor, oil and splash resistant, washable for [[printed circuit board]] assembly
* Operating environment&nbsp;— minimum and maximum operating temperature and other environmental considerations, such as effects of humidity and salt
* Assembly&nbsp;— Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
* Mounting&nbsp;— sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
* Switching time&nbsp;— where high speed is required
* "Dry" contacts&nbsp;— when switching very low level signals, special contact materials may be needed such as gold-plated contacts
* Contact protection&nbsp;— suppress arcing in very inductive circuits
* Coil protection&nbsp;— suppress the surge voltage produced when switching the coil current
* Isolation between coil contacts
* Aerospace or radiation-resistant testing, special quality assurance
* Expected mechanical loads due to [[acceleration]]&nbsp;— some relays used in [[aerospace]] applications are designed to function in [[Shock (mechanics)|shock]] loads of 50 [[G-force|''g'']], or more.
* Size&nbsp;— smaller relays often resist mechanical vibration and shock better than larger relays, because of the lower inertia of the moving parts and the higher natural frequencies of smaller parts.<ref name="keller"/> Larger relays often handle higher voltage and current than smaller relays.
* Accessories such as timers, auxiliary contacts, pilot lamps, and test buttons.
* Regulatory approvals.
* Stray magnetic linkage between coils of adjacent relays on a printed circuit board.
There are many considerations involved in the correct selection of a control relay for a particular application, including factors such as speed of operation, sensitivity, and [[hysteresis]]. Although typical control relays operate in the 5 [[millisecond|ms]] to 20 ms range, relays with switching speeds as fast as 100 [[microsecond|μs]] are available. [[Reed relay]]s which are actuated by low currents and switch fast are suitable for controlling small currents.

As with any switch, the contact current (unrelated to the coil current) must not exceed a given value to avoid damage. In high-[[inductance]] circuits such as [[electric motor|motors]], other issues must be addressed. When an inductance is connected to a power source, an [[inrush current|input surge current or electromotor starting current]] larger than the steady-state current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging arc across the separating contacts.

Consequently, for relays used to control inductive loads, we must specify the maximum current that may flow through the relay contacts when it actuates, the ''make rating''; the continuous rating; and the ''break rating''. The make rating may be several times larger than the continuous rating, which is larger than the break rating.