Relay application considerations

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

• Number and type of contacts – normally open, normally closed, (double-throw)
• Contact sequence – "Make before Break" or "Break before Make". For example, the old style telephone exchanges required Make-before-break so that the connection didn't get dropped while dialing the number.
• Contact current rating – small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
• Contact voltage rating – typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V
• Operating lifetime, useful life - 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 between the contacts, eventually leading to contacts that weld shut or contacts that fail due erosion by the arc.
• Coil voltage – machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils,
• Coil current - Minimum current required for reliable operation and minimum holding current, as well as, effects of power dissipation on coil temperature, at various duty cycles. "Sensitive" relays operate on a few milliamperes
• Package/enclosure – open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant, washable forprinted circuit board assembly
• Operating environment - minimum and maximum operating temperature and other environmental considerations such as effects of humidity and salt
• Assembly – Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
• Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
• Switching time – where high speed is required
• "Dry" contacts – when switching very low level signals, special contact materials may be needed such as gold-plated contacts
• Contact protection – suppress arcing in very inductive circuits
• Coil protection – 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 – some relays used in aerospace applications are designed to function in shock loads of 50 g or more
• Size - 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. 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. These considerations include factors such as speed of operation, sensitivity, andhysteresis. Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 us are available. Reed relays 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 motors, other issues must be addressed. When an inductance is connected to a power source, an 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 itself larger than the break rating.

Derating factors

Type of load	% of rated value
Resistive	75
Inductive	35
Motor	20
Filament	10
Capacitive	75

Control relays should not be operated above rated temperature because of resulting increased degradation and fatigue. Common practice is to derate 20 degrees Celsius from the maximum rated temperature limit. Relays operating at rated load are affected by their environment. Oil vapor may greatly decrease the contact life, and dust or dirt may cause the contacts to burn before the end of normal operating life. Control relay life cycle varies from 50,000 to over one million cycles depending on the electrical loads on the contacts, duty cycle, application, and the extent to which the relay is derated. When a control relay is operating at its derated value, it is controlling a smaller value of current than its maximum make and break ratings. This is often done to extend the operating life of a control relay. The table lists the relay derating factors for typical industrial control applications.

Undesired arcing

Main article: Arc suppression

Switching while "wet" (under load) causes undesired arcing between the contacts, eventually leading to contacts that weld shut or contacts that fail due to a buildup of contact surface damage caused by the destructive arc energy.

Inside the 1ESS switch matrix switch and certain other high-reliability designs, the reed switches are always switched "dry" to avoid that problem, leading to much longer contact life.

Without adequate contact protection, the occurrence of electric current arcing causes significant degradation of the contacts, which suffer significant and visible damage. Every time a relay transitions either from a closed to an open state (break arc) or from an open to a closed state (make arc & bounce arc), under load, an electrical arc can occur between the two contact points (electrodes) of the relay. In many situations, the break arc is more energetic and thus more destructive, in particular with resistive-type loads. However, inductive loads can cause more destructive make arcs. For example, with standard electric motors, the start-up (inrush) current tends to be much greater than the running current. This translates into enormous make arcs.

During an arc event, the heat energy contained in the electrical arc is very high (tens of thousands of degrees Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current. The extremely high temperature of the arc cracks the surrounding gas molecules creating ozone, carbon monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter. This action causes the material in the contacts to degrade quickly, resulting in device failure. This contact degradation drastically limits the overall life of a relay to a range of about 10,000 to 100,000 operations, a level far below the mechanical life of the same device, which can be in excess of 20 million operations.