Selecting the Proper Relay



Relays are very important electrical devices, and choosing the proper one is very crucial. Relays usually incorporate an electromagnet, which is activated by a current or signal in one circuit to open or close another circuit. There are numerous factors that should be taken into consideration when choosing the proper relay for a job, including the contact system, magnetic system, the environment, and the physical requirements for installation.


The contacts in an electromagnetic relay make or break connections in electric circuits. The contacts carry the inrush as well as the nominal load current. Their arrangement, mechanical construction and suitability of the contact materials all determine how well they physically perform. Electrically, two important factors influence contact performance – the first being the magnitude of load current and open circuit voltage. The second is the specific characteristics of the circuitry.


The coil of the relay’s magnetic system causes the contacts to open or close when power is applied to the coil. With the exception in special cases, the coil must be adequately energized by the voltage of the power source to operate the contact system at all times, and the power source should not drift to cause the coil to malfunction. While coil function is inter-related with the functions of other relay components, the coil design has a major effect on relay sensitivity, operating speed and power consumption.


There is a wide choice of terminals to choose from: pc mount, screw-type, threaded stud, quick-connect, pierced or wire solder lug, taper tab, octal base, and other plug-in types meet every connection requirement.


There are certain environments that require dust-proof or hermetically sealed enclosures. Relays are cleaned ultrasonically to remove all possible contaminants prior to sealing. When indicated, hermetically sealed relays are tested with Radiflo Leak Test equipment. These high-voltage breakdown tests automatically check in sequence all of the relay’s possible voltage breakdown points.

Relay Contact Life

Relay contacts are available in a variety of metals and alloys, sizes and styles. A universal contact does not exist. The person using the relay should select contact materials, ratings, and styles to meet, as accurately as possible, the requirements of a particular application. Failure to do this can result in contact problems, as well as early contact failure.

In some applications, the contacts may be subjected to punishing current surges which can drastically reduce their life. For example, a 40 watt, 120V AC incandescent lamp has a current rating of .33 ampere. The resistance of the filament when cold, however, is so low that initial inrush current may be as much as 6 amps.

Contact Materials

Fine silver has the highest electrical and thermal properties of all metals, and is the best general purpose material available. Unfortunately, it is affected by sulfidation. The rate of sulfidation indoors in a metropolitan area is approximately 70 micrograms per square centimeter per day. This sulfidation forms a film on the surface of the silver which increases contact interface resistance. Silver has a tendency to stick and weld in certain applications. Because of this, silver is alloyed with other metals to offer advantages that are not possible with just silver alone.

Contact Life

The electrical life expectancy of general purpose and power relays is generally rated to be 100,000 operations minimum, while mechanical life expectancy may be one million, 10, or even 100 million operations.

Why is electrical life rated so low compared with mechanical life? This is because contact life is application dependent. The electrical rating applies to contacts switching their rated loads. When a set of contacts switches a load of less than rated value, contact life may be significantly greater.

Rated electrical life also takes into consideration arc destruction of the contacts. By use of appropriate arc suppression, contact life may be lengthened. An arc will ignite if both minimum arc voltage and current are exceeded.

Contact life ends when the contacts stick or weld, or when excessive material is lost from one or both contacts and a good electrical make is not possible. These conditions are the result of cumulative material transfer during successive switching operations, and of material loss due to splattering.