Relay application considerations

1. Selection of an appropriate relay for a particular application requires evaluation of many different factors:
2. Number and type of contacts - normally open, normally closed, changeover (double-throw)
3. In the case of changeover, there are two types. This style of relay can be manufactured two different ways. "Make before Break" and "Break before Make". The old style telephone switch required Make-before-break so that the connection didn't get dropped while dialing the number. The railroad still uses them to control railroad crossings.
4. Rating of contacts - small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
5. Voltage rating of contacts - typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V
6. Coil voltage - machine-tool relays usually 24 VAC or 120 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few mill amperes
7. Package/enclosure - open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil-splash resistant
8. Mounting - sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
9. Switching time - where high speed is required
10. "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts
11. Contact protection - suppress arcing in very inductive circuits
12. Coil protection - suppress the surge voltage produced when switching the coil current
13. Isolation between coil circuit and contacts
14. Aerospace or radiation-resistant testing, special quality assurance
15. Expected mechanical loads due to acceleration - some relays used in aerospace applications are designed to function in shock loads of 50 g or more
16. Accessories such as timers, auxiliary contacts, pilot lamps, test buttons
17. Regulatory approvals
18. Stray magnetic linkage between coils of adjacent relays on a printed circuit board.