In electric power systems and industrial automation, ANSI Device Numbers can be used to identify equipment and devices in a system such as relays, circuit breakers, or instruments.
The device numbers are enumerated in ANSI/IEEE Standard C37.2 Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
Today, microprocessor-based relays can perform many protective functions in one device.
Suffix numbers are used to distinguish multiple "same" devices in the same equipment such as 51–1, 51–2.
The subsequent letters are: 'C' security processing function (e.g. VPN, encryption), 'F' firewall or message filter, 'M' network managed function, 'R' rotor, 'S' switch and 'T' telephone component.