(IA/ICTL/IAC) field relay (power system device function numbers) A relay that functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an alternating-current (ac) machine indicating. (IA/ICTL/IAC) field relay (power system device function numbers) A relay that functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an alternating-current (ac) machine indicating. A protective relay is a device that is used to protect electrical equipment from damage or failure. It is designed to detect abnormal conditions, such as a power surge or a short circuit, and respond by opening or closing electrical contacts. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. : 4 The first. The protection and control devices in electrical equipment can be referred to by numbers, with appropriate suffix letters when necessary, according to the functions they perform. These numbers are based on a system that is adopted by a standard for automatic switchgear by Institute of Electrical. Please note before using selection table! There are two methods for indicating protection relay functions in common use. The other is given in IEC 60617 and uses. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems.