No	Name	Description	Notes
21	distance relay	A device that functions when the circuit admittance, impedance, or reactance increases or decreases beyond a predetermined value.
22	equalizer circuit breaker	A device that serves to control or make and break the equalizer or the current-balancing connections for a machine field, or for regulating equipment, in a multiple-unit installation.
23	temperature control device	A device that functions to control the temperature of a machine or other apparatus, or of any medium, when its temperature falls below or rises above a predetermined value.	An example is a thermostat that switches on a space heater in a switchgear assembly when the temperature falls to a desired value. This should be distinguished from a device that is used to provide automatic temperature regulation between close limits and would be designated as device function 90T.
24	volts per hertz relay	A device that operates when the ratio of voltage to frequency is above a preset value or is below a different preset value. The relay may have any combination of instantaneous or time delayed characteristics.
25	synchronizing or synchronism-check relay	A synchronizing device produces an output that causes closure at zero-phase angle difference between two circuits. It may or may not include voltage and speed control. A synchronism-check relay permits the paralleling of two circuits that are within prescribed limits of voltage magnitude, phase angle, and frequency.
26	apparatus thermal device	A device that functions when the temperature of the protected apparatus other than the load-carrying windings of machines and transformers as covered by device function number 49 or of a liquid or other medium exceeds a predetermined value; or when the temperature of the protected apparatus or of any medium decreases below a predetermined value.
27	undervoltage relay	A device that operates when its input voltage is less than a predetermined value.
28	flame detector	A device that monitors the presence of the pilot or main flame in such apparatus as a gas turbine or a steam boiler.
29	isolating contactor or switch	A device that is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.
30	annunciator relay	A nonautomatically reset device that gives a number of separate visual indications upon the functioning of protective devices and that may also be arranged to perform a lockout function.
31	separate excitation device	A device that connects a circuit, such as the shunt field of a synchronous converter, to a source of separate excitation during the starting sequence.
32	directional power relay	A device that operates on a predetermined value of power flow in a given direction such as reverse power flow resulting from the motoring of a generator upon loss of its prime mover.
33	position switch	A device that makes or breaks contact when the main device or piece of apparatus that has no device function number reaches a given position.
34	master sequence device	A device such as a motor-operated multi-contact switch, or the equivalent, or a programmable device, that establishes or determines the operating sequence of the major devices in equipment during starting and stopping or during sequential switching operations.
35	brush-operating or slip-ring short-circuiting device	A device for raising, lowering, or shifting the brushes of a machine; short-circuiting its slip rings; or engaging or disengaging the contacts of a mechanical rectifier.
36	polarity or polarizing voltage device	A device that operates, or permits the operation of, another device on a predetermined polarity only or that verifies the presence of a polarizing voltage in equipment.
37	undercurrent or underpower relay	A device that functions when the current or power flow decreases below a predetermined value.
38	bearing protective device	A device that functions on excessive bearing temperature or on other abnormal mechanical conditions associated with the bearing, such as undue wear, which may eventually result in excessive bearing temperature or failure.
39	mechanical condition monitor	A device that functions upon the occurrence of an abnormal mechanical condition except that associated with bearings as covered under device function 38, such as excessive vibration, eccentricity, expansion, shock, tilting, or seal failure.
40	field relay	A device that functions on a given or abnormally high or low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an ac machine indicating abnormally high or low field excitation.