Nema Device Numbers - Apprenticeship Pertinent With Descriptions
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- Master Element
- An initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time-delay relays to place an equipment in or out of operation.
- Stopping Device
- A control device used primarily to shut down equipment and hold it out of operation. This device may be manually or electrically actuated, but excludes the function of electrical lockout (86) on abnormal conditions.
- Unit Sequence Switch
- A switch that is used to change the sequence in which units may be placed in and out of service in multiple-unit equipments.
- Over-Speed Device
- Usually a direct-connected speed switch which functions on machine over speed.
Synchronous-Speed Device is a device such as a centrifugal switch, a slip-frequency relay, a voltage relay, and undercurrent relay, or any type of device that operates at approximately the synchronous speed of a machine.
- Under-Speed Device
- A device that functions when the speed of a machine fall below a pre-determined value.
Speed or Frequency Matching Device is a device that functions to match and hold the speed or frequency of a machine or of a system equal to, or approximately equal to, that of another machine, source, or system.
- Shunting or Discharge Switch
- A switch that serves to open or to close a shunting circuit around any piece of apparatus (except a resistor, such as a machine field, a machine armature, a capacitor, or a reactor).
- Distance Relay
- A relay that functions when the circuit admittance, impedance, or reactance increases or decreases beyond a predetermined value.
- Volts-Per-Hertz Relay
- A relay that functions when the ratio of voltage to frequency exceeds a preset value. The relay may have an instantaneous or a time characteristic.
- Synchronizing or Synchronism-Check Device
- A device that operates when two ac circuits are within the desired limits of frequency, phase angle, and voltage to permit or cause the paralleling of these two circuits.
- Undervoltage Relay
- A relay that operates when its input voltage is less than a predetermined value.
- Isolating Contactor
- A device that is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.
- Annunciator Relay
- A non-automatically reset device that gives a number of separate visual indications of the functions of protective devices, and which may also be arranged to perform a lockout function.
- Directional Power Relay
- A relay that operates on a predetermined value of power flow in a given direction or upon reverse power flow such as that resulting from the motoring of a generator upon loss of its prime mover.
- Position Switch
- A switch operates when a device or apparatus reaches a given position.
- Master Sequence Device
- A device such as a motor operated multi-contact switch, or programming device9such as a (computer) that establishes or determines the operating sequence of the major devices in equipment during starting, stopping and other sequential switch operations.
- Bearing Protective Device
- A device that functions on excessive bearing temperature or another abnormal mechanical condition associated with the bearing, such as undue wear, which may eventually result in excessive bearing temperature.
- Field Relay
- 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 ac machine indicating abnormally low field excitation.
- Field Circuit Breaker
- A device that functions to apply or remove the field excitation of a machine.
- Manual Transfer or Selector Device
- A manually operated device that transfers the control circuits in order to modify the plan of operation of the switching equipment or of some of the devices.
- Reverse-Phase or Phase-Balance Current Relay
- A relay that functions when the poly-phase currents are of reverse-phase sequence or when the poly-phase currents are unbalanced or contain negative phase sequence components above a given amount.
- Incomplete Sequence Relay
- A relay that generally returns the equipment to the “normal” or “off” position and locks it out if the normal starting, operating, or stopping sequence is not properly completed within a predetermined time. If the device is used for alarm purposes only, it should preferably be designated as 48A (alarm).
- Instantaneous Over-Current Relay
- A relay that functions instantaneously on an excessive value of current.
- AC Time Over-Current Relay
- A relay that functions when the AC input current exceeds a predetermined value, and in which the input current and operating time are inversely related through a substantial portion of the performance range.
- Overvoltage Relay
- A relay that operates when its input voltage is more than a predetermined value.
- Pressure Switch
- A switch that makes electrical contact when a certain set pressure has been reached on its input.
- Ground Detector Relay
- A relay that functions on failure of the insulation of a machine, transformer; or of other apparatus to ground; or on flashover of a DC machine to ground.
- The assembly of fluid, electrical, or mechanical control equipment used for regulating the flow of water, steam, or other medium to the prime mover for such purposes a starting, holding speed or load, or stopping.
- AC Directional Over-Current Relay
- A relay that functions on a desired value of AC over-current flowing in a predetermined direction.
- A variable resistance device used in an electric circuit, which is electrically operated or has other electrical accessories, such as auxiliary, position, or limit switches.
- Alarm Relay
- A relay other than an annunciator (30) that is used to operate or to operate in connection with, a visual or audible alarm.
- AC Reclosing Relay
- A relay that controls the automatic reclosing and locking out of an ac circuit interrupter.
- Frequency Relay
- A relay that responds to the frequency of an electrical quantity; operating when the frequency or rate of change of frequency exceeds or is less than a predetermined value.
- Carrier or Pilot-Wire Receiver Relay
- A relay that is operated or restrained by a signal used in connection with carrier-current or DC pilot-wire fault relaying.
- Lockout Relay
- An electrically operated hand or electrically reset relay or device that functions to shutdown or hold an equipment out of service, or both, upon the occurrence of abnormal conditions.
- Differential Protective Relay
- A protective relay that functions on a percentage, phase angle or other predetermined difference between two electrical quantities.
- Auxiliary Motor or Motor Generator
- Used for operating auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers etc.
- Regulating Device
- A device that functions to regulate quantities such as voltage, current, power, speed, frequency, temperature, and load at a certain value or between certain limits.
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