Module 3—Introduction to Circuit Protection, Control, and Measurement
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ACTUATOR—The part of a switch that is acted upon to cause the switch to
change contact connections;
e.g., toggle, pushbutton, and rocker.
AMMETER—A meter used to measure
ARC EXTINGUISHER—The part of a circuit breaker that confines and divides the arc that occurs when
the contacts of the circuit breaker open.
ARMATURE—In a relay, the movable portion of the relay.
BREAK—In a switch, the number of breaks refers to the number of points at which the switch opens the
circuit; e.g., single break and double break.
BURNISHING TOOL—A tool used to clean and polish contacts on
CONTINUITY—An uninterrupted, complete path for current.
DAMPING—The process of smoothing out
oscillations. In a meter, damping is used to keep the pointer of the meter from overshooting the correct reading.
D’ARSONVAL METER MOVEMENT—A name used for the permanent-magnet moving-coil movement used in most meters.
DIRECT SHORT—A connection between two points in a circuit, such as between a component and ground.
ELECTRODYNAMIC METER MOVEMENT—A meter movement using fixed field coils and a moving coil; usually used in
ELECTROMAGNETISM—The relationship between magnetism and electricity.
METER MOVEMENT—A meter movement that uses the electrostatic repulsion of two sets of charged plates (one fixed and
the other movable). This meter movement reacts to voltage rather than to current and is used to measure high
FERRULES—The cylindrical metallic ends of a cartridge fuse.
FREQUENCY METER—A meter used
to measure the frequency of an ac signal.
GALVANOMETER—A meter used to measure small values of current by
electromagnetic or electrodynamic means.
HOT WIRE METER MOVEMENT—A meter movement that uses the expansion
of a heated wire to move the pointer of a meter; measures dc or ac.
IN-CIRCUIT METER—A meter permanently installed in a circuit; used to monitor circuit operation.
LOADING EFFECT—The effect of a voltmeter upon the circuit being measured which results in an
inaccurate measurement. Loading effect is minimized by using a voltmeter with an internal resistance many times
higher than the resistance of the circuit being measured.
MAGNETIC TRIP ELEMENT—A circuit breaker trip
element that uses the increasing magnetic attraction of a coil with increased current to open the circuit.
MEGGER—Common name for a megohmmeter.
MEGOHMMETER—A meter that measures very large values of resistance;
usually used to check for insulation breakdown in wires.
METER—A device used to measure an electrical
quantity; e.g., current, voltage, and frequency.
METER MOVEMENT—The part of a meter that moves.
MOVING-IRON METER MOVEMENT—Same as moving-vane meter movement.
MOVING-VANE METER MOVEMENT—A meter movement
that uses the magnetic repulsion of the like poles created in iron vanes by current through a coil of wire; most
commonly used movement for ac meters.
MULTIMETER—A single meter combining the functions of an ammeter, a
voltmeter, and an ohmmeter.
NONTRIP-FREE CIRCUIT BREAKER—A circuit breaker that can be held ON during an
OHMMETER—A meter used to measure resistance.
OUT-OF-CIRCUIT METER—A meter which is not permanently installed in a circuit. Usually portable and
self-contained, these meters are used to check the operation of a circuit or to isolate troubles within a circuit.
PARALLAX ERROR—The error in meter readings that results when you look at a meter from some position other than
directly in line with the pointer and meter face. A mirror mounted on the meter face aids in eliminating parallax
POINT BENDER—A tool used to adjust the contact spacing on a relay.
POLE—(1) One end of a magnet. (2)
The number of points at which current can enter a switch; e.g., single pole, double pole, and three pole.
POLE PIECE—A piece of ferromagnetic material used to control the distribution of magnetic lines of force; i.e.,
concentrate the lines of force in a particular place or evenly distribute the lines of force over a wide area.
RANGES—The several upper limits a meter will measure as selectable by a switch or by jacks; e.g., a voltmeter may
have ranges of 1 volt, 2.5 volts, 10 volts, 25 volts, and 100 volts.
RECTIFIER—A device used to convert ac
to pulsating dc.
RELAY—An electromagnetic device with one or more sets of contacts which changes position
by the magnetic attraction of a coil to an armature.
RELUCTANCE—The resistance of a magnetic path to the flow of magnetic lines of force through it.
ROTARY SWITCH—A multicontact switch with contacts arranged in a circular or semi-circular manner.
SENSITIVITY—(1) For an ammeter: the amount of current that will cause full-scale deflection of the meter. (2) For
a voltmeter: the ratio of the voltmeter resistance divided by the full-scale reading of the meter, expressed in
SHORT CIRCUIT—An unintentional current path between two components in a circuit or between a component and
ground which is usually caused by a malfunction in the circuit.
SHUNT RESISTOR—A resistor in parallel. In
an ammeter, shunt resistors are used to provide range capability.
SNAP-ACTING—Changing position quickly
with the aid of a spring.
SOLENOID—An electromagnetic device that changes electrical energy into
mechanical motion; based upon the attraction of a movable iron plunger to the core of an electromagnet.
SWITCH—A device used to open or close a circuit.
TEST EQUIPMENT—A general term applied to devices used to
test electrical and electronic circuits.
THERMAL TRIP ELEMENT—A circuit breaker trip element that uses the
increased bending of a bimetallic strip caused by increased current to open a circuit.
TRIP ELEMENT—A single circuit breaker trip element that combines the action of a thermal and a magnetic trip
THERMOCOUPLE METER MOVEMENT—A meter movement that uses the current induced in a thermocouple by
the heating of a resistive element to measure the current in a circuit; used to measure ac or dc.
a switch, the number of different circuits each pole can control; e.g., single throw and double throw.
TRIP-ELEMENT—The part of a circuit breaker that senses any overload condition and causes the circuit breaker to
open the circuit.
TRIP-FREE CIRCUIT BREAKER—A circuit breaker that will open a circuit even if the
operating 5 mechanism is held in the ON position.
TROUBLESHOOTING—The process of locating and repairing
faults in electrical or electronic equipment.
VOLTMETER—A meter used to measure voltage.
SWITCH—A rotary switch in which the contacts are arranged on levels. Each level is electrically independent but
mechanically connected by the shaft of the switch.
WATT-HOUR METER—A meter used to measure electrical
WATTMETER—A meter used to measure electrical power.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics