A panel is a board on which switches, fuses, and circuit breakers for controlling and protecting a number of similar branch circuits are mounted; installed in a cabinet and accessible from the front only. Also called a panel board.
The panel consists of:
Neutral line: a circuit conductor that normally completes the circuit back to the source; tied back to the substation.
Analogy: hot wire delivers power from a battery to a lightbulb; the neutral wire completes the circuit by returning electrons to the power supply.
Neutral wires take used electricity from the load back to the power supply.
Ground conductor: conductor intentionally connected to a ground connection; goes to the grounding electrode (metal ground plate or rod embedded in earth) to establish a ground.
Ground fault: electricity takes an unplanned path to ground; current increases and the breaker trips.
Causes include damaged appliances, incorrect wiring, or worn insulation.
Ground fault interrupter (GFI, GFCI): a circuit breaker that senses currents caused by ground faults and instantaneously shuts off power before damage or injury occurs.
Bus (bus bar): a heavy conductor, usually solid copper, used for collecting, carrying, and distributing large electric currents.
Fuse: protects the circuit from fire; contains a strip of fusible metal that melts under excess current, interrupting the circuit. Types include plug fuse, cartridge fuse, knife blade fuse, and time delay fuse. These are not normally used anymore and have been replaced by the main circuit breaker.
Circuit Breaker: a switch that automatically interrupts an electric circuit to prevent excess current from damaging equipment or causing fire. A circuit breaker may be reused without replacement of any component.
Some equipment may not require a grounding wire if it is double insulated (will not conduct electricity on the outside of the equipment), e.g., fans, TVs, toasters, radios. Some equipment that may require grounding wires include refrigerators, washing machines, air conditioners, and freezers.
Load and Demand Concepts
Load: the power delivered by a generator or transformer or the power consumed by an appliance or device.
Load Factor: the average load divided by the peak load in a specified time period. ext{Load Factor} = rac{P{ ext{avg}}}{P{ ext{peak}}}
Current load: the total load on an electric system or circuit if all connected apparatus and equipment are energized simultaneously.
Maximum demand: the greatest load delivered to an electrical system or circuit over a specified interval of time.
Demand factor: the ratio of the maximum demand to the connected load of an electrical system, used in estimating the required capacity of the system to account for the probability that only a portion of the connected load may be applied at any time. ext{Demand Factor} = rac{D{ ext{max}}}{P{ ext{connected}}}
Diversity Factor: the ratio of the sum of the maximum demands on the various parts of an electrical system to the maximum demand on the whole. ext{Diversity Factor} = rac{
\sumi Di}{D{ ext{total}}} where $Di$ are the maximum demands of individual parts and $D_{ ext{total}}$ is the maximum demand on the whole.
Lightning and Protection
Lightning affects power lines by direct and indirect effects; transient high voltages may cause flashover on electrical equipment.
Direct lightning strikes may intercept line conductors, towers, or shielding wires; probability of direct strike increases with line height (high voltage lines more at risk than MV/LV lines).
Indirect effects: when lightning strikes the ground or near a line, electromagnetic fields propagate and induce voltage impulses via inductive and capacitive coupling with conductors, potentially causing overvoltages on power lines.
Induced overvoltages can be significant on MV and LV lines due to relatively low critical flashover voltages compared to HV lines; indirect effect risk is often higher than direct strikes.
Protective measures include:
Lightning rods
Lightning arresters
Spark gaps
Electrical wiring and shielding practices to minimize induction and flashover
Wires, Cables, and Conductors (Definitions and Types)
Conductor: a substance, body, or device that conducts heat, sound, or electricity.
Cable: a single insulated conductor, or a bound or sheathed combination of conductors insulated from one another.
Armoured Cable (BX): Electrical cable consisting of two or more insulated conductors protected by a flexible helically wound metal wrapping.
Non-metallic sheathed cable (Romex): Electric cable consisting of two or more insulated conductors enclosed in a non-metallic, moisture-resistant, flame-retardant sheath.
Coaxial cable: a cable for transmitting high-frequency signals, consisting of an insulated conducting tube enclosing an insulated conducting core.
Wire: a pliable metallic strand or a twisted or woven assembly of such strands, often insulated, used as a conductor.
Insulator: a material that is a poor conductor of electricity, used to separate or support conductors to prevent undesired current flow.
Breakdown Voltage: the minimum applied voltage at which a given insulator breaks down and permits current to pass.
Dielectric Strength: the maximum voltage that can be applied to a given material without causing breakdown, usually expressed in volts or kilovolts per unit thickness.
Dielectric: a medium that transmits electric force without conduction; an insulator.
Junctions, Openings, and Protectors
Junction Box: an enclosure for housing and protecting electric wires or cables that are joined together in connecting or branching circuits.
Knockout: a removable panel opening in a casing or box to provide an opening into the interior.
Grommet: a rubber or plastic washer inserted in a hole to prevent grounding of a wire passing through the hole.
Brushing: an insulating and protective lining for conductors passing through a hole.
Conduit: a tube, pipe, or duct for enclosing and protecting electric wires or cables.
PVC Conduit: rigid nonmetallic raceway of circular cross section with fittings for installation of conductors.
Rigid Metal Conduit (RMC): heavy-walled tubular steel conduit joined by screwing into a threaded hub with lock nuts and bushings.
Flexible Metal Conduit (FMC): a flexible helically wound metal conduit for connections to motors or vibrating equipment.
Duct: an enclosed runway for housing conductors or cables.
Bus Duct (Busway): a rigid metal housing for a group of buses insulated from each other and the enclosure.
Cable Tray: an open metal framework for supporting insulated electrical conductors.
Raceway: a channel designed to hold and protect wires/cables; surface raceways for exposed installation in dry, non-hazardous, non-corrosive locations.
Multi-outlet assembly: a surface-mounted raceway designed to house wires for a circuit and a series of receptacles.
Underfloor Raceway: a raceway for installation under a floor, often used in office buildings to allow flexible placement of power, signal, and telephone outlets.
Electrical Switches and Outlets
Switch: a device for making, breaking, or directing an electric current; faceplates are typically placed over them as a protective plate surrounding an outlet or light switch.
Types of Switches:
Air Switch: push-button switch that uses air pressure instead of electricity to activate the device; used in garbage disposals; reduces shock risk when hands are wet.
Knife Switch: a form of air switch with a hinged copper blade between contact clips; used in heavy industries and laboratories for control and test panels where visible disconnects are required.
Float Switch: switch controlled by a conductor floating in a liquid; a level sensor used to detect liquid level in a tank; can control a pump, provide an indicator, alarm, or control other devices; one type uses a mercury switch inside a hinged float.
Mercury Switch: a quiet switch that opens/closes a circuit by moving a sealed glass tube of mercury to uncover or cover contacts; used in relays for industrial applications requiring frequent high-current switching.
Key Switch: switch operated only by inserting a key.
Dimmer: a rheostat or similar device to regulate light intensity without substantially altering spatial distribution.
Toggle Switch: a switch with a lever or knob moving through a small arc to open or close the circuit.
Poles and Throws:
Pole: number of separate circuits a switch can control (single-pole controls one circuit; double-pole controls two).
Throw: number of output connections each pole can have.
Three-way / Two-way Switch:
To control a light from two locations, use two SPDT switches.
SPDT is called a 2-way switch in the European Union and a 3-way switch in North America.
Outlets and Plugs
Outlet: a point on a wiring system where current is taken to supply an electric device; generally consists of the outlet box, the convenience outlet, the receptacle, and the faceplate.
Grounding outlet: includes an additional contact for a ground connection.
Plug: a male fitting for making an electrical connection to a circuit by insertion into a receptacle.