Switches and Over Current Protective Devices Class Instructor ALPHER E. DE VERA, uap

DEVICES SwitchesFuse Circuit Breakers Panelboard Unit Sub – Station SwitchboardReceptacles Outlet and Boxes Emergency / Stand By Power Equipment

A. SWITCHES Switches are electrical devices intended for on / off control of an electrical circuit. Rated for current, voltage, duty, poles and throw, including fusibility and enclosure.

A. SWITCHES The current rating is the amount of current that the switch can carry continuously and interrupt safely. Switches are rated 250V, 600V or 5KV. Switches for normal use in light and power circuits are called general use safety switches and are rated ND ( normal duty ).

A. SWITCHES Switches intended for frequent interrupting are rated HD ( heavy duty ). Switches intended to be opened only occasionally, such as service switches, are rated LD ( light duty ). Switch may be constructed with ( fusible ) or without ( non – fusible ) fusing. Switch may be constructed with ( fusible ) or without ( non – fusible ) fusing.

A. SWITCHES NEC states that “ No grounded neutral conductor should be broken, thus most switches must carry the neutral through unbroken, by means of solid link within the switch, known as “ Solid Neutral.

SWITCH POLES AND THROWS Single Pole – Single Throw Switch Two Pole – Single Throw Switch Three Pole – Solid Neutral Switch Single Pole – Double Throw Switch * Center Type – With center “ off “ position or hand off control * Center Type – With center “ off “ position or hand off control automatic switch automatic switch * 3 – Way Switch – Two different locations * 3 – Way Switch – Two different locations

SPECIAL SWITCHES Time Controlled Devices Precise low speed miniature drive motor or timer permitting electric connection. Useful for areas frequented by power outages. Remote Control Switches Contactor that latches mechanically useful for lighting control and for energy conservation techniques. Automatic Transfer Switch Essential part of stand by power arrangement, a double throw switch generally 3 – pole to restore power in case of power failure.

CIRCUIT PROTECTIVE DEVICES To protect the insulation, wiring, switches and other apparatus from overload and short circuit currents. Acts as automatic means for opening circuits as well as protective devices. Two most common devices are : 1. Fuse2. Circuit Breaker 1. Fuse2. Circuit Breaker

B. 1. FUSE Simple device consisting of fusible link or wire of low melting temperature. Two kinds are cartridge fuse ( enclosed in insulating fiber tube ) and plug fuse ( enclosed in porcelain cup ).

B.1. FUSE When the fuse is subjected to excess current, the energy loss in the link generates heat and melt it. Plug fuse are normally used in residences all rated 5 – 30 amp while cartridge fuse are 5 – 600 amp.

B.2. CIRCUIT BREAKERS Safety shut-off in electric circuit: a device that can automatically stop the flow of electricity in a circuit if there is too much current to operate safely. Circuit breakers are easier to use than fuses because they just need to be reset instead of replaced. Circuit is a route for electricity: a route around which an electrical current can flow, beginning and ending at the same point.

B.2. CIRCUIT BREAKERS An electro mechanical device with the same protective function as a fuse and in addition, acts as a switch. It can be used in lieu of a switch and fuse combination to both protect and disconnect circuit.

B.2. CIRCUIT BREAKERS Most breakers are equipped with both thermal and magnetic trips. Thermal trip – causes an element to move when heat generation is excessive and trip the latching mechanic of the breaker, opening the breaker contacts along with the circuit. Magnetic Trip – actuate the core which trips the circuit breaker latch, where thermal and magnetic action have inverse time characteristics like heavier load means faster trip of action. Two types of circuit breakers are molded case breaker and large air breaker.

FUSE AND CIRCUIT BREAKERS A. FUSE Instantaneous in operation Energy limiting device causing self distraction B. CIRCUIT BREAKERS Provided with time delay avoiding immediate power failure. Can be reset after tripping by merely operating its handle.

D.1. SWITCHBOARDS Free standing assemblies of switches, fuses and circuit breakers which provide switching and feeder protection to a number of circuits connected to a main source. Represented in a single line diagram.

D.2. UNIT SUB - STATIONS Also called “ transfer load centers “, an assembly of fuse and circuit breaker combined with step – down transformer, meter controls, bus work and secondary switchgear. Available for indoor or outdoor use. Useful in supplying power from a primary voltage line to any large facility, usually located at the basement.

D.3. PANELBOARDS Serve basically the same as “ switchboard “ except on a smaller scale. Accepts relatively large blocks of power and distributes it in smaller blocks. Compose of main fuses to which hare connected in circuit protective devices ( breakers and fuses ) which feed to smaller circuits.

D.3. PANELBOARD Compose of buses and breakers mounted on insulating board that in turn is mounted inside an enclosing cabinet. The line terminal of each circuit is connected to the bus bars of the panelboard. The load terminal of the device then feeds the outgoing branch circuit.

D.3. PANELBOARD Panelboards are described by specific type, bus arrangement, branch breakers, main breaker, voltage and mounting Panelboards are classified as : a. Flush typeb. Surface type a. Flush typeb. Surface type

E. RECEPTACLES Identified by the number of poles and wires and whether or not designed for connection of a separate grounding wire. The equipment grounding pole is not counted in the number of poles but is counted in wires.

E. RECEPTACLES Poles must not be confused with the system ground or neutral. Available from 10 – 400 amp., 2- 4 poles and 125V – 600V.

F. OUTLETS AND BOXES Galvanized stamped sheet metal provided with conduit knockouts conveniently placed around the periphery or at the back.

F. OUTLETS AND BOXES Most common is the 4” square sand rectangular boxes used for fixtures, junctions, and devices. Junction box are enclosure for housing and protecting electric wires or cables that are joined together in connecting or branching electric circuits

G. EMERGENCY / STAND BY POWER EQUIPMENT A. Emergency System Provide power and illumination essential for safety and security of property. Emergency power for exit lighting, elevators, fire alarm, fire pumps especially for health care facilities. B. Stand By System Provide power to selected loads in the event of failure of normal source. Intended to protect against property damage and financial loss not involved in safety considerations. Useful for water and sewage treatment plants and industrial facilities.