Pilot and Overcurrent Devices

  • Motors are often controlled with simple stop/start buttons

  • A start button is a momentary make N/O contact

  • A stop button is a momentary break N/C contact

  • To add more stop buttons they must be series with each other

  • To add more start buttons they must be parallel with each other

  • Pilot devices signal a change by opening or closing a set of contacts: may be momentary or maintained

  • Automatic pilot devices detect a change and conver this change to the opening or closing of a set of contacts

  • Temperature switches are automatic pilot devices that control contacts by detecting a temperature change

  • Thermocouples are voltage devices that indicate a temperature by producing a change in voltage

  • Resistive temperature devices use resistance to indicate a change in temperature

  • Infrared sensors are non-contacting sensors, measures the temperature of a surface by virtue of its thermal radiation

  • Bimetallic devices take advantage of the expansion of metals when they are heated

  • Silicon diode sensors can be used over the widest variety of temperature ranges

  • Pressure switches is a switch with both N/O and N/C contacts connected to an actuator, some may be dual-stage and operate contacts at two predetermined pressures.

  • Pressure switches are drawn as if there is no pressure in the system

  • All switches are drawn de-energized or without any forces acting upon them

  • Limit switches and proximity switches have several applications:

  1. Measuring the travel of a piece of equipment for positioning

  2. Stopping or changing directions

  3. Measuring the levels of solids in a container or pile

  • Limit switches require the actuator, level, cam, or rod to contact the object being sensed

  • Proximity switches can detect the object or material without contacting it

  • Types of proximity switches are:

  1. Magnetic

  2. Inductive

  3. Capacitive

  4. Photoelectric

  • Magnetic proximity switches operate by detecting the presence of an external magnetic field

  • Inductive proximity switches operate by creating a magnetic field and then sensing when a ferrous metal enters that field

  • Capacitive proximity switches operate by creating an electrostatic field and then sensing when an object enters that field

  • Photoelectric proximity switches operate by sensing light reflected off an object or sensing when an object interrupts a beam of light

  • Float switches indicate the liquid level of a vessel or tank, may be level actuated or a tethered bulb

  • The switching is done in the bulb itself where as level actuated is done at the termination point

  • Current flow creates heat and magnetism, if too much current flows, the heat generated could damage even the conductor itself, this is an overcurrent scenario

  • Overcurrent scenarios can be caused by either short-circuits or overloads

  • A short-circuit occurs in a circuit if a live ungrounded conductor contacts:

  1. Another ungrounded conductor

  2. A grounded conductor

  3. Equipment that is bonded to ground

  • Current in a short-circuit can be extremely high because the impedance of the path is typically very low

  • Overload conditions is when the overcurrent is moderately higher than the normal current for that circuit, this prolonged heat can damage the circuit and conductors

  • An overcurrent device is any device capable of automatically opening an electric circuit under both pretermined overload and short-circuit conditions

  • Overcurrent devices have two ratings:

  1. A continuous current rating: the amount of current a device will carry without exceeding a specified temperature rise

  2. An interrupting rating: the maximum amount of current a device can safely interrupt, typically very high amounts

  • The two types of overcurrent devices are:

  1. Fuses: current-senstivie device that consists of a conductor called the element, surrounded by an arc-quenching materiall called the filler

  2. Circuit breakers: electromechanical devices that can be operated manually to de-energize a circuit and can also operate automatically in reponse to overcurrent conditions

  • Fuses must carry its normal load continuously without deteriorating (passive functionality) and open the circuit under overcurrent conditions (active functionality)

  • There are plug and cartridge fuses

  • Non-time delay fuses operate quickly when a circuit exceeds the current rating, they do not allow for brief overloads

  • The total operating time of a non-time-delay fuse can be less than one half an electrical cycle, very high short-circuit ccurrents can be cleared in less than one-quarter of an electrical cycle

  • Time delay fuses have a thermal cut out which responds with inverse time characteristics, higher current value less time it takes to open the circuit, vice versa. And a fuse element which opens the circuit during very high fault currents

  • Circuit breakers may be instantaneous trip or inverse-time

  • Instantaneous trip respond only to short-circuit conditions

  • Inverse-time circuit breakers use a bimetallic tripping element that works similar to time-delay fuses, giving the breaker inverse time characteristics

  • Circuit breakers have trip-free mechanisms which allows the breaker to trip even if held or locked in the on position

  • If fault current exceeds a device rating it may rupture causing damage to equipment

  • The factors affecting fault current are:

  1. KVA kilo-volt amperes rating of the transformer

  2. The voltage rating of the transformer secondary supplying the circuit %IZ

  3. The percent impedence voltage of the transformer

  • The closer a fault is to source the less impedence there will be since the length of the fault path is shorter