Pilot and Overcurrent Devices 030204c Flashcards
What are holding contacts also referred to as?
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Seal-in contacts.
Sealing contacts.
Maintaining contacts.
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What is a pilot device?
Pilot and Overcurrent Devices ILM
Signals a change in condition by opening or closing a set of contacts.
Can be either a maintained or momentary type switch.
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What are automatically operated pilot devices?
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Change states without requiring a person to operate them.
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Automatic pilot devices can be used to detect changes in:
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Temperature.
Pressure.
Liquid levels.
Thickness of materials.
Light.
Rotation.
Specific gravity.
Vibration.
Linear movement.
Flow of liquid.
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What are some examples of temperature sensors?
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Thermocouples.
Resistance temperature detectors (RTDs).
Thermistors.
Infrared sensors.
Bimetallic devices.
Silicon diodes.
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What is a thermocouple?
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A device that measures temperature by producing a change in voltage.
As temp goes up voltage goes up.
Flame proving device.
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How do infrared sensors function?
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Do not need to touch the surface being measured.
Measures the temperature of the surface by its thermal radiation.
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What type of sensor can measure the temperature of moving objects?
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Infrared Sensors.
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How do Bimetallic devices work?
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Two metals are bonded together and mechanically linked to a set of contacts.
When heated one side of the strip will expand more causing the strip to bend and activate the contacts.
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What type of sensors can be used in a wide range of temperatures including cryogenic temperatures of -150°C?
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Silicon Diode sensors.
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What is the difference between a limit switch and a proximity switch?
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Limit switches require the actuator, lever, cam, or rod to contact the object or material being sensed.
Proximity switches can detect the object or material without contacting it.
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What are some types of proximity switches?
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Magnetic.
Inductive.
Capacitive.
Photoelectric.
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How does a magnetic proximity switch work?
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Detects the presence of an external magnetic field.
Example: Security system sensors or hall effect sensors.
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How do Inductive proximity switches work?
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Operate by creating a magnetic field and then sensing when a conductive metal object enters that field.
Example: Sensors under road construction materials at traffic lights.
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How do capacitive proximity switches work?
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Create an electrostatic field and then sense when an object enters that field.
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How do photoelectric proximity switches work?
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Sense light reflected off an object or sense when an object interrupts a light beam.
Example: Garage door sensors.
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What are two types of float switches?
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Lever actuated by a mechanical float and rod.
Tethered bulb.
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When does an overcurrent condition occur?
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Occurs when the current in the circuit exceeds the normal continuous operating amount.
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What is an overload condition?
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When current is a moderate percentage above the normal current for the circuit for long enough to cause damage.
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What ratings do all overcurrent devices, fuses and circuit breakers need to have?
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Voltage rating.
Continuous current rating.
Interrupting rating.
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What does the continuous current rating indicate?
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Indicates the amount of current the device can carry without exceeding a specified temperature rise.
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What does the interrupting rating indicate?
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Indicates the maximum amount of current the device can safely interrupt when it opens during a short-circuit condition.
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What are the two basic types of overcurrent devices?
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Fuses.
Circuit breakers.
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What are the two basic types of cartridge and plug fuses?
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Non-time delay fuses. (Single-element)
Time-delay. (Dual-element)
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