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2nd Period Electrical Lab > Pilot and Overcurrent Devices 030204c > Flashcards

Pilot and Overcurrent Devices 030204c Flashcards

1
Q

What are holding contacts also referred to as?

A

Seal-in contacts, sealing contacts, or maintaining contacts.

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2
Q

What is a pilot device?

A

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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3
Q

What are automatically operated pilot devices?

A

Detect a change and convert this change to the opening or closing of a set of contacts.
Does not require a person to operate them.

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4
Q

Automatic Pilot switches are designed to detect changes in what?

A

Temperature
Pressure
Liquid levels
Thickness of materials
Light
Rotation
Specific gravity
Vibration
Linear movement
Flow of liquid.

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5
Q

What are some examples of temperature sensors?

A

Thermocouples
Resistive temperature devices including resistance temperature detectors (RTDs) and thermistors
Infrared sensors
Bimetallic devices
Silicon diodes.

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6
Q

What is a thermocouple?

A

Voltage devices that indicate temperature by producing a change in voltage

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7
Q

How do infrared sensors function?

A

Do not need to touch the surface being measured. Measures the temperature of the surface by its thermal radiation.

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8
Q

What type of sensor can measure the temperature of moving objects?

A

Infrared Sensors.

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9
Q

How do Bimetallic devices work?

A

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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10
Q

What is an example of a bimetallic switch?

A

Non-programmable thermostats used for residential heating control.

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11
Q

What type of sensors can be used in a wide range of temperatures including cryogenic temperatures of -150°C?

A

Silicon Diode sensors.

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12
Q

What is a typical pressure switch?

A

Both normally open and normally closed contacts connected with an actuator.

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13
Q

What are some common uses of pressure switches?

A

Air Compressors
Signal broken fan belt or clogged air filters
Prove exhaust flow by measuring the pressure differences across a barrier
Operate contacts at a predetermined pressure differential.

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14
Q

What are some applications for limit switches and proximity switches?

A

Measuring the travel of a piece of equipment for positioning
Stopping or changing directions
Measuring the levels of solids in a container or pile.

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15
Q

What is the difference between a limit switch and a proximity switch?

A

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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16
Q

What are some designs of proximity switches?

A

Magnetic
Inductive
Capacitive
Photoelectric

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17
Q

How does a magnetic proximity switch work?

A

Detects the presence of an external magnetic field.

Often used in security systems or Hall effect sensors to detect the position or speed of the rotor of a motor.

18
Q

How do Inductive proximity switches work?

A

Operate by creating a magnetic field and then sensing when a ferrous (conductive) metal object enters that field.

An example is the sensors placed under the road construction materials at traffic lights to detect the presence of vehicles.

19
Q

How do capacitive proximity switches work?

A

Operate by creating an electrostatic field and then sensing when an object enters that field.

Can read through walls of enclosures so used for registering levels of liquids in a vessel.

20
Q

How do photoelectric proximity switches work?

A

Operate by sensing light reflected off an object or sensing when an object interrupts a light beam.

An example would be garage door sensors that look for an object in the way when closing.

21
Q

What are two types of float switches?

A

Lever actuated by a mechanical float and rod

Tethered bulb.

22
Q

What scenarios are considered to be called overcurrent conditions?

A

When too much current flows in a conductor that it can produce enough heat to damage the conductor’s insulation or even the conductor itself.

Occurs when the current in the circuit exceeds the circuit’s designed continuous capacity.

23
Q

When does a short circuit occur?

A

If a live ungrounded conductor contacts another ungrounded conductor, a grounded conductor, or equipment that is bonded to ground.

24
Q

What can cause a short circuit condition?

A

Insulation failure
Incorrectly connected wires
Tool dropped across the terminals of a device.

25
Q

What is an overload condition?

A

A condition in which the level of overcurrent is a moderate percentage above the normal current for the circuit.

26
Q

What can cause an overload condition?

A

Adding too many loads to a circuit or by a single load drawing too much current.

27
Q

What ratings do all overcurrent devices, fuses and circuit breakers need to have?

A

A continuous current rating
An interrupting rating.

28
Q

What does the continuous current rating indicate?

A

Indicates the amount of current the device will carry without exceeding a specified temperature rise.

29
Q

What does the interrupting rating indicate?

A

Indicates the maximum amount of current the device can safely interrupt when it opens during a short-circuit condition.

30
Q

What are the two basic types of overcurrent devices?

A

Fuses
Circuit breakers.

31
Q

What is a fuse?

A

Current-sensitive device that consists of a conductor of reduced cross-section, called the element, surrounded by an arc quenching, heat-conducting material called filler.

32
Q

What are the two basic types of cartridge and plug fuses?

A

Non-time delay fuses
Time-delay.

33
Q

What type of fuse is also known as a single-element fuse?

A

Non-time delay fuses.

34
Q

What type of fuses are also known as dual-element fuses?

A

Time-delay fuses.

35
Q

What are the two parts of a time delay/dual-element fuses?

A

A thermal cut-out
A fuse element or link.

36
Q

What are the two types of circuit breakers?

A

Instantaneous-trip
Inverse-time

37
Q

Which type of circuit breaker has both a bimetallic tripping element as well as an electromagnetic element?

A

Inverse-time

38
Q

How fast can a non-time delay fuse clear a short-circuit condition?

A

Less than half of an electrical cycle.

39
Q

How fast can a non-time delay fuse clear a high short circuit condition?

A

Less than a quarter of an electrical cycle.

40
Q

What factors affect the maximum available fault current?

A

Kilo-volts-amperes (kVA) rating of the transformer
Voltage rating of the transformer secondary supplying the circuit.
Percent impedance voltage of the transformer.

41
Q

How does the distance of a short circuit from the source impact the fault current?

A

The closer the short circuit occurs to the source the lower the impedance.

2nd Period Electrical Lab (9 decks)

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