Electrical Safety and Relay Basics

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Questions and Answers

What is the typical voltage rating for control relays?

300 VAC or 600 VAC (correct)

50 VDC

24 VAC

15000V

What is the typical coil current range for 0-24 VDC coils?

1-10 mA

10-30A

40-200 mA (correct)

1-5 A

Which of these is NOT a typical mounting method for relays?

Sockets

Plug board

Adhesive mount (correct)

Rail mount

For switching very low level signals, what specific contact material might be necessary?

Gold plated Signup and view all the answers

What is a common accessory for relays?

Timers Signup and view all the answers

Approximately how many employees were killed annually due to electrical contact, according to the statistics mentioned?

289 Signup and view all the answers

What is the primary cause of secondary injuries resulting from electrical shock?

Involuntary muscular reactions Signup and view all the answers

Which of the following is NOT a direct result of electricity passing through the body?

Chemical burns Signup and view all the answers

What is the potential consequence of not being alert to electrical hazards in the workplace?

Increased likelihood of accidents Signup and view all the answers

What are the three basic components of an atom?

Electrons, protons, and neutrons Signup and view all the answers

What is the charge of an electron?

Negative (-) Signup and view all the answers

What are free electrons?

Electrons that have detached from their orbit Signup and view all the answers

What causes the flow of electrons that forms the basis of electricity?

The movement of free electrons between atoms Signup and view all the answers

In an AC circuit containing both resistance and capacitive reactance, what is the phase relationship between current and voltage?

Current leads voltage by more than 0 degrees and less than 90 degrees Signup and view all the answers

What effect does increasing the resistive component of an AC circuit have on the phase difference between current and voltage, while keeping other parameters constant?

Decreases the phase difference, making it less out of phase Signup and view all the answers

What is the primary effect of inductance in an AC circuit?

It opposes changes in current flow, creating inductive reactance. Signup and view all the answers

What two parameters influence the value of inductive reactance in an AC circuit?

Inductance and frequency. Signup and view all the answers

How is impedance (Z) represented as a quantity?

As a vector with both magnitude and direction. Signup and view all the answers

What is the primary function of a relay?

To amplify an electrical signal to control a separate circuit. Signup and view all the answers

On an impedance vector diagram, at what angle is capacitive reactance plotted?

-90 degrees. Signup and view all the answers

In a series RLC circuit, what is the phase relationship between current and voltage in an inductive AC circuit?

Current lags voltage by 90 degrees. Signup and view all the answers

In a relay, what does the term 'normal' state refer to?

The state when the coil is de-energized and the relay is not in use. Signup and view all the answers

Which of the following is NOT a typical application of a solenoid?

Measuring impedance in a circuit. Signup and view all the answers

In a series RLC circuit, what is the phase relationship between current and voltage in a capacitive AC circuit?

Current leads voltage by 90 degrees. Signup and view all the answers

What type of contact is generally preferred for high-current applications in relays?

Open-air contacts. Signup and view all the answers

What makes a relay useful when dealing with large amounts of current?

Its low coil current controlling a higher contact current. Signup and view all the answers

What type of limit switch uses a lever pushed by a machine part?

Lever-actuated limit switch Signup and view all the answers

Which type of switch uses a magnetic or high-frequency electromagnetic field to sense the approach of a metallic machine part?

Proximity switch Signup and view all the answers

What is a common drawback of open-air contacts in relays?

Tendency to corrode and spark. Signup and view all the answers

Which of the following uses a light source and photocell to detect machine position?

Optical switch Signup and view all the answers

Which of the given options below best describes one of the main advantages of Mercury or reed contacts compared to open-air contacts?

They do not corrode or spark. Signup and view all the answers

In a SPST relay, what does the acronym SPST refer to?

Single Pole, Single Throw. Signup and view all the answers

A switch, which monitors the rotary speed of a shaft is called a:

Speed switch Signup and view all the answers

What is used to actuate a pressure switch?

Gas or liquid pressure Signup and view all the answers

Which switch type uses a small roller bearing to prevent the lever from being worn off?

Lever-actuated limit switch Signup and view all the answers

What is the primary function of process switches in industrial applications?

To monitor physical quantities and trigger actions Signup and view all the answers

How do complex proximity switches that use a coil of wire monitor a metallic part?

They monitor changes in the current flowing through the coil Signup and view all the answers

In a parallel circuit, how is the current through each branch calculated?

By dividing the circuit voltage by the resistance of that branch. Signup and view all the answers

What is the total resistance of a parallel circuit if all resistors have the same resistance?

The resistance of one resistor divided by the number of resistors. Signup and view all the answers

How can the total current in a parallel circuit be calculated?

By dividing the circuit voltage by the total resistance. Signup and view all the answers

What is another term for a series-parallel circuit?

Compound circuit Signup and view all the answers

What is the minimum number of resistors needed to form a series-parallel circuit?

Three Signup and view all the answers

According to Ohm's Law, what is the formula to calculate current (I) given voltage (E) and resistance (R)?

I = E / R Signup and view all the answers

Using the power formulas, how can current (I) be calculated if power (P) and voltage (E) are known?

I = P / E Signup and view all the answers

What formula calculates voltage (E) using current (I) and resistance (R)?

E = I * R Signup and view all the answers

Study Notes

Jadeer Learning Manual - Electrical Area 2

This manual covers plant electrical installation at level 1
The document's issue date is November 6, 2024
The next review date is October 1, 2027
The discipline is electrical technician
The job/area is area 2
The learning module is JAD-LC-A2-YP-EMT-LM
The revision date is 2024.01

Basics of Electricity

Electrical hazards are dangerous
Between 1992 and 2001, 4309 individuals lost time at work due to electrical injuries
In 1992-2001, there were an average of 4,309 work-related electrical deaths
Electrical injuries cost workers time away from work and can be long and painful
The goal is worker health and quality of work life
3 milliamperes of current through the body can lead to injuries

Learning content

Table of contents
Learning objectives
Basic introduction to electricity
DC and magnetism
Understanding of AC circuits
Series parallel circuits
Understanding of electrical formulas
Understanding of the power triangle

Objectives

Basic understanding of DC and magnetism
Basic understanding of AC Circuits & Series Parallel Circuits
Understanding of Power Triangle

Basic Introduction of Electricity

All matter is made up of molecules composed of atoms
Atoms have a nucleus with electrons orbiting around it
The nucleus is made up of positively charged protons and neutral neutrons
Electrons are negatively charged

Additional information

Conductors permit electrons to flow freely
Conductors: copper, gold, silver, aluminum
Insulators oppose current flow
Insulators: plastic, rubber, glass, mica, ceramic
Semiconductors exhibit qualities of both conductors and insulators

Electric Charge

Elements are defined by the number of electrons in orbit around the nucleus and by the number of protons in the nucleus
An atom with an equal number of electrons and protons is electrically neutral
Electrons which leave their orbits due to external force create positive and negative charges

Attraction and Repulsion of Electric Charges

Opposite charges attract
Like charges repel
Invisible lines of force are in the fields surrounding objects with charges, causing attraction or repulsion

Current

Electricity is the flow of electrons through a conductor
Current is measured in amperes (amps)
Direct Current (DC) flows consistently in one direction
Alternating Current (AC) reverses its direction periodically

Voltage

Voltage (or potential difference) is the force that drives current
Voltage is measured in volts
A difference in potential between two points is referred to as voltage

Resistance

Resistance (opposition to current flow) is a circuit, component, or material property
Resistance is dependent on composition, length, cross-section, and temperature of the resistive material
Resistance is measured in ohms

Ohm's Law Triangle

Simplifies calculations involving voltage, current, and resistance in a circuit.
Using the triangle you can quickly determine the correct formula

Basics of Electrical (DC and Magnetism)

Magnetism is integral to electricity - producing electricity
Permanent magnets are characterized by attracting iron and assuming a north-south orientation
Invisible magnetic lines of flux leave the north pole and enter the south pole.

Interaction between magnets

Similar poles repel and opposing poles attract

Electromagnetism

Electric current produces a magnetic field
The direction of current flow and the direction of the magnetic field are related.
The relationship can be visualized using the left-hand rule.
Electrical devices such as motors, circuit breakers, and contactors utilize electromagnetic principles

Electromagnets

A coil of wire carrying a current acts as a magnet
The strength of the field is increased by adding more turns, increasing the current or using a core of soft iron
Electromagnets are used in various electrical devices (e.g., motors, circuit breakers).

Understanding of AC Circuits (AC Current)

Current changes direction periodically
Sine waved are used to represent AC Voltage and Current

Basic AC Generator

Consist of a magnetic field, an armature, slip rings, and brushes.
The armature is a number of conductive wires wound in loops that rotate through the magnetic field
When a conductor moves through a magnetic field, a voltage is generated.
The process continuous as long as the generator is running.

Frequency

Frequency is the number of cycles per second of voltage
Measured in Hertz (Hz).
The standard power line frequency in many countries is 60 Hz.

Four-pole AC Generator

The frequency is the same as the number of rotations per second with a magnetic field.
An increase in the number of poles causes an increase in the number of cycles completed in a revolution

Amplitude

The peak or maximum value for each half of the sine wave
The peak-to-peak value is the range of positive and negative peaks
The effective value is also referred to as RMS
Instruments are designed to measure AC voltages/currents that mostly display the effective value.

Inductive Reactance

The opposition to current flow is called inductive reactance
It is proportional to both inductance and frequency

Current and Voltage Phases

In purely resistive circuits, current and voltage are in phase.
Current lags the voltage in an Inductive circuit by 90 degrees.
Current leads the voltage in a capacitive circuit by 90 degrees.

Capacitive Reactance

Capacitors also oppose current flow in AC circuits by capacitive reactance,
It is inversely proportional to the frequency and capacitance.

Calculating Impedance

Impedance (Z) is the total opposition to current flow in an AC circuit
Impedance is represented as a vector with magnitude and direction.
Resistance, inductive reactance, and capacitive reactance are at right angles to each other
Reactance is plotted at 90 degrees

RLC Circuits

Circuits that contain resistance, inductance, and capacitance
Calculating total impedance- use formula with resistance and reactance.
The resistance, inductance, and capacitance are in series
The values of resistance, inductance, and capacitance change when the frequency changes

Parallel Circuit Resistance/Current

The total resistance is found by adding all the current in a series configuration
The total current is found adding the current in a parallel configuration

Transformer

A device which steps up or down voltage
Uses mutual induction to transfer energy between electrical circuits
Composed of primary and secondary coils
Coils are frequently wound on the same iron core

Residential Transformer Applications

Single-phase, three-wire supply is common
Voltage between either hot wire and neutral is 120V
Voltage between two hot wires is 240V
120V - general purpose and lighting
240V - heating, cooling, cooking

Three-Phase Power

Used when power requirements are higher than in single phase applications
Three overlapping AC cycles are used
Each wave represents a phase
Each phase is offset by 120 degrees

Three-Phase Transformers

Have three interconnected coils in both the primary and secondary
Connected in either wye or delta configuration

Power in AC Circuits

The force required to make electricity flow through a conductor is called a difference in potential, electromotive force (emf) or voltage
Voltage is measured in volts
A difference in potential between two points is referred to as voltage.
The vector sum of true power and reactive power is known as apparent power.
Apparent power (S) is measured in volt-amperes (VA).
True power is sometimes called effective power, and is equal to the current squared times the resistance(I2x R)
Measured in Watts (W)

Power Factor (PF)

The power factor is the ratio of true power to apparent power.
It is also equal to the cosine of the phase angle

Introduction to Electrical Switches

An electrical switch is used to interrupt electron flow.
Switches are binary devices (either fully on or off)
Different types of switches exist
Process switches are designed for machine control rather than manual operation
Various mechanical, electronic, and optical switch types

List of Switch Types

Hand Switches: Toggle, Pushbutton, Selector
Process Switches: Limit, Proximity, Speed, Pressure, Temperature, Level, Flow, Nuclear

Contact "normal" state and make/break sequence

Normal position of the switch is when there is no influence or force on the switch.

Generic Symbology

Normally Open (NO) contacts are open when unactuated
Normally Closed (NC) contacts are closed when unactuated

Electro-mechanical Switches

Introduces contactors, relays
Lists types of contactors and relays

Studying That Suits You

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Description

Test your knowledge on electrical safety measures and relay operations. This quiz covers typical voltage ratings, coil currents, mounting methods, and essential concepts of electricity. Perfect for students in electrical engineering or related fields.