EEE 102: Protective Devices and Electrical Safety

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Questions and Answers

What opens the contacts in a circuit breaker based on a magnetic field?

Excess voltage

Mechanical reset

Electric shock

Sufficient magnetic force created by excess current (correct)

What is the primary concern when working with electricity?

Reliability

Safety (correct)

Efficiency

Cost

What provides a current path when voltage is applied across two points on the body?

The environment

Voltage

Resistance

You (correct)

What is the cause of electrical shock?

Current Signup and view all the answers

What determines the path of the current through the body?

Points across which the voltage occurs Signup and view all the answers

What determines the severity of the resulting electrical shock?

Amount of voltage and the path that the current takes through the body Signup and view all the answers

What is dependent on voltage and resistance?

Current Signup and view all the answers

What factors affect the human body's resistance to electrical current?

Body mass, skin moisture, and points of contact of the body with a voltage potential Signup and view all the answers

What is the typical range of resistance of the human body?

10 kΩ to 50 kΩ Signup and view all the answers

Why should you avoid contact with any voltage source?

Because it can be harmful to humans Signup and view all the answers

What should you do before working on circuits?

Turn the power off Signup and view all the answers

Why should you remove rings, watches, and other metallic jewelry when working on circuits?

To prevent electrical shock Signup and view all the answers

What should you do with capacitors before touching any part of a circuit?

Always shut off power and discharge them Signup and view all the answers

Why should you wear safety glasses when working on electrical circuits?

To protect against flying debris Signup and view all the answers

What should you do when working on electrical circuits?

Do not work when tired or taking medications that make you drowsy Signup and view all the answers

Why should you keep your tools adequately maintained?

To ensure proper function and safety Signup and view all the answers

What should you do when handling instruments?

Never handle them when your hands are wet Signup and view all the answers

What should you do when making circuit connections?

Always connect to the point with the highest voltage as your last step Signup and view all the answers

What should you do with cables and wires?

Keep them as short as possible Signup and view all the answers

What should you do if someone is in contact with an energized conductor?

Switch the power off immediately Signup and view all the answers

Why should you use a lockout/tagout procedure?

To avoid someone turning the power on while you are working on a circuit Signup and view all the answers

What should you avoid in the workplace or laboratory?

Having drinks or food near equipment Signup and view all the answers

What is the primary function of fuses and circuit breakers?

To deliberately create an open circuit when the current exceeds a specified number of amperes Signup and view all the answers

What happens when a fuse is 'blown'?

It must be replaced Signup and view all the answers

What do fuses and circuit breakers protect against?

Damage to a circuit due to excess current or overheating of wires and other components Signup and view all the answers

What are the two basic categories of fuses in terms of their physical configuration?

Cartridge type and plug type (screw-in) Signup and view all the answers

How do circuit breakers detect excess current?

Through both the heating effect of the current and the magnetic field it creates Signup and view all the answers

What happens to the contact in a circuit breaker once it is opened?

It is held open mechanically until manually reset Signup and view all the answers

What is the purpose of a bimetallic spring in a circuit breaker?

To open the contacts when the rated current is exceeded Signup and view all the answers

What is shown in Figure 6.1?

Typical fuses and circuit breakers and their symbols Signup and view all the answers

Study Notes

Protective Devices

Fuses and circuit breakers are used to deliberately create an open circuit when the current exceeds a specified number of amperes due to a malfunction or other abnormal condition in a circuit.
The fundamental difference between a fuse and a circuit breaker is that when a fuse is "blown," it must be replaced, but when a circuit breaker opens, it can be reset and reused repeatedly.
Both devices protect against damage to a circuit due to excess current or prevent a hazardous condition created by the overheating of wires and other components when the current is too great.

Types of Fuses

There are two basic categories of fuses in terms of their physical configuration: cartridge type and plug type (screw-in).
Cartridge-type fuses have various-shaped housings with leads or other types of contacts.
A typical plug-type fuse is shown in Figure 6.1(b).

Circuit Breakers

Typical circuit breakers are shown in Figure 6.1(c), and the symbol is shown in part (e).
A circuit breaker detects excess current either by the heating effect of the current or by the magnetic field it creates.
In a circuit breaker based on the heating effect, a bimetallic spring opens the contacts when the rated current is exceeded.
Once opened, the contact is held open mechanically until manually reset.

Electrical Safety

Safety is a primary concern when working with electricity.
The possibility of an electric shock or a burn is always present, so caution should always be used.
You provide a current path when voltage is applied across two points on your body, and the current produces an electrical shock.

Electric Shock

Current through your body, not the voltage, is the cause of electrical shock.
It takes voltage across a resistance to produce current.
When a point on your body encounters a voltage and another point encounters a different voltage or the ground, such as a metal chassis, there will be current from one end to the other.

Effects of Current on the Human Body

The amount of current is dependent on voltage and resistance.
The human body has a resistance that depends on many factors, which include body mass, skin moisture, and points of contact of the body with a voltage potential.
Table 6.1 shows the effects of various current values in milliamperes.

Body Resistance

Resistance of the human body is typically between 10 kΩ and 50 kΩ and depends on the points between which it is measured.
The moisture of the skin also affects the resistance between two points.
The resistance determines the voltage required to produce each effect listed in Table 6.1.

Safety Precautions

Avoid contact with any voltage source.
Turn the power off before you work on circuits when touching circuit parts is required.
Do not work alone.
A telephone should be available for emergencies.
Do not work when tired or taking medications that make you drowsy.
Remove rings, watches, and other metallic jewelry when you work on circuits.
Do not work on equipment until you know proper procedures and know potential hazards.
Make sure power cords are in good condition, and grounding pins are not missing or bent.
Keep your tools adequately maintained.
Make sure the insulation on metal tool handles is in good condition.
Handle tools properly and maintain a neat work area.
Wear safety glasses when appropriate, particularly when soldering and clipping wires.
Always shut off power and discharge capacitors before you touch any part of a circuit.
Know the location of the emergency power-off switch and emergency exits.
Never try to override or tamper with safety devices like an interlock switch.
Always wear shoes and keep them dry.
Do not stand on metal or wet floors when working on electrical circuits.
Stand on a rubber mat if possible.
Never handle instruments when your hands are wet.
Never assume that a circuit is off.
Double-check it with a known good meter before handling.
Set the limiter on electronic power supplies to prevent currents larger than necessary to supply the circuit under test.
When making circuit connections, always connect to the point with the highest voltage as your last step.
Avoid contact with the power supply terminals.
Always use wires with insulation and connectors or clips with insulating shrouds.
Keep cables and wires as short as possible.
Connect polarized components properly.
Be aware of and follow all workplace and laboratory rules.
Do not have drinks or food near equipment.
If another person cannot let go of an energized conductor, switch the power off immediately.
If that is impossible, use nonconductive material to separate the body from the contact.
Use a lockout/tagout procedure to avoid someone turning the power on while you are working on a circuit.

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Description

This quiz covers protective devices and electrical safety, including fuses and circuit breakers, and their applications in preventing electrical malfunctions.