RME Review - Control Equipment

What is the primary function of an industrial relay in a control circuit?

• To supply power to the motor directly

• To measure the current in the circuit

• To protect the circuit from overload

• To isolate and control the operation of high-power circuits using a low-power signal

Answer explanation

The primary function of an industrial relay is to isolate and control high-power circuits using a low-power signal, allowing safe operation without direct electrical connection to the high-power components.

How does the coil in an industrial relay function?

• It generates a magnetic field to open or close the relay contacts

• It provides overload protection to the relay

• It converts AC voltage to DC voltage

• It acts as the main switch in the circuit

Answer explanation

The coil in an industrial relay generates a magnetic field when energized, which opens or closes the relay contacts, allowing or interrupting the flow of electricity in the circuit.

What is the primary function of a circuit breaker?

• To measure the current in a circuit

• To protect the circuit against overcurrent

• To protect the circuit against short circuit

• To protect electrical circuits from both overcurrent and short circuits

Answer explanation

The primary function of a circuit breaker is to protect electrical circuits from both overcurrent and short circuits, ensuring safety by interrupting the flow of electricity when these conditions occur.

Which type of circuit breaker is commonly used for control circuit?

• miniature circuit breaker

• air circuit breaker

• oilt type circuit breaker

• SF6 circuit breaker

Answer explanation

The miniature circuit breaker (MCB) is commonly used for control circuits due to its compact size and ability to protect against overloads and short circuits, making it ideal for low voltage applications.

What happens to the fuse element when the current exceeds the fuse's rated capacity?

• It increases the circuit's resistance

• It melts and breaks the circuit

• It allows more current to pass through

• It reduces the voltage in the circuit

Answer explanation

When the current exceeds the fuse's rated capacity, the fuse element melts due to overheating, breaking the circuit. This prevents further current flow, protecting the electrical system from damage.

Why is a circuit breaker preferred over a fuse in motor control circuits?

• Circuit breakers are cheaper than fuses.

• Circuit breakers can be reset after tripping, while fuses need replacement.

• Circuit breakers are smaller in size than fuses.

• Circuit breakers reduce the motor's starting current.

Answer explanation

Circuit breakers are preferred in motor control circuits because they can be reset after tripping, unlike fuses which must be replaced. This feature allows for quicker recovery and less downtime in operations.

In a motor control circuit, what happens if the fuse used is rated lower than the motor's operating current?

• The motor will run at a reduced speed.

• The fuse will blow frequently during normal operation

• The motor will not start.

• The fuse will act as an overload relay.

Answer explanation

If the fuse is rated lower than the motor's operating current, it cannot handle the normal current draw, causing it to blow frequently during operation. This interrupts the circuit and prevents the motor from running properly.

Why is a fuse sometimes preferred over a circuit breaker in certain applications?

• A fuse can handle higher voltages than circuit breakers.

• A fuse provides faster protection against short circuits.

• A fuse is more durable than a circuit breaker.

• A fuse can be reset easily after tripping.

Answer explanation

A fuse provides faster protection against short circuits because it melts quickly when excessive current flows, interrupting the circuit almost immediately. This rapid response can prevent damage to electrical components.

Which type of fuse is commonly used in motor control circuits for high current applications?

• Cartridge fuse

• Glass fuse

• High Rupturing Capacity (HRC) fuse

• Blade fuse

Answer explanation

The High Rupturing Capacity (HRC) fuse is designed to handle high current applications and can interrupt large fault currents without damage, making it ideal for motor control circuits.

How does an overload relay respond to excessive current?

• It trips the circuit after a delay

• It immediately cuts off the motor

• It reverses the motor rotation

• It reduces the motor's speed

Answer explanation

An overload relay is designed to protect motors from excessive current by tripping the circuit after a delay. This delay allows for temporary surges without immediate shutdown, ensuring the motor can handle brief overloads.