EC (Relays) Ch 4

Question 1

What is the difference between inrush current and holding current in the coil of a control relay?

Answer 1

Inrush current is the initial surge of current when the coil is first energized, much higher than normal. Holding current is the lower current required to keep the coil energized once the magnetic circuit is established.

Question 2

What are the three ratings generally published concerning control relay contacts?

Answer 2

1. Voltage rating
2. Current rating
3. Horsepower (HP) or load type rating (e.g., resistive, inductive, motor load)

Question 3

Explain contact wipe and how contact bounce affects circuit operation.

Answer 3

Contact wipe is a sliding motion that occurs as contacts close, helping to clean the contact surfaces. Contact bounce is when contacts rapidly open and close momentarily during closure, causing voltage spikes, false signals, or undesired circuit activation.

Question 4

What is the symbol for a relay coil?

Answer 4

(__) or CR inside parentheses

Question 5

What is the symbol for a Normally Open (NO) contact?

Answer 5

──o o──

Question 6

What is the symbol for a Normally Closed (NC) contact?

Answer 6

──o/ o──

Question 7

Describe the interlock circuit diagram.

Answer 7

1. Start PB (NO) connects to Relay coil (CR)
2. CR energizes and closes NO contact in parallel with Start PB (seal-in circuit)
3. A Stop PB (NC) is in series to break the circuit
4. Red push-to-test light wired across CR contacts to show relay status

Question 8

What advantages does a time-delay relay offer over a standard control relay?

Answer 8

1. Adds timing control (delay ON/OFF)
2. Useful for sequencing, startup delays, or safety interlocks
3. Provides automation without PLCs

Question 9

Describe how a latching relay operates.

Answer 9

A latching relay stays in its last energized state even after the control signal is removed. It ‘latches’ in place using mechanical or magnetic hold and requires a separate signal to unlatch/reset.

Question 10

How many poles are generally available on contactors?

Answer 10

Usually 1 to 4 poles

Question 11

What is the range in current-carrying capacity for contactors?

Answer 11

From a few amps to over 1000 amps, depending on application

Question 12

What is one advantage of using a plug-in relay?

Answer 12

Relays can be changed without disturbing the circuit wiring.

Question 13

What is the dropout voltage of relay coils?

Answer 13

85% of the rated voltage