7.3.16 Electrical Test Equipment

Question 1

Why is it essential to measure electrical circuits in aircraft maintenance?

Answer 1

To ensure serviceability and troubleshoot issues.

Question 2

What are the most important electrical measurements?

Answer 2

Current, voltage, resistance, and in some cases, power.

Question 3

What principle is most commonly used for electrical measurements?

Answer 3

Electromagnetism.

Question 4

What are the two fundamental assumptions of electromagnetism?

Answer 4

1. The strength of an electromagnetic field is proportional to the amount of current that flows in the coil. 2. Voltage, resistance, and power relate to a flow of current, and if the amount of current is known, the other values can be found.

Question 5

What devices use the principle of electromagnetism?

Answer 5

Ammeter, voltmeter, and ohmmeter.

Question 6

What is the most widely used meter type?

Answer 6

Permanent Magnet Moving Coil (PMMC) meter.

Question 7

How does a PMMC meter operate?

Answer 7

Its pointer deflects an amount proportional to the current flowing through its moving coil across a permanent magnet.

Question 8

Who designed the DC ammeter that the PMMC meter is based on?

Answer 8

Jacques-Arsène d’Arsonval.

Question 9

What creates a reference magnetic field in a PMMC meter?

Answer 9

A horseshoe-shaped permanent magnet.

Question 10

What material are the hairsprings in a PMMC meter made from?

Answer 10

Phosphor bronze.

Question 11

What is full-scale current?

Answer 11

The amount of current that must flow through the meter coil to cause a full-scale deflection.

Question 12

What is meter sensitivity?

Answer 12

The reciprocal value of the full-scale current, representing the total amount of resistance for each volt needed to produce a full-scale current.

Question 13

What is the typical sensitivity of many multimeters?

Answer 13

20,000 ohms per volt.

Question 14

What is the function of a shunt in an ammeter?

Answer 14

To increase the amount of current it can measure by bypassing excess current.

Question 15

What happens if an ammeter is connected in parallel?

Answer 15

It gives an incorrect measurement and can damage the ammeter.

Question 16

What is a multi-range ammeter?

Answer 16

An ammeter that can measure several different maximum current readings or ranges.

Question 17

How is a voltmeter connected in a circuit?

Answer 17

In parallel with the load to be measured.

Question 18

What is a multiplier resistor?

Answer 18

A resistor placed in series with the ammeter movement to multiply a meter’s basic range.

Question 19

What is the purpose of an ohmmeter?

Answer 19

To measure the resistance of a load or external resistance.

Question 20

What must be done before using an ohmmeter?

Answer 20

Calibrate the device.

Question 21

What happens when the test leads of an ohmmeter are separated?

Answer 21

The pointer of the meter will return to the left side of the scale, indicating infinity.

Question 22

What happens when the test leads of an ohmmeter are separated?

Answer 22

The pointer of the meter will return to the left side of the scale (infinity) due to the interruption of current and spring tension.

Question 23

Why is the ohmmeter indication considered ‘backwards’?

Answer 23

Because maximum indication (infinity) is on the left of the scale, while voltage and current meters have zero on the left.

Question 24

What must be done before connecting an ohmmeter to a circuit?

Answer 24

The power switch of the circuit to be measured must always be in the Off position to prevent damage to the meter.

Question 25

How is the current flow through the ohmmeter affected?

Answer 25

The amount of current that flows through the meter coil depends on the total resistance of resistors R1 and R2 and the resistance of the meter.

Question 26

What happens if resistors R1 or R2 are replaced with larger values?

Answer 26

The current flow decreases further, leading to a lower pointer deflection and a higher scale indication of circuit resistance.

Question 27

What is the nature of the ohmmeter scale?

Answer 27

The ohmmeter scale is non-linear and reversed compared to voltmeters and ammeters, due to the current-operated nature of the device.

Question 28

What is the purpose of multi-range ohmmeters?

Answer 28

They allow for different operational ranges, indicated by R × 1, R × 10, R × 100, etc., with readings multiplied by the range setting.

Question 29

What distinguishes a series ohmmeter from a shunt ohmmeter?

Answer 29

In a series ohmmeter, the resistance to be measured is in series with the meter movement, while in a shunt ohmmeter, it is in parallel.

Question 30

What is a safety ohmmeter designed for?

Answer 30

Safety ohmmeters are specifically designed for testing resistance in volatile and explosive devices, using a very small current.

Question 31

How does a moving coil meter function in AC circuits?

Answer 31

It requires a rectifier to convert AC to DC, as the meter movement is restricted to direct current.

Question 32

What is a multimeter?

Answer 32

A multimeter can function as a voltmeter, ammeter, or ohmmeter by connecting it to different external resistor networks.

Question 33

What is the advantage of digital multimeters over moving coil meters?

Answer 33

Digital multimeters are more common and preferred due to their compact design and ease of use.

Question 34

What safety precautions should be taken when using a multimeter?

Answer 34

De-energise the circuit before connecting or disconnecting, and never apply power while measuring resistance.

Question 35

What is a current clamp?

Answer 35

A current clamp allows measurement of current in a conductor without physical contact, using the concept of a current transformer.

Question 36

What are the general safety precautions for electrical test meters?

Answer 36

Use calibrated meters, inspect test leads for damage, and ensure proper function and range for measurements.