Electrical General Test Equipment Operation, Function and Use

What happens when a voltmeter is connected across two points in a circuit?

Current is shunted.

What is the fundamental principle behind the operation of the meter?

Electromagnetism.

What is the purpose of a shunt resistor?

If the meter is used to measure more than its current sensitivity, then a shunt resistor is used to bypass current that exceeds the current sensitivity.

What is electrical damping?

It is a method of damping, where a moving coil is wound on an aluminium frame and as the coil moves in the field of the permanent magnet, current is set up in the aluminium frame. The magnetic field produced opposes the magnetic field of the permanent magnet. As a result, the pointer will swing more slowly to its proper position and come to rest with very little oscillation.

Define damping.

It is a method used to bring the pointer of a meter to rest after it has been set in motion. It causes a reduction in the oscillation of the pointer.

What is electromagnetism?

It is the interaction between the magnetic fields of a permanent magnet and the coils of a rotating magnet.

What two things does a basic ohmmeter contain?

A battery and a variable resistor. The battery produces current causing a deflection of the pointer proportional to the external resistance being measured. The variable resistor adjusts the current so that the pointer is at exactly zero. This called zero adjustment.

What causes a great rotation of the coils?

A great current flowing through the coils and producing a strong magnetic field.

What is a back-off scale on the ohmmeter?

A scale where the resistor values decrease from left to right.

How are the ohmmeter leads connected?

Across the resistance to be measured.

What is a common example of mechanical damping?

Air damping.

How is excess voltage that is greater than the full deflection of a voltmeter dropped?

By adding series resistance called multiplier resistance RM.

What is the advantage of the multi-range ammeter?

It can be used over a wide range of voltages. However, in order to accomplish this, each range must utilise a different shunt resistance.

What is the Megger or megohmmeter?

It is an ohmmeter used to measure insulation resistance and other high resistance values, as well as ground, continuity and short circuit testing.

Define resistance.

It is the actual DC resistance of the wire used to construct the meter coil.

Define the current sensitivity of a meter.

It is the amount of current required to drive the meter to a full-scale deflection.

In the multiple range voltmeter, how many multiplier resistors are used for each range?

One RM for each range.

What is the purpose of a meter?

The purpose of the meter is to measure quantities existing in the circuit. When a meter is connected it must not change the characteristics of the circuit.

How do externally excited meters operate?

They get their power source from the circuit they are connected to.

What is Ohm's law?

V = IR

What are the most common analog meters used today?

Voltmeter, ammeter and ohmmeter.

In which terms is voltmeter sensitivity defined?

In terms of resistance per volt (ohms/volt).

How do self-excited meters operate?

From a power source within the meter.

What is measured with the ohmmeter?

Resistance.

What happens when the ohmmeter leads are open?

The meter is at full-scale deflection indicating an infinite resistance or an open circuit.

What is the difference between the ammeter, voltmeter and ohmmeter?

All of them are current measuring devices, however, the ammeter and voltmeter differ in the way they are connected to the circuit and the ohmmeter differs in that it is self-excited.

Why should a high resistance voltmeter be used when taking a voltage measurement?

Because a low resistance voltmeter will draw off a significant amount of current and as a result, the circuit will be shunted.

What are the two primary elements that the megger consists of?

- DC generator, which supplies the current needed to make the measurement. - Instrument portion, which indicates the value of resistance being measured.

What is the d'Arsonval meter movement?

It is a current measuring device which is used in the ammeter, voltmeter and ohmmeter.

What advantage does the megger have over the ohmmeter?

Its advantage is its capacity to measure resistance with a high breakdown voltage and this ensures that during testing the insulation will not leak due to electrical stress.

How is the value of a shunt resistor determined?

Rsh = (Im x Rm)/Ish

What happens when the ohmmeter leads are shorted?

The pointer will be at the full right hand position indicating a short circuit or zero resistance.