Print this page Introduction to Electrical Measurements
A. Bhatia, B.E.
Course Outline
When an electrical circuit is in operation, electricity flows through the wires and other objects in the circuit. Three basic parameters - current, potential difference (voltage) and resistance are of primary importance to achieve the desired functionality of electrical networks. Engineers concerned with design of electrical systems keep necessary provisions for in-circuit meters to monitor the operation of a piece of electric or electronic equipment and determine the reason the equipment is not functioning properly.
This 4- hour course provides you some rudimentary understanding of the measuring instruments and demonstrates their correct and intelligent use. This course material is based entirely on Naval Education and Training Materials (NAVEDTRA 14175), Electricity and Electronic Training Series; Module-3 “Circuit Measurement” and covers Chapter 1. The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.
Learning Objective
At the conclusion of this course, the student will be able to:
- State two ways circuit measurement is used;
- State how a d’Arsonval meter movement reacts to DC;
- State the electrical quantity measured by ammeter, voltmeter, ohmmeter, megger, multi-meter and how these are connected to a circuit;
- State the meaning of term “damping” as it applies to meter movements;
- Identify three meter movements that measure DC or AC without use of a rectifier;
- State the reason for using the highest range when connecting an ammeter to a circuit;
- State the method used to allow a voltmeter to measure different ranges;
- List the safety precautions for use of measuring instruments;
- State two types of ohmmeters and the way in which each can be identified;
- State the primary reason for using a megger and the method of using it;
- State how a multi-meter differs from other meters and how a multi-meter is changed from a voltage measuring to a current measuring device; and
- State the purpose of hook-on type voltmeter and the two types of frequency meters.
Intended Audience
This course is aimed at students, professional electrical & electronics engineers, service technicians, energy auditors, operational & maintenance personnel, facility engineers and general audience.
Course Introduction
Electrical current is a measure of the flow of charges per unit time past a point in a circuit. An instrument known as “ammeter” is used to measure electrical current in amperes or amps. To measure the current flowing through some point of a circuit, the circuit must be broken open at that point and the ammeter inserted so that the current to be measured actually flows through the meter too. It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure.
Electrical potential (voltage) is a measure of the work required to move an electrical charge between two points. The potential difference, or change in electric potential, between two points is measured with a voltmeter and is measured in units of volts (V) or millivolts (mV = 10-3 V). Common usage refers to a potential difference relative to ground (0.0 V) as simply the voltage, though it is prudent to call it by its correct name to emphasize the way it is measured. The potential difference across a circuit element is measured by placing the two leads of a voltmeter on the two sides of the element. In contrast the ammeter, a voltmeter must be connected in parallel.
Course Content
Circuit Measurement (Chapter 1, NAVEDTRA 14175)
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Course Summary
Circuit measurement is used to monitor the operation of a piece of electric or electronic equipment and determine the reason the equipment is not functioning properly. In-circuit meters monitor the operation of equipment and out-of-circuit meters can be used on more than one device.
Most common electrical measurement instruments are based on electrical current.
Current is measured using an ammeter placed in series with the branch of the circuit you wish to test. Because we do not wish to modify the circuit by imposing an instrument of inappropriate characteristics, an ammeter is a very low-resistance device. If a significant potential difference exists between the input probes of an ammeter, the high current will at best blow a fuse and, at worse, destroy the instrument. ALWAYS place the ammeter in series with other circuit components.
Voltage is measured between two points - the voltmeter is placed in PARALLEL with a circuit component. A voltmeter has a very high resistance so as to reduce any 'parallel-circuit' modification of the system. Among the specifications for voltmeters is 'input impedance', often expressed in ohms/volt.
Electrical resistance (impedance) is measured by, first, shutting down any current in the circuit and, then, by placing the two probes of an ohmmeter on the two points between which electrical resistance is to be measured. This instrument allows a low, measured current to flow through the component and simultaneously measures the voltage drop, calculating and displaying the ratio (V/I), or R. Limitations: an ohmmeter can not function properly if some unknown current also flows through the component. The circuit can not be active.
Resistance can also be measured if the current through a component is known and the voltage drop across the component measured. This requires simultaneous us of an ammeter with a voltmeter. Many sensitive resistance measurements use this method because it eliminates the effect of contact resistance - possibly large resistance exists where electrical current flows from one component into another.
When working on circuits, you must follow general safety precautions of electrical and electronic devices. Always de-energize and discharge the circuit completely before you connect or disconnect an instrument….. Remember “Safety First”.
Quiz
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.
