Chapter 3: Test Instruments
Chapter Overview
Chapter 3 will cover test instruments. Properly and safely using different test instruments is a mandatory and crucial skill for any electrician. Understanding what test instrument to utilize in individual situations is an important aspect of being a productive and employable journeyman wireman. It is also very useful if you wish to go home at the end of the day.
Key points to remember:
- You should be familiar with and be properly trained before using a new piece of equipment. That means reading and becoming familiar with the operator’s manual.
- Tools should always be inspected before and after each use.
- Digital meters are becoming more prevalent in the industry, but analog meters still exist.
Common Meters
Common meters used in the electrical field that you will encounter include digital multimeter (DMM), ammeters, megohmmeters, phase sequence indicators (rotation meters), and branch circuit identifiers. Multimeters typically include multiple functions including the ability to check for the presence of voltage (measured across or in parallel), current (measured in-line or in series) and electrical resistance (only measured after verifying zero voltage). They are the most common type of meter that you will work with throughout your career.
Ammeters offer the ability to check the current draw for a specific circuit. A Clamp-on ammeter works based on the property of electromagnetism to safely measure circuit current without physically breaking the circuit. Hence the tendency to be referred to as a Current transformer (CT). The advantage here is safety, as you will not have to disconnect a live circuit and physically place your meter in line with the circuit conductors to accurately determine the amount of current flowing in the circuit at any one time.
How to Use a Clamp Meter
Megohmmeters are a specialized type of meter used to test the integrity of conductor insulation. A megohmmeter detects insulation degradation (deterioration) by measuring high resistance values using high voltage signals. Extreme caution should also be taken when working around high voltage equipment. Megohmmeters or “meggers” as they are they are more commonly referred to as in the electrical industry, can even damage wires and potentially equipment such as motors do to the high potential energy that is introduced into the system. This potential hazard is somewhat mitigated with the use of “very high frequency” or VHF type meters that use significantly smaller voltage to accomplish the same task, that is to verify the integrity of wires and their insulation after final installation.
Phase sequence indicators, commonly referred to as rotation meters, are used to indicate “the direction of rotation” of a particular electrical system. Looking ahead to three-phase systems, it is important to understand the idea that the order that each individual “phase” of a three-phase system is energized first, will determine if the system is oriented clockwise or counterclockwise. Swapping the orientation of any two wires results in the reversal of this order and thereby the reversal of the “direction of rotation”.
Phase Sequence/Motor Rotation Tester on 3-Phase Motor
Branch circuit identifiers are niche meters used to test and identify unknown electrical circuits. They typically consist of a transmitter that is attached to an electrical circuit and a receiver that picks up the associated signal frequencies to determine the origin of unknown circuits. This is especially helpful for electricians working around sensitive electrical systems that should not be shut off such as is the case with critical hospital and data center electrical circuits.
Looking Deeper
It is important to familiarize yourself with the proper and safe operation of all tools you are working with to maintain safe working conditions when working in the field. Your ability to identify and properly utilize various electrical measuring devices will directly translate into your maintained safety throughout your electrical career. An example is “ghost” or stray voltage, which is present in most electrical systems. There are examples of ghost voltage measured as high as 23 volts AC without there being any real danger of electrical shock. Conversely, mistaking 230 volts AC for 23 volts could be the difference between life and death.
Keep in mind
Always check your electrical meter on a known live circuit to ensure that it is functioning correctly. Never leave your meter on without turning it off, as some meters enter a ‘sleep’ mode to conserve battery life. This could mistakenly cause you to believe the meter indicates zero volts when checking an energized system.
Electrical measuring device that detects the amount of current flow imposed on the system without breaking the circuit based on the strength of the magnetic field created by the intensity of the current flow in the system
Electrical testing device that measures insulation integrity of electrical conductors after installation to ensure proper and safe operation usually before new building certification.
Electrical testing device used to verify the direction of rotation of three-phase systems.
Negligible voltage induced into most electricals systems; although not considered dangerous, its presence or lack thereof is a great indicator of the quality of the electrical measuring device being used.