Like a comet lighting up the night sky, the hand-held digital multimeter (DMM) shines brightly in the field of test equipment, helping engineers quickly make the right measurement. It is an indispensable tool for test engineers, loaded with powerful features yet simple to use.
DMMs continue to dazzle you with their functionality, size, accessories and ease of use. These durable instruments also offer unseen benefits that protect both you and the meter from dangerous levels of energy. But how do you know your meter offers this protection?
Safety organizations sanction equipment designed and tested to meet these standards, and manufacturers list the information with their product specifications. Some of the published standards include:
o IEC 1010-1 Safety Requirements for Electrical Equipment, Control and Laboratory Use–General Requirements–The IEC 1010-1 supersedes the IEC 348.
o ANSI/ISA S82.01-1988 Safety Standard for Electrical and Electronic Test, Measurement, Controlling and Related Equipment–General Requirements
o UL 1244 Standard for Safety: Electrical and Electronic Measuring and Testing Equipment
Look for equipment that offers protection from high-voltage transients, such as those generated from starting motors or load switching that last several microseconds. The meter should protect you and itself from contact with a high-energy source while it is in the resistance or continuity mode. It also should recover from overloads of 500 Vrms or more.
Your meter also should not measure voltage while the leads are connected to the current jacks. To prevent this potential disaster, some manufacturers block the current jacks when the voltage mode is selected. The simplest method, especially in high-voltage situations, is for you to use a meter without current-measuring capabilities.
You need a different form of protection if the probes are already connected to the current jacks and you inadvertently attempt to measure voltage. The meter needs a very low-value shunt resistor that acts like a short and protects the test equipment.
Unfused meters or low-energy fuses of less than 250 V could explode with the wrong connection. High-energy fuses or fusible resistors connected to the milliamp and 10-A range protect you and the meter from high-energy overloads.
However, if you must perform high-voltage or high-current measurements, turn off all power while making connections and make sure it cannot be turned on by anyone but you. Multimeters used for measuring power circuits must contain adequate protection, including fuse protection on all current input jacks.
Copyright 1995 Nelson Publishing Inc.