Safety standards are written for a reason: to ensure we all go home safely at the end of the day. When it comes to electrical safety, the hazards can be deadly and manifest themselves quickly, often leaving virtually no response time.
Engineers, electricians, and technicians must follow electrical safe work practices when it comes to using multimeters, including inspection before use. The most effective method to ensure safety, and also the method required by OSHA (Occupational Safety and Health Administration), is for employees to demonstrate their ability to select, inspect, use, and maintain their test equipment.
OSHA regulations and the NFPA (National Fire Protection Association) 70E® Standard for Electrical Safety in the Workplace® provide procedural guidance when it comes to test equipment inspection. For example, NFPA 70E states that only "qualified persons" are allowed to perform tasks that include the use of test equipment on systems 50 volts and greater. A digital multimeter is the most commonly used instrument for these kinds of measurements.
Since 2007, OSHA regulations require technicians to demonstrate their skills to their employer to be considered a qualified person. Thus, employers must verify an individual's ability to safely use digital multimeters.
Properly rated for the circuit
A digital multimeter, or any electrical measurement tool, must be properly rated for the circuit on which it is to be applied. Technicians must be able to understand and explain these ratings. Accessories like test probes, flexible clamps, or others are included in these ratings. The ratings should always be printed on the tools you’re working with.
The first step is to identify the nominal system voltage of the circuit to be tested. This is the voltage class assigned to systems and equipment and can be found on nameplates and drawings. Typical nominal voltages found in plants are 120/240, 208Y/120 and 480Y/277.
Technicians need to be aware of the extreme danger of using an inadequately rated digital multimeter. Using a 1000-volt-rated digital multimeter on medium-voltage circuits has, unfortunately, happened more than once with tragic results.
Nationally Recognized Testing Laboratory (NRTL)
In addition to proper voltage and current ratings, test tools must be listed by a Nationally Recognized Testing Laboratory (NRTL) and properly labeled with the NRTL's mark. These marks should show up on your tools and accessories just like the CAT rating does.
OSHA lists which NRTLs have been approved to test and verify that tools meet their standards. This testing reasonably assures that products are safe for use in the environments their ratings are designed for. Once the equipment meets the testing laboratory's criteria, the tool can be labeled with the NRTL's recognized mark. Test equipment without a label on it should not be used.
Among the most common testing laboratory marks found on test tools are Underwriters Laboratories Inc. (UL), Canadian Standards Association (CSA), and TUV. Technicians must demonstrate their ability to recognize and identify these labels and explain their importance.
Electrical workers must also be familiar with the Category Rating found on digital multimeters and other test tools and accessories. The "CAT" rating indicates the tool's ability to withstand transient overvoltage conditions that could destroy the meter and cause injury.
Tools used in distribution systems should be at least CAT III rated. CAT IV offers a greater degree of protection. Most industrial digital multimeters are rated CAT III for use on 1000 volt and below systems and CAT IV for use on 600 volt and below systems. Electrical workers should be able to identify the CAT rating needed for their job.
The ratings should also be easy to find on the tool, near the NRTL’s mark. These ratings are the voltage levels the laboratory has certified the tool to be able to withstand.
Designed for environment and use
Technicians must verify that the test instruments and their accessories are designed for both the environment and the way they will be used. For example, when examining a digital multimeter for proper design, ask, "Will this digital multimeter be used in a hazardous location?" When taking voltage readings, it is possible for a very small electrical arc to be drawn when placing a test probe on, or removing a test probe from, an energized point.
The National Electrical Code® (NEC) identifies environments as Hazardous (Classified) locations if explosive atmospheres are present. Intrinsically safe test tools are designed for use in these kinds of locations and technicians must look for the identification, if applicable. Usually, you want to look for a red tool or an EX symbol on the tool.
This is also a good reason to place and remove test probes at a ninety-degree angle to the terminal, and not let the probe "slide" from one terminal to another.
A visual inspection should be done, only the test tool itself, but all associated test leads, cables, power cords, probes, and connectors. Look for any obvious external defects. It is not uncommon to find damaged test leads or probes, which must be replaced before use.
One good method is to slowly pull test leads between your fingers as you perform a visual inspection of the lead. Your fingers can often feel if the insulation has been damaged, even if you can’t see it. All test leads should have a shroud around the end that is inserted into the digital multimeter. This prevents accidental shock should the test lead become unplugged from the tool while the probe is still on an energized component.
Test probes (both voltage and current probes) will have a voltage , amperage, and category rating, just like the rest of your tools. Look for the IEC (International Electrotechnical Commission) symbol for "double-insulated" (one square box inside of another). This symbol tells you that one single insulation failure will not result in personnel being exposed to dangerous shock levels.
Do not discount the use of clamps, flex clamps, and test probes for current measurements when it comes to visual inspections. Such devices should be marked with a maximum current rating. They should also have the NRTL label. Many test probes are double insulated and marked with the double insulated symbol.
Once you’ve done the visual inspection, never hesitate to remove a tool from service if there is any question about its condition. Make sure some method, such as tagging, is used to ensure someone else does not inadvertently use the defective test equipment before repairs are completed.
The three-point test method, or live-dead-live testing
One of the most critical safety tasks performed by a technician is verifying the absence of voltage during the lockout/tagout process at voltages of 50 volts or more. A test tool that fails to operate properly during this test could result in a catastrophic accident. That’s why it is vital that technicians properly perform the "three-point" or a “live-dead-live” test when verifying the absence of voltage during their qualification activities.The process to verify absence of voltage is:
- Verify the test tool works properly when the function switch is placed to "voltage" by testing for voltage on a known energized source, or by using an electronic proving unit, and observing the correct reading on the meter face.
- Test the circuit to be verified by measuring phase-to-phase and phase-to-ground across all phases. Zero energy must be indicated.
- Ensure the test tool still indicates voltage properly by placing the test probes, once again, on a known, energized source or the electronic proving unit.
Note: Proving units verify the proper operation of the meter without the need for cumbersome Personal protective equipment (PPE).
When it comes to safety, never assume any test tool or its accessories is operating correctly. Always verify proper operation before you begin work.
Personal protective equipment (PPE)
PPE is your last line of defense in dangerous situations. The NFPA has specific rules and recommendations around the level and type of PPE needed for each situation you may find yourself in. Selecting the proper PPE to conduct your work safely is a huge step in ensuring you go home safely at the end of a workday.
Stay up to date on training and standards
Creating a strong culture of safety involves every person and department in a company, but at the end of the day, it’s up to you to make the safest choices. As the person doing the work, you should know the proper steps to take, the right PPE to wear, and what your tools can handle. All of that knowledge takes a foundation of proper safety training, conducted and updated regularly to keep it front of mind, is your best line of defense.
Proper inspection takes time, and experienced personnel may not appreciate being audited in the field. However, mistakes cannot afford to be made and demonstrating safe work practices with test equipment is a mandatory component of the qualification process.