First step: Choose the right tool
- Always select a multimeter suitable for the measurement location.
- Make certain the meter has the proper Overvoltage Installation Category rating (CAT II, CAT III or CAT IV) for the application. Always choose a tool rated for the highest category in which it could potentially be used. CAT ratings are discussed in more detail below.
- Example: To measure 480 V in an electrical distribution feeder panel, use a meter rated at CAT III-600 V, CAT-III-1000 V or CAT IV-600 V.
Inspect a multimeter before using it
- Start with a visual inspection; look for signs of physical damage.
- Never assume a meter is working properly. Before working on a live high-energy circuit, you must verify that your meter is working. Use a known voltage source, or a proving unit such as the Fluke PRV240. This is a requirement of NFPA70E (U.S.) and GS38 (Europe).
- When not in use, keep a multimeter, its test probes and accessories in a protective case.
Inspect test probes
- Unplug test probes from the multimeter’s input jacks; verify they are shrouded and show no physical damage.
- When inserted into the jacks, the connection should feel firm and secure.
- Run test probes between your fingers as you visually inspect them; feel for any signs of damaged insulation.
- Use test leads with minimal exposed metal at the tip to protect against electrical shorting.
- Damaged test probes cannot be repaired; they must be replaced. Never use damaged test probes.
Avoid electrical shock
- Always assume every electrical component in a circuit is energized until positively discharged.
- Shock occurs when the human body becomes part of an electrical circuit, so be cognizant of body positioning when working in electrical environments.
- The severity of shock depends on the:
- Amount of electric current (in milliamps, or mA).
- Length of time the body is exposed to current flow.
- Path the current takes through the body.
- Area exposed to electric contact.
- Condition of the body exposed to current (wet hands, for instance, have less resistance to current flow than dry hands).
- Use personal protective equipment (PPE), both on-body (gloves, headwear) and near-body (insulated rubber mats). It’s required when working on or near energized, exposed electrical circuits of greater than 50 V.
- Never work alone when working on or near exposed, energized equipment.
- Avoid taking measurements in humid or damp environments.
- Ensure no atmospheric hazards (flammable dust or vapor, for instance) are present in the area.
- Watch for visual warnings in the display of multimeters; they can alert operators to irregularities such as unsafe voltages (30 V or greater) at the test probes.
Know the hazards
- Transient overvoltage: A type of voltage surge; a brief, undesirable, erratic energy spike that can reach thousands of volts. Prime generators of spikes include motors, lightning strikes, unfiltered electrical equipment and power being switched on and off. Lightning strikes can cause extreme transients on outdoor transmission lines. Transients are an almost unavoidable hazard of testing electrical equipment.
- Arc flashes, arc blasts: Electric current discharged across an air gap. Caused by a) excess voltage ionizing the air between two conductors or b) accidental contact between two conductors.
- Caution: An arc flash can happen when a power line transient (lightning strike, power surge) occurs while a digital multimeter is being used to measure voltage in an electrical system. CAT-rated meters (explained next) are designed to minimize or reduce the possibility of an arc flash occurring inside the meter.
Understand CAT ratings
Always choose a tool rated for the highest category in which it could potentially be used. Reasons why:
- A CAT rating (CAT II, CAT III and CAT IV) refers to the magnitude of transient (temporary energy spike) that a tool can withstand.
- These categories are based on the fact that a dangerous, high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (ac resistance) of an electrical system.
- Location is key to CAT ratings. As a transient travels through an electrical system, impedance will minimize it.
- Generally, the closer you are to a power source, the higher a CAT number will be. Example: Outdoor mains conductors, vulnerable to lightning strikes, will be rated CAT IV.
- A higher CAT number refers to an electrical environment with higher available power and higher potential energy transients. Accordingly, a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.
Understand voltage ratings
- Within a category, a higher voltage rating denotes the ability to withstand a higher transient.
- Example: A CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter. Yet a CAT II-1000 V meter does not offer more protection than a CAT III-600 V meter.
|CAT IV|| |
Three-phase at utility connection, any outdoor conductors
Limited only by the utility transformer feeding the circuit
››50 kA short circuit current
|CAT III|| |
Three-phase distribution, including single-phase commercial lighting
‹50 kA short circuit current
|CAT II|| |
Single-phase receptacle connected loads.
‹10 kA short circuit current.
|Measurement Categories IAW IEC/EN 61010-031|
Complete the circle of safety by choosing the right tool
- When choosing a meter for a task, analyze the worst-case scenario of your job and determine what category your use or application fits into. Then:
- Choose a meter rated for the highest category you could be working in.
- Look for a multimeter with a voltage rating for the category matching your needs.