Electrical measurements require the system to be energized. The meter and test leads are connected to a source of considerable energy, and mistakes can have disastrous consequences. The more serious hazards include:
- Electrocution from contact with exposed live connections at the test probes or plugs.
- Arcing when a dislodged test lead comes in contact with a conductive object.
- Fire or explosion when a meter intended to measure voltage has its leads plugged into the jacks for measuring current.
- Electrocution from conductive paths caused by moisture or contaminants.
- Electrocution, fire or explosion caused by voltage transients breaking down insulation.
- Explosion because improper equipment was used in a hazardous atmosphere.
Where to use them
Figure 1. Not every meter can be used everywhere.
Location is everything
The international standard for test equipment safety is IEC 61010-1 (formerly IEC 1010-1). Table 1 shows the four measurement categories, sometimes called installation or location categories, the standard defines, as well as typical locations within a plant that fall under each category. The categories are based on the length of the conductors from the electrical service connection.
Higher measurement categories apply to testing performed closer to the point of origin, where a voltage impulse delivers greater energy to connected meters. Therefore, a meter rated for a higher category features larger electrical clearances and creepage distances to withstand these impulses (see Table 2). At lower category locations, the facility wiring tends to attenuate impulses, and meter construction requirements are less demanding. When in doubt about a location, assume it falls under the higher category.
The first important rating is the measurement range. For voltage measurement, this is the maximum voltage that can be applied to the test terminals. This doesn't mean, however, that measurements can always be made safely at that voltage.
Meters also should be marked with a safety category rating, which consists of a rated voltage and a measurement category, such as "600V CAT II." This marking specifies the maximum voltage that can be applied to the instrument for any type of measurement when it's connected in the category location. The latest IEC standard requires a meter to be capable of being set to any function or range with the leads connected to the maximum rated voltage without creating a hazard. Equipment can be dual-rated, for example 1000V CAT II / 600V CAT III. If a meter doesn't have a marked category rating, it's a CAT I device.
Often the rated voltage is lower than the meter's measurement range. Figure 2 shows two meters with the same measurement range of 750 VAC or 1,000 VDC. The lower meter is rated for CAT III over its full range. The top one, however, is limited to 300 V in a CAT III location, and can be used on higher voltage only in a CAT II location.
Select a meter with a safety rating that exceeds the anticipated application requirements for both the category and voltage. For most industrial work, 600V CAT III will be the minimum acceptable. The latest models from several manufacturers have ratings of 1,000V CAT III/600V CAT IV. Don't forget that these ratings also apply to test leads, clamp-on adapters and any other electrical accessories. The lowest rating on any of these determines the limit for the measurement.
The IEC standard is voluntary, and IEC doesn't monitor compliance. Meters also should carry the listing mark of a nationally recognized testing laboratory, such as UL, ETL or CSA. Listing under UL standard 3111-1 or the newer 61010B-1 ensures that the meter complies with the IEC standard. Other UL standards, or the general terms "UL Listed" or "designed to meet," don't carry the same assurance. The European CE marking doesn't imply that a third party has verified the design. Without third-party listing, there's no way to know if the manufacturer's rating was established in accordance with IEC requirements
Meters used in explosive atmospheres are outside the scope of these standards, and must have an additional "intrinsically safe" rating. Check with the manufacturer regarding the specific application.
Use the right unit
Table 1. IEC measurement categories
Modern meters have safety features that go a long way toward minimizing hazards. Features to look for include:
- Shrouded test lead plugs that eliminate exposed connections, even when not inserted.
- Audible alarms that sound when test lead connections don't agree with the switch setting.
- High-interrupting fuses in current-measuring circuits to prevent destructive current if the meter is improperly connected.
- More rugged case designs that minimize breakage.
- Hangers on the meter case that free both hands to make measurement without distraction.
- Test probe clips that eliminate the need to hold the probes. They make it easier to access test points safely.
- Higher safety category ratings for greater resistance to voltage impulses.
Listing by a nationally recognized testing agency to ensure the meter meets minimum safety standards.