Safety Standards for Testing Instruments

The International Electrotechnical Commission (IEC) is an organization that develops international safety standards for electrical equipment. The IEC standards reduce safety hazards that can occur from unpredictable circumstances when using electrical test equipment such as test instruments and meters. For example, voltage surges on a power distribution system can cause a safety hazard when a test instrument is being used in an electrical system.

A voltage surge is a higher‐than‐normal voltage that temporarily exists on one or more power lines. Voltage surges vary in voltage amount and time present on the power lines. One type of voltage surge is a transient voltage. A transient voltage (voltage spike) is a temporary, undesirable voltage in an electrical circuit. Transient voltages typically last for a very short time, but they are large in magnitude and quite erratic. Transient voltages are caused by unfiltered electric equipment, lightning strike, capacitor bank, or generators being switched ON and OFF.

It is possible for transient voltages to reach several thousand volts. On a 120 V power line, a transient voltage can reach 1000 V (1 kV) or higher. High transient voltages occur close to a lightning strike or when large loads are disconnected (see Figure 1). For example, when a large motor (100 HP) is turned OFF, a transient voltage moves down the power distribution system. When a test instrument or meter is connected to a point along with the system in which a high transient voltage is present, an arc can be created inside the meter. An arc can lead to a high-current short in the electrical distribution system even after the original high transient voltage has gone. A high‐current short can turn into an arc blast.

 

Figure 1. Transient voltages can lead to electrical shocks and/or damage to test equipment and meters

 

An arc blast is a type of explosion that happens when the air around electrical equipment becomes ionized and conductive. The amount of current drawn and the potential damage caused depends on the specific location of the arc blast in the power distribution system. All electrical distribution systems have current limits determined by fuses and circuit breakers located along the system. The current rating (size) of the fuses and circuit breakers decreases further away from the main distribution panel. The further away from the main distribution panel, the less likely a high transient voltage is to cause damage.

Caution

Other than a laser, an electric arc is the hottest heat source in existence. Electric arcs are capable of producing temperatures up to 10,000°F. Temperatures of such intensity are capable of producing serious burns at distances up to 20′ and can be fatal at distances up to 8′.

 

CAT Ratings

IEC standard 1010 classifies the applications in which test instruments may be used into four overvoltage installation categories (Category I – Category IV). The four categories are typically abbreviated as CAT I, CAT II, CAT III, and CAT IV. The CAT ratings determine what magnitude of transient voltage a test instrument or other electrical appliance can withstand when used on a power distribution system. For example, a test instrument or other electrical measurement tool specified for use in a CAT III installation must withstand a 6000 V transient (2 ms rise time with a 50 ms, 50% duration) voltage without resulting in a hazard. When a test instrument or other meter is operated on voltages above 600 V, the test instrument must be capable of withstanding an 8000 V transient voltage. Also, a test instrument or meter that is designed to withstand a transient voltage can be damaged, but the transient cannot result in a hazard to the technician or the facility. To protect technicians from transient voltages, protection must be built into all test equipment.

Safety standards such as IEC 61010‐1 2nd edition, the harmonized North America standard, and the UL standard 61010‐1 vary but are closely matched. The requirements of the standards are used to rate test equipment for minimizing hazards such as shock, fire, and arc blast, among other concerns. A test instrument designed to these standards offers a high level of protection. A measurement category rating such as CAT III or CAT IV indicates acceptable usage on three‐phase permanently installed loads and three-phase distribution panels in a building or facility. All exposed electrical installations and the power panels of a facility are considered high‐voltage areas. Measurement categories such as CAT III and CAT IV ratings are important criteria for test instruments and meters used in industrial applications (see Figure 2).

 

Figure 2. The IEC 1010 standard defines the applications in which test instruments and meters can be used according to the four categories.

 

Power distribution systems are divided into categories based on the magnitude of transient voltage that test instruments must withstand when used on the power distribution system. Dangerous high‐energy transient voltages such as a lightning strike are attenuated (lessened) or dampened as the transient travels through the impedance (AC resistance) of the system and system grounds. Within an IEC 1010 standard category, a higher voltage rating denotes a higher transient voltage withstanding rating. For example, a CAT III‐1000 V (steady‐state) rated test instrument has better protection compared to a CAT III‐600 V (steady‐state) rated test instrument. Between categories, a higher voltage rating (steady‐state) might not provide higher transient voltage protection. For example, a CAT III‐600 V test instrument has better transient protection compared to a CAT II‐600 V test instrument. A test instrument must be chosen based on the IEC overvoltage installation category first and voltage second.

 

Independent Testing Organizations

National, state, and local standards and codes are used in order to protect people and property from electrical hazards. A code is a standard regulation or minimum requirement. A standard is a recognized reference or practice. Codes and standards ensure that electrical equipment is built and installed safely, and every effort is made to protect people from electrical shock. The IEC sets standards but does not test or inspect for code and standard compliance.

A test instrument with a symbol and listing number of an independent testing lab such as Underwriters Laboratories Inc.®(UL), Canadian Standards Association (CSA), or other recognized testing organization indicates compliance with the standards of the organization. A manufacturer can claim to “design to” a standard with no independent verification. To be UL listed or CSA certified, a manufacturer must employ the services of an approved agency to test a product’s compliance with a standard (see Table 1). For example, UL 3111‐1 or CSA C22.2 No. 1010.1‐92 indicates that the IEC 1010 standard is met.

 

Recognized Testing Laboratories (RTLs) And Standards Organizations

Underwriters Laboratories, Inc.® (UL)	American National Standards Institute (ANSI)
British Standards Institution (BSI)	CENELEC European Committee for Electrotechnical Standardization
Canadian Standards Association (CSA)	Verband der Elektrotechn k und Informationstechnik (VDE)
Japanese Standards Association (JSA)	International Electrotechnical Commission (IEC)
The Institute of Electrical and Electronics Engineers, Inc. (IEEE)	National Institute of Standards and Technology (NIST)
National Electrical Manufacturers Association (NEMA)	International Standards Organization (ISO)

Table 1. Test instruments have symbols listing the nationally recognized testing laboratories and standards organizations that the meters are in compliance with.

 

Caution

Before using any electrical test instruments or meters, always refer to the user’s manual for proper operating procedures, safety precautions, and technical limits. Conditions can change quickly as voltage, and current levels vary in individual circuits

 

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