It’s not unheard of for an electrician, for example, to find himself the victim of a transient power spike that knocks him flat before he knows it’s coming, thanks to a big motor kicking on or off somewhere upline. Occasionally a workman may end up working with lines holding significantly more juice than he realizes. It's also not unheard of for someone to work on a 12,000-volt line thinking he's on a 480 line.

The task of defining new international safety standards for electrical test equipment was recently addressed by the IEC (International Electrotechnical Commission). The IEC 348 standard that was used for years to design electrical test equipment has been replaced by IEC 1010, offering a significantly higher level of safety to people working in high-voltage, high-current environments.

The real issue for test equipment circuit protection is not just the maximum steady state of voltage range, but a combination of both steady state and transient overvoltage withstand capacity. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionized and conductive. The result is an arc blast, a disastrous event which causes more electrical injuries every year than the better known hazard of electric shock.

The most important single concept to understand about the new standard is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Within a category, a higher voltage rating denotes a higher transient withstand rating.

Ranging from Category I protected low energy circuits to Category IV installations with high-powered lines exposed to outdoor environments, the rigorous IEC international standards are designed to keep you safe. Meters designed to the new standards are more capable of withstanding the hazards caused by transients and other dangers in today’s electrical systems.

Category I – typically covers electronic equipment

• Protected electronic equipment
• Equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an appropriately low level
• Any high-voltage-low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a copier.

Category II – single-phase receptacle connected loads

• Appliance, portable tools and other household and similar loads
• Outlet and long branch circuits
• Outlets at more than 10 meters from CAT III source
• Outlets at more than 20 meters from CAT IV source

Category III – three-phase distribution, including single-phase commercial lighting

• Equipment in fixed installations, such as switchgear and polyphase motors
• Bus and feeders in industrial plants
• Feeders and short branch circuits, distribution panel devices
• Lighting systems in larger buildings
• Appliance outlets with short connections to the service entrance

Category IV – three-phase at utility connection, any outdoor conductors

• “Origin of installations,” such as where low-voltage connection is made to utility power
• Electricity meters, primary overcurrent protection equipment
• Outside and service entrance, service drop from pole to building, run between meter and panel
• Overhead line to detached building, underground line to well pump

Category II conditions are most prevalent, but that shouldn’t lull those testing electricity lines and sources into complacency. As soon as you move from inside a house or garage outside or into an industrial setting, chances are you’re dealing with Category III or Category IV. As soon as you get into large industrial motors, you’re in Category IV territory, and extra precautions should be taken when dealing with Category IV.

As you can see, the concept of categories is not new and exotic. It is simply an extension of the same common-sense concepts that people who work with electricity professionally apply every day.