402 Electrical Systems your meter is set to measure resistance and you apply the test leads to an energized circuit, catastrophic results may occur. Use extra caution when using 1000:1 or 100:1 high- voltage probes. Make sure that you have the correct type of probe and that the voltage reaching the test equip- ment will be less than its rated voltage. Follow the old electrician’s rule: “Use only one hand when making a measurement and keep the other hand in your pocket.” Th is lessens the possibility of a com- plete circuit being made from one hand to the other, with the current going through your heart. Be extremely cautious of test lead placement. If the lead slips, it could come in contact with something that could cause a short circuit. Always move carefully and deliberately when performing a test. 21.1.3 Overvoltage Categories (CAT Level) Th e International Electrotechnical Commission (IEC) is a nonprofi t, nongovernmental international standards organization that prepares and publishes standards for electrical, electronic, and related technologies. Th e IEC standards for overvoltage protection categories are used PROCEDURE Preparing to Perform an Electrical Test The following are general steps to take before performing an electrical test or taking an electrical measurement: 1. If possible, de-energize the equipment before testing. Follow appropriate lockout/tagout procedures. 2. Wear appropriate PPE based on the test, equipment, and environment. 3. Select the appropriate test equipment based on the type of test and expected measurement values. 4. Inspect the test equipment for wear and damage. 5. Verify that the test equipment has an adequate voltage rating and CAT level. 6. Check that the leads are connected properly and that the test selector switch is set correctly. by test equipment manufacturers. Th ese manufacturers certify through testing that their test equipment adheres to IEC standards. Overvoltages, or transients, are undesirable voltage spikes in excess of the intended voltage. If the test equip- ment you are using is not capable of dealing with the transients that may be present, the test equipment could explode, causing injury or death. A table with descriptions and examples of the four CAT levels is shown in Figure 21-7. Test equipment must be labeled at the required CAT level or at a higher CAT level. For example, if the test requires a CAT II rat- ing, equipment labeled for CAT II, CAT III, or CAT IV can be used safely. 21.1.4 Electrical Testing Safety Rules Before testing: ■ De-energize the circuit whenever possible. Live circuits present additional risks. When de-energizing a circuit, observe the proper lockout/tagout procedures. ■ Use the proper PPE. ■ Be certain you are using the correct test equipment for the job. ■ Check your test equipment settings and connec- tions before each measurement. ■ Inspect your test leads for wear and tear before use and do not use any test leads that are damaged or in need of replacement. During testing: ■ Know the expected voltage and current before connecting test equipment to the device. Make sure test equipment is connected properly and set to the correct range. ■ Avoid holding the test equipment while making measurements. Th is minimizes your exposure to an arc fl ash. Hang or rest the meter if possible. Use the “kickstand” if available. ■ Use only one hand when performing the test. Keep one hand in your pocket while making measurements. ■ Watch your test lead placement to avoid a short circuit. ■ Stay alert and practice safe procedures when making measurements and working on electrical equipment. Copyright Goodheart-Willcox Co., Inc.