basic  voltage  categories:  (1)  voltage  measurements below 300 volts, and (2) voltage measurements above 300 volts. VOLTAGES BELOW 300 VOLTS Most of the voltage measurements that you will make will be below 300 volts. Almost all the newer electronic systems operate at less than 28 volts, except for the main input ac power. Table 3-2 lists the safety procedures  for  measuring  voltages  below  300  volts. Follow them! In addition to the procedures in table 3-2, you must obey the four safety precautions in table 3-4 when you take measurements on energized equipment. The following excerpt from a mishap report shows the importance of following these precautions: “A technician could not get the alligator clip on the test lead to stay on the probe, so he held the clip to the probe with his right hand. He violated safety precautions by continuing to  hold  the  clip  and  the  probe  while  he  was energizing the test lead with 1,200 volts. Soon 3-4 Step Procedure 1 ALWAYS notify and obtain permission from your commanding officer (afloat) or your supervisor (ashore) to work on energized equipment. (Some commands require you to complete a checklist before doing this.) 2 ALWAYS study the schematic and wiring diagrams of the equipment on which you will be working. Note the location of the test points and the location of any other high-voltage points that you should be careful not to touch. 3 ALWAYS remove all metal items, such as, watches, belt buckles, rings (even wedding bands), and other items that have exposed metal. If you are wearing a security badge, put it in your pocket. 4 ALWAYS ensure that you are wearing electrical safety shoes, if they were issued, and that you are standing on insulating rubber matting. If you must insert your hand into the enclosure of the energized equipment, wear electrical safety rubber gloves rated for the appropriate voltage. See table 3-3. 5 NEVER work alone. Have a co-worker stand by. Make sure that your co-worker knows where to secure the power in case of emergency. Table 3-2.—Procedures to Follow When Measuring Voltages Below 300 Volts Class Maximum Safe Voltage 0 750 volts I 3,000 volts II 4,000 volts III 5,000 volts Table 3-3.—Rubber Glove Ratings No. Safety Precaution 1 ALWAYS use test probes with safety guards or barriers on the probe tips to prevent your hand from inadvertently touching the probe tip. 2 ALWAYS use insulated alligator clips. 3 ALWAYS keep your body clear of any metal parts of the equipment on which you are working. 4 ALWAYS try to keep one hand in your pocket or behind your back when you are taking a measurement to avoid creating a ground path for electric current flow through your body from hand to hand. Table 3-4.—Safety Precautions For Measuring Voltages Below 300 Volts