Design a Safe Hipot Test Bench

Here are some simple steps you can take to prevent shocks from your tester:

• Locate your hipot test area away from the normal flow of people and materials.
• Mark the area with tape on the floor that indicates a restricted area.
• Allow only qualified people into the hipot test area.
• Post signs indicating “Danger, High Voltage” around the area.
• Don’t put any other work area within 3 ft of the hipot test bench. The more space you can spare around your hipot area, the better.

Figure 1 shows a layout for a hipot test bench. The work surface and operator chairs should be made of nonconducting materials, which won’t create unwanted ground paths that can harm people. You should install a ground rod driven into the floor close to the test bench or connect your safety ground to the building’s ground system. Connect the hipot tester’s safety ground connection to the ground rod using 18 AWG or heavier wire. Or, you can rely on the safety ground wire of the hipot tester’s power cord as your safety ground.

Figure 1. Place all equipment on a nonconductive work surface and keep the hipot tester’s from panel away from the DUT. Mount palm switches that require two hands for hipot operation. (Courtesy of Associated Research.)

Some hipot testers can perform ground bond tests that tell you whether the safety ground can withstand any current that might flow to ground should a DUT’s insulation fail.1 If possible, place a barrier between test operators and the DUT. For example, you might build an acrylic box with a hinged cover to fit over the DUT. Use the hipot tester’s interlock terminals to make sure the tester can’t supply voltage while the test operator has access to the DUT. You also can install a switch that closes only when the cover is closed (Fig. 2).

Figure 2. Install a microswitch that prevents the hipot tester from operating while the DUT cover is open. (Courtesy of QuadTech.)

For additional safety, you can force test operators to keep both hands away from the DUT by connecting palm switches in series to the hipot tester’s interlock terminals. Mount the palm switches at least 3 ft apart (Fig. 1) so the test operator must use both hands to activate the hipot tester. Install the switches near the outside edge of the test bench, in easy reach of the operator.2

The most elaborate way to prevent improper use of a hipot tester is to use a light curtain—an infrared light beam that will open a switch whenever anyone breaks any part of the light beam. Connect the light curtain’s output to the hipot tester’s interlock terminals; this will prevent the hipot test from turning on the high voltage whenever an operator places anything—including hands—through the infrared beam. Figure 3 shows how to place a light curtain between the test operator and the DUT. You also must make sure that nobody can reach the DUT by going around the light curtain.

Figure 3. A light curtain disables the hipot tester when anything breaks the curtain’s light beam. (Courtesy of QuadTech.)

Proper layout of the test bench also reduces accidents. Note the layout of the equipment in Figure 1. A test operator can reach the hipot tester’s front panel without reaching over the DUT.

If the test bench has a shelf, never place the hipot tester above the DUT unless you have a barrier that has an interlock switch around the DUT.

Even with all these precautions, you must still allow for the possibility of an accident. Design the bench for the easiest way to remove power in case of emergency. Do not require the test operator to use a rocker or toggle switch. Instead, add another palm switch to the test bench that removes all AC power—except for lights—from the test bench. T&MW

FOOTNOTES
1. A Basic Guide to Electrical Product Safety Testing, Associated Research, Lake Forest, IL, 1998,
p. 13.
2. Electrical Safety Guide, QuadTech, Marlborough, MA, 1998, p. 19.

FOR FURTHER READING
EN 50116, Information technology equipment: Routine electrical safety testing in production, CENELEC, European Committee for Electrotechnical Standardization, Brussels, Belgium, August 1996.
General Instructions for Manually Operated Bench Type Testing. Slaughter, Ardmore, OK, 1994.