ONLINE TOOLBOX

 Reference Book

 Software Downloads

 Links To Related Sites

 Industry News

 Training Opportunities

 Glossary Of Terms

 Online Calculators

 Technical Information

 EPAct

 Rebate Information

 Factory Cross Reference

HOW-TO TIPS FOR HI-POT TESTING

Hi-potential DC testing requires precise measurements at hazardous voltage levels.

• Published in Southwire's Power Cable Update newsletter in February 1998
• Reprint permission granted

Doing high-potential DC testing? Here are some tips for getting better- and safer-results. "Above 40 or 50kV, leakage paths become critical," says Sid Ticker, senior applications engineer. "It doesn't take much to give you a false reading - and safety is essential."

Prevent Corona Discharge

A 15kV cable with 220 mils of insulation may be tested at 64kV. When you're measuring leakage current in microamperes, you need to prevent corona discharge that may cause false current readings.

• Wash down the ends of the cable and all test connections with a residue-free cable cleaner to remove dust and moisture.
• Wrap the test connections with several layers of plastic wrap. The generic brands seem to suppress leakage better than the high-cling, top-line brands.
• Close off cable ends with plastic wrap, a plastic bag, a close-fitting glass jar, or even a high voltage rubber glove.
• In extremely humid conditions, you can purge the plastic bag with SF6 gas so moist air is displaced by a non-conducting gas.
• Make sure that any terminations and splices on the line can handle the test voltages. A flash-over could damage the splice. If there is any question, check with the manufacturer of the splices or terminations and make sure they can be tested at the same level as the cable without excessive leakage current.

Apply Voltage Gradually

As a rule of thumb, limit voltage increases to 1kV per second or 75 percent of the rated current output of the test equipment, whichever is less.

• If you're doing a time-leakage test, record the output current every 15 seconds for the first minute and then every minute till the test time has elapsed.
• Plot the readings and look at the shape of the curve. It should increase gradually and be more or less linear with increases in voltage. You shouldn't have any sharp increases from one voltage level to the next.

Have Qualified Safety Supervision

• After testing a conductor, let the residual voltage discharge to 20 to 50 percent of the test value before discharging it fully with a manual ground. Leave the conductor grounded for one to four times the duration of the test.
• Don't use string or cord to tie back cable ends. It may contain moisture that can form a current path to any framework it's tied to.
• Be sure the test area is secured, to prevent anyone from wandering into the area. Don't assume there's no one in any intermediate access vaults. Never apply test voltages without communication between both ends of the test cable.

"High-potential tests produce dangerous voltages and possible hazards," says Ticker. "They should be supervised by qualified engineers who are familiar with hi-pot testing."