Using Voltage to Detect Insulation Defects

Here are some guidelines for using voltage to find insulation problems.

Test SituationRecommendation

When air gaps between conductors can be less than 0.1"

High-voltage breakdown testing is appropriate when air gaps of less than 0.1 inches can occur between conductors. The testing often needs to be done above 500 volts to really be helpful.

Voltage must be more than 50 volts to be useful

Testing at less than 50 volts can detect leakage or contamination, but the low test voltage generally does not detect common insulation breakdown between conductors.

Test at voltages above the rated working voltage

Brief high-voltage testing at three times the rated voltage of cable assemblies and wire harnesses has not been known to damage or degrade insulation. A common aerospace guideline is to test at two times the operating voltage plus 1000 volts.

Insulation resistance testing

Testing High-voltage Insulation Resistance (IR) is often needed when the cabling is used for sensitive low-power signals.

AC testing

High-voltage AC tests for large assemblies can create a shock hazard. For this reason, and also because of slower test speed and higher cost, it is not as popular as DC testing. In some circumstances, AC testing detects problems not found with DC tests, particularly when used with long application times. AC tests are known to degrade PVC insulation. When using a DC test to replace an AC test, raise the voltage by at least 40 percent (1.414 RMS multiplier).

Shock hazard

High-voltage DC tests of cables shorter than 500 ft. do not need to be a shock hazard. A tester designed to test larger assemblies at higher currents can still be a hazard with wrong settings or if it malfunctions. IEC479-1 and EN61010-1:1993 specify a non-hazardous energy source as being less than 45 micro coulombs (this is 45 / 1500 = .03 micro-farads at 1500 VDC). This limit includes energy from both the tester and the assembly under test.

Which wires require testing

Test voltages must be applied between all relevant combinations of wires, including shields, to test insulation.

Voltage ramp rate in DC testing

Controlling the voltage ramp rate is a common way to limit the current needed to bring each wire up to the hipot voltage. A modern hipot tester can limit the inrush current and detect failures as the voltage is rising which makes the ramp rate specification unnecessary.

Burning out shorts and arcing

A high-voltage test that allows high currents causing visible arcing that burns out a short and then passes the wired assembly as good, cannot be relied upon for defect removal because carbon tracks and potential insulation damage may occur.

Voltage accuracy

Voltage accuracy must be within 10 percent or a higher voltage setting must be used that assures testing to within 10 percent of the specified test voltage.

Finding stray strands

High-voltage testing, unless done at many thousands of volts, does not detect stray strands unless they are within .050 of an inch of another conductor/contact.