Does repeating the Hipot test damage your cable?
Cirris cable hipot testers are designed to limit the current and the charge that is applied during the hipot test. * Consequently, Cirris hipot testers are recognized as safe for the tested device. Note however, there are many kinds of hipot testers. Not all hipot testers limit the current and charge like the Cirris testers.
Some electrical cables have electrical components such as integrated circuits, diodes, capacitors and resistors. During the hipot test, the Cirris cable tester can raise voltage simultaneously on all the legs of a component or circuit so that no voltage differential is seen across it. A test technician should review the test program to ensure it correctly takes into account sensitive components.
Why limit current and charge
During a high voltage test, a cable tester applies high voltage to each connector contact or interconnected net of the tested device while holding other connections at ground. During the Dielectric Withstand Voltage (DWV) Test, the Cirris cable tester monitors for a sudden spike in the current that indicates an arc or other sudden breakdown condition. If the current level exceeds the current trip setting (also referred to as a DWV threshold current setting) the Cirris cable tester immediately shuts off the voltage and fails the tested device. During the Insulation Resistance (IR) Test, a Cirris tester switches to a lower current range to accurately measure high levels of insulation resistance. Again, the tester immediately shuts off high voltage with any significant rise or small spike in the current.
Some test specs may allow for higher currents than the Cirris tester will support. A lower current threshold is a tighter specification. Some Cirris customers have been concerned about rejecting good cables because they can’t set the trip current setting to the maximum allowed by the test specification. However, practice shows that good cables are not rejected when using a lower current level. There is no need to allow a current spike to continue. Once an arc or breakdown initiates, the current increases until the current threshold is reached regardless of where the current threshold is set.
Why do some hipot testers allow higher current thresholds? Some hipot testers are designed with the capability to do destructive hipot tests, where the dielectric voltage strength of insulators is exceeded and monitored during an extended breakdown period. While exceeding the dielectric strength, high current flow causes heat, which in turn allows evaluation of the potentially damaging effects of high temperatures. In other cases, hipot testers have high current limits to accommodate noise in the measurement system. Some have suggested higher test current thresholds can be used to “blow the high resistance short out”. While this may be possible, we would suggest that damage is more likely to occur under high current conditions. The error should always be diagnosed and corrected at its root cause.
Cables and conductors with very capacitive nets, such as very long cables, also require higher charge and current limits when performing a hipot test. When a voltage is raised quickly on a highly capacitive net, there is an inrush of current, even though the net is electrically isolated from other nets. The voltage ramp time can be increased on a DC hipot test, allowing the tester to raise a net to the hipot voltage without high current flow. However, the internal charge, Q, on a net with capacitance, c, increases as the voltage increases according to the equations Q = cv. Note that if charge is not limited, the energy stored in the cable could allow a destructive high current flow during a hipot failure. In an AC hipot test, the voltage changes with the AC frequency. Therefore, higher current is required to perform AC hipot on a highly capacitive net.
Prudence when hipot testing
High voltage settings for the hipot test should be selected that are below the actual dielectric breakdown voltages of the components being tested. Note the actual dielectric breakdown voltage is often much higher than the rated or working voltage of the components.
Rerunning a failed hipot test over and over is not considered a good industry practice. This is the case of a bad cable possibly being made worse each time a hipot arc occurs. It may be necessary to rerun a hipot test to diagnose an error. While it is extremely unlikely that a repeated low current arc of a Cirris Tester could do permanent damage, the reason for the hipot failure should be investigated. Once you believe you have fixed a hipot error, you must rerun a hipot test to ensure the hipot error has been corrected. MIL-STD-202 Method 301, which dictates practices for the Dielectric Withstand Voltage (DWV) test on electrical components and parts used in the military states,
When a component part is faulty …, application of the test voltage will result in either disruptive discharge or deterioration. … Deterioration due to excessive leakage currents may change electrical parameters or physical characteristics.
We have on occasion seen instances where the hipot test may fail once and then, without any change to the device under test, will not fail again. This indicates a semi-conductive element was dissipated, in other words, a bad cable becomes good with the hipot test. Note however, when a cable passes the Cirris hipot test, there is no possible negative effect due to limited current and charge of the Cirris Hipot tester. It is true that if the hipot voltage is misapplied, the discharge could damage a static sensitive component. However, a normal discharge that occurs from one wire net to another does not generate enough heat to damage an insulator.
Because of the complexities and criticalness of military equipment, and potentially because of the variances in current limits of test equipment doing the DWV test through the years, MIL-STD-202 Method 301, further states:
1.1 Precautions. The dielectric withstanding voltage test should be used with caution particularly in inplant quality conformance testing, as even an overpotential less than the breakdown voltage may injure the insulation and thereby reduce its safety factor. Therefore, repeated application of the test voltage on the same specimen is not recommended. In cases when subsequent application of the test potential is specified in the test routine, it is recommended that the succeeding tests be made at reduced potential.
While this statement indicates a possible negative effect from repeat hipot testing, Cirris has performed experiments to observe any effects of repeated hipot in a device that passes the hipot test. With a Cirris hipot tester, we have never been able to create a change in the device under test, even when the hipot test voltage is purposely set near the maximum dielectric strength of the device under test, and the device is hipoted thousands of times.
* Current and charge limits for Cirris testers.