HiPot Testing: Dielectric Withstand Testing for Cables, Breakers, and Switchgear

High-potential testing, HiPot, or dielectric withstand testing, proves that insulation can survive a voltage significantly above its rated operating level without breaking down. Unlike insulation resistance or power factor testing, which measure the condition of insulation and produce values to trend, HiPot is a pass/fail proof test. The insulation withstands the applied voltage for the specified duration, or it doesn't. There is no grey zone in the result, though there is considerable judgement in choosing the right voltage and interpreting what happens to the current during the test.

Southern Switch field crews perform HiPot testing on cables, circuit breakers, switchgear, and rotating machinery as part of both acceptance testing and maintenance test programs throughout Florida and the Southeast.

What HiPot actually tests

In service, insulation is stressed continuously at system voltage. Defects, voids, delamination, moisture intrusion, mechanical damage, contamination, weaken the insulation locally, reducing the voltage the insulation can withstand before it conducts. A defect that does not cause a failure at operating voltage may still fail at a moderate overvoltage. HiPot exploits this by applying a controlled overvoltage under test conditions, where a breakdown is a test failure rather than an uncontrolled in-service fault.

The tradeoff is that HiPot is slightly stressful to the insulation even when it passes. This is especially true for aged cable insulation, where repeated HiPot testing at high voltage can accelerate degradation. This does not make HiPot a bad test, it is the most direct way to confirm insulation integrity, but it does inform voltage selection and frequency, particularly for in-service equipment with unknown history.

AC HiPot vs. DC HiPot

HiPot testing is performed with either alternating current (AC) or direct current (DC), and the two are not equivalent. AC HiPot applies a sinusoidal voltage at power frequency (60 Hz), which stresses the insulation in the same way the energized system does. The peak voltage during an AC HiPot test is 1.414 times the RMS test voltage, so a 10 kV RMS AC HiPot subjects the insulation to 14.1 kV peak stress. AC HiPot is the preferred method for switchgear, circuit breakers, and rotating machinery because it replicates service stress accurately and is more sensitive to surface contamination and tracking defects.

DC HiPot applies a steady-state direct current voltage. It does not replicate AC service stress, the field distribution in the insulation under DC differs from AC because DC field distribution follows resistivity while AC follows permittivity, but DC HiPot equipment is lighter, more portable, and easier to manage in the field than AC equipment at medium voltages. For this reason, DC HiPot has historically dominated field cable testing. The DC test voltage is set higher than the equivalent AC test to partially compensate for the different stress profile: NETA ATS Table 100.1 and IEEE 400 specify DC test voltages for cable that are typically 1.6 to 2.5 times the AC rated voltage, depending on cable type and age.

One important limitation of DC HiPot on cross-linked polyethylene (XLPE) cable is that DC can cause charge accumulation in the polymer matrix, space charge, that does not discharge quickly and can actually damage the cable or cause delayed failures after the test. For XLPE and EPR cable specifically, very low frequency (VLF) testing has largely replaced DC HiPot as the acceptance and proof-test method, because VLF applies AC stress at 0.1 Hz rather than 60 Hz, eliminating the space charge problem while remaining portable. For mass-impregnated paper cable (PILC) and older EPR, DC HiPot remains standard. See our article on VLF Cable Testing and Tan Delta for a detailed comparison.

Test voltage selection

Test voltage depends on the equipment type, voltage class, whether the test is acceptance or maintenance, and whether the equipment is new or in service. The key principle is that acceptance test voltages are higher than maintenance test voltages, because new equipment is expected to withstand a more aggressive proof test, while in-service equipment with years of aging is tested at a reduced level to avoid introducing new damage.

NETA ATS and NETA MTS are the primary references for field test voltages on substation equipment. For circuit breakers, NETA MTS Table 100.1 lists specific DC HiPot voltages by voltage class, for example, a 15 kV class circuit breaker is tested at 36 kV DC across the open contact gap and 27 kV DC from each terminal to ground. For cables, IEEE 400 and NETA ATS Table 100.3 specify DC test voltages by rated voltage and insulation type, with columns for new cable acceptance and in-service maintenance. For medium-voltage switchgear, NETA ATS Table 100.2 governs. These are starting points; some utilities specify their own higher or lower standards based on operational experience, and manufacturer documentation governs for equipment still under warranty.

A common rule of thumb for equipment not covered by a specific table is the 2E + 1000 V formula from IEEE C37 and IEC standards, twice the rated phase-to-phase voltage in volts plus 1000 V, applied AC RMS. This formula originates in factory acceptance test requirements and is conservative for field use; NETA maintenance test voltages are typically set below factory acceptance levels.

Circuit breaker HiPot

For circuit breakers, HiPot testing covers three insulation zones independently: across the open contact gap (from one terminal to the other with the breaker open), from each terminal to ground (with the breaker open), and from line to load through the closed contacts (with the breaker closed, to check bushing and tank insulation). The open-contact gap test is the most demanding because the insulation across the gap must withstand full system voltage during normal switching, it is the zone most exposed to arcing and contact erosion damage.

Failures in circuit breaker HiPot most commonly appear as flashover across contaminated or cracked insulation, breakdown through moisture-saturated insulation, or tracking paths that have developed from repeated arcing. In oil circuit breakers, carbon contamination in the oil creates conduction paths that lower the withstand voltage progressively. HiPot after maintenance, after a contact replacement or oil change, confirms that the repair restored full dielectric integrity before the breaker returns to service.

Cable HiPot

Cable is the most common application for field HiPot. New cable is tested at acceptance to verify it survived installation without jacket damage, termination problems, or splice issues. In-service cable is proof-tested periodically to confirm continued integrity. The entire cable run, conductor, insulation, shield, and jacket, is stressed simultaneously, which means any defect anywhere in the run can produce a failure. This is both the strength and the limitation of cable HiPot: a pass result covers the entire length, but a failure does not immediately tell you where the defect is.

When a DC HiPot failure occurs in cable, fault location testing, time-domain reflectometry (TDR), arc reflection, or decay method, is used to identify the fault location before excavation or repair. The fault location process adds time and cost to any HiPot failure, which is one reason utilities increasingly prefer VLF tan delta testing for periodic in-service evaluation: tan delta identifies degraded cable sections without needing a failure event to locate the problem.

Switchgear and rotating machinery

Medium-voltage switchgear is HiPot tested at acceptance to verify that bus insulation, instrument transformer insulation, and cable terminations all meet the required withstand level. The test is run with all primary connections made and secondary circuits isolated. Failures typically trace to inadequate air clearances after assembly, contaminated bus insulation, or termination problems.

For motors and generators, AC HiPot is preferred over DC because the winding insulation in rotating machinery responds differently to DC stress than to AC, DC can charge the insulation to a level that produces failures when the machine is next energized rather than during the test. IEEE 95 governs HiPot testing of rotating machines. For new machines, the factory applies 2E + 1000 V AC for one minute. For field acceptance after rewind, NETA ATS specifies 75% of the factory test level. For in-service maintenance, 65% of factory level is typical.

Reading the current during the test

During a DC HiPot test, the leakage current displayed on the test set is the primary diagnostic tool. At the moment voltage is applied, a large capacitive charging current flows and then decays within seconds. After charging current subsides, the steady-state leakage current represents the actual condition of the insulation. Good insulation shows a stable, low leakage current, typically in the microamp range for cable. Insulation that is deteriorating or has a defect shows elevated leakage current, or current that continues to rise rather than stabilizing. A rising current during a timed withstand test is a warning that breakdown may be approaching, even if the insulation has not yet failed.

In AC HiPot, the equivalent signal is the charging current, a largely capacitive current that is expected to be stable. An increase in current during the hold period, particularly a sudden step increase, indicates partial discharge activity or the beginning of a breakdown path. Some AC HiPot sets include partial discharge measurement capability, allowing defect detection below the breakdown threshold.

How HiPot fits alongside megger and power factor testing

HiPot, insulation resistance (megger), and power factor testing are complementary, not interchangeable. Megger testing measures the DC resistance of the insulation bulk, it is sensitive to moisture and contamination but requires significant degradation before it catches localized defects. Power factor testing measures dielectric loss and is sensitive to general insulation aging and oil quality in transformers and bushings. HiPot proves that the insulation, in whatever condition it is in, can hold off a specific overvoltage for a specific time. A piece of equipment can pass a megger test and a power factor test and still fail HiPot if it has a localized defect that is invisible to bulk measurements. For this reason, a complete acceptance test program uses all three methods, each answering a different question about insulation condition.