Does the cable survive far more voltage than it will ever carry?
Insulation resistance measures how good the insulation is. A hi-pot test asks a harder question: does it hold at a voltage well above normal? The cable is stressed under control so a weak spot fails on the test, not on a live runway circuit.
A high-potential (hi-pot) test, also called a dielectric withstand test, applies a voltage well above the cable's rated voltage between the conductor and earth for a set time, with the circuit de-energised. It is a proof test with a simple question: does the insulation hold, or does it break down? A cable that passes has shown it can take more than service will ever ask of it, so a weak joint or a damaged length fails on the test rather than on a live airfield circuit.
1How it differs from insulation resistance
Both tests apply voltage between the conductor and earth, but they answer different questions. Insulation resistance uses a modest DC voltage and measures the insulation in megohms. A hi-pot test uses a much higher voltage and proves the insulation withstands it. The insulation-resistance test comes first, because a cable that already reads low should not be stressed further.
| Test | Voltage | Question it answers | Result | Risk to the cable |
|---|---|---|---|---|
| Insulation resistance | Modest DC, set by the cable rating | How good is the insulation? | A value in megohms, kept as a baseline | Low |
| Hi-pot (withstand) | Well above the rated voltage, per the specification | Does it hold at high stress? | Pass or fail, plus the leakage current | Higher: it is a stress test |
2The sequence on site
The specification sets the test voltage, the hold time, the ramp rate and the leakage limit, taken from the cable data and the standard in use. The steps are the same each time:
- Isolate and prove dead. Lock out the CCR and disconnect it from the loop.
- Take out everything that is not cable. Disconnect the isolation transformers, lamps and connectors so that only the cable and its joints are stressed.
- Cap the far end and clear the area. Cap the far end, put up barriers and warning signs, and keep everyone clear of the cable and its far end.
- Pass insulation resistance. A cable that already reads low is not stressed further.
- Ramp up, hold, ramp down. Ramp up at the set rate, hold for the set time while watching the current, then ramp down.
- Discharge, earth and retest. Discharge and earth the cable, then measure insulation resistance again and compare it with the reading before the test.
3Why the current is watched
A cable acts as a capacitor, so raising the voltage first draws a charging current. It scales with how fast the voltage rises, which is why a slow, controlled ramp matters:
When the voltage stops rising, the surge dies away and only a small leakage current through the insulation remains, following Ohm's law:
The tester has a current trip that cuts the voltage if the current exceeds a set limit, which is what a breakdown looks like. The ramp is also watched for a current that keeps climbing in the hold, which points to moisture or deterioration before the insulation fails outright.
4DC, or very low frequency AC
The specification names the type of voltage. DC testers are small and easy to carry to site. Very low frequency (VLF) AC, at about 0.1 Hz, gives a stress closer to what the cable sees in service with equipment that is still portable, and power-frequency AC is closest of all but needs heavy kit. Many cable standards discourage DC on aged extruded cable, so follow the cable maker's advice and the project specification rather than a habit.
5Reading the result
| What you see | What it usually means | What to do |
|---|---|---|
| Voltage held, current settles low and steady | Insulation withstands the test | Ramp down, discharge, retest insulation resistance, record |
| Surge on the ramp that settles | Normal capacitive charging | No action; slow the ramp if it trips the limit |
| Current keeps climbing in the hold | Moisture, damage or deteriorating insulation | Stop and discharge; do not energise; find the cause |
| Breakdown and tester trip | A weak spot has failed | Locate with TDR or bridge methods, repair, retest from the start |
| Readings wander or will not repeat | Surface leakage across dirty or damp cable ends | Clean and dry the terminations, then repeat |
6When it is used, and when it is not
A hi-pot test is called for on new cable and new joints at acceptance, and after a repair where the specification asks for one. It is a stress, so it can finish off insulation that was already marginal. Run it only where the specification calls for it, at the specified level and time, and do not repeat it without reason. It is a cable test, not an equipment test: the regulator, isolation transformers, connectors and lamps have their own tests and ratings, and are kept out of it.
7Safety, and the record
The voltages are lethal even at tiny currents. Use barriers and warning signs, keep a clear exclusion zone around the whole cable and its far end, and make sure everyone on the circuit knows the test is under way. The cable stores charge, and after it is earthed some voltage can slowly return (dielectric absorption), so discharge it through the tester's discharge path, keep the earth connected, and only then handle or reconnect it. Record the test voltage, hold time, leakage current, conditions, tester and calibration date, and the result, with the cable records.
Frequently Asked Questions
Q1What is the difference between a hi-pot test and an insulation-resistance test?
An insulation-resistance test applies a modest DC voltage and reads the insulation in megohms, so it measures how good the insulation is. A hi-pot test applies a voltage well above normal for a set time and asks only whether the insulation withstands it without breaking down. Insulation resistance is done first, and the hi-pot test is a proof test that follows it.
Q2Why are the CCR, transformers and lamps kept out of a hi-pot test?
The test voltage is chosen for the cable and is higher than those items are built to take. The regulator, isolation transformers, connectors and lamps are disconnected so that the voltage stresses only the cable and its joints, and so that nothing rated lower is damaged. Equipment is tested separately to its own requirements.
Q3Why does the current jump when the voltage is first raised?
A cable behaves like a capacitor, and raising the voltage charges it. The charging current is proportional to how fast the voltage rises, so a slow, controlled ramp keeps the surge small. It then settles to a small steady leakage current. A current that keeps climbing during the hold is a warning of weak insulation.
Q4Is the test done with DC or AC?
Either can be specified. DC testers are small and easy to carry to site. Very low frequency AC at about 0.1 Hz gives a stress closer to service, and many cable standards discourage DC on aged extruded cable. Follow the cable maker's advice and the project specification for the type, level and time.
Q5Can a hi-pot test damage a good cable?
A sound cable should not be harmed by a correctly specified test, but the test is a stress and it can finish off insulation that was already marginal. For that reason it is run only where the specification calls for it, at the specified level and time, and not repeated without reason. The failure is better found on the test than on a live circuit.
Q6What happens when the hold time ends?
The voltage is ramped down, then the cable is discharged and earthed. It stores charge like a capacitor, and after earthing some voltage can return slowly, an effect called dielectric absorption. So the earth stays connected until the cable is handled or reconnected, and insulation resistance is measured again to confirm nothing changed.
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