It is an electrical safety test that product insulation passes, to ensure they can withstand high-voltage or short circuits.
During the test, the insulator passes through a very high voltage, for a specific time more than they will encounter during use.
For the device under test (DUT) to be safe for use, it must withstand the test without defect or breakdown. Secondly, it must not allow excess leakage current to flow out of the product surface.
Importance of High voltage withstand Test
- It helps to determine if the insulation of a product can withstand over-voltage without defect.
- It helps to detect nicked or crushed insulation, tolerance errors in cables, stray wire strands, and conductive or corrosive contaminants around the conductors.
- Helps protect users from the electric shock that may arise from poor insulation.
- It also detects scrapes, pinholes, spacing, poor creepage, and clearance distances made during production.
- In the production line, hi-potential testing helps to check whether the construction of a production unit is the same as the construction of the unit under testing.
Some failures that may be detected by a production line hipot test include a transformer wound in such a way that the creepage and clearance have been reduced.
Such failures are usually because of a new operator in the winding department.
Hi-potential testing formula
The formula for voltage withstand test is twice the normal operating voltage plus 1000 volt. The reason for adding 1000V is because of the various degrees of high power surges the insulator will pass during use.
And research shows that over-voltage can be as high as 1000V.
Hi-potential test voltage, which one to use, AC or DC?
You can use an AC or DC power supply for the test, both have an advantage, and drawback. But to be on the safer side, always check the voltage of the appliance you want to test.
If you are using AC current, use an AC tester. Else, use a direct-current tester.
When using an AC or alternating current, the insulation will experience the highest stress when the voltage is at its peak, either positive or negative peak of the sine wave.
Note: When using DC current, make sure that the test voltage is below 2 root (1.414) times the AC test voltage. This will make the DC voltage equal to the AC voltage peak.
Dielectric withstand test equipment
This test is done with a hipot testers. There are different brands, some from fluke, megger and other companies. You can buy a quality tester here on Amazon.
Dielectric Withstand Test Procedure
- Isolate the circuit or cable you want to test by opening the circuit breakers or switches.
- Place a 5ft barrier away from the equipment you want to test to prevent unqualified persons from coming close.
- Connect the ground lead of the tester to the earth electrode terminal.
- Connect the high-voltage lead to one of the isolated circuit phases.
- Switch ON the analyzer and set the meter to 1000V. Press the “test button” on the meter and record the resistance reading after one minute.
- Switch the HIPOT tester from high potential test mood to voltage measuring mode to ensure that the circuit phase conductor and the voltage of the tester correspond to zero volts.
- Repeat the above test for all the circuit phase conductors you want to test.
- Once you are through, disconnect the tester and reconnect the circuits.
To pass the test, the appliance must withstand the stress test unaffected.
During high potential testing, you may likely be exposed to some risk. To prevent electrical shock and other hazards, ensure your testing equipment adheres to these tips.
- The total hipot energy should not be above 350 MJ.
- The total current should not pass the 5 mA peak (3.5 mA RMS).
- Fault current should not stay above 10 MS.
- When testing the cable, don’t touch it during the test
- Do not remove the cable while testing is ON.
- Always wear insulating gloves
- Never allow children to use testing equipment.
- A qualified electrician should carry out the test
Advantages and disadvantages of using AC voltage for dielectric withstand test
- It does not require ramping of voltage since the polarity changes
- Discharging the device under testing (DUT) is not needed
- Measures both reactive and real current
- It stresses insulation in both polarities
- Generally accepted by safety agencies
If the circuit under test has a large value of Y capacitor, the AC tester may show a failure depending on the current trip setting.
Advantages and disadvantages of using DC voltage.
- The leakage current measurement is purely real
- Requires lower current level with less risk to the operator
- Usually easier to perform on capacitive DUTs
DC tester is more expensive because generating DC voltage is hard.
Difference between Hi-potential and continuity test
Hipot is the opposite of a continuity test.
Continuity tester checks to ensure that current flows from one point to another along the circuit, while Hipot checks to make sure current did not flow to the surface of the insulator.
Hi-potential testing duration
According to IEC 60950 standard, the test time for the product should be one minute.
Difference between dielectric withstand test and insulation resistance test
The difference between insulation resistance and dielectric withstand test lies in their measurement. The insulation resistance test measures the amount of current that flows through an insulator. The current is usually less than 1000V and is measured in ohm.
While hipot detector measures about 5000 to 6000V depending on the requirement and also returns values in milliampere.
What is the difference between Megger and Hi-potential testing?
The difference between megger and hipot is that megger is a megohmeter that measures insulation resistance, while hipot tester measures leakage current.
Is hipot testing destructive?
No, it is a non-destructive test. It is a safety test that measures the insulation of a product to make sure it is safe for consumer use.