Electrical insulation is an essential part of electrical circuits. It ensures the safety and quality of the electrical circuits. Without good insulation, you can face electric shocks, and as a result, you can get fatal injuries. So, you must perform electrical safety tests to verify insulation ability. For this, you can do a Hipot test. Let’s get into details.
What is Hipot testing?
The term “Hipot” stands for high potential. You can also call it a dielectric (insulation barrier) withstand test. Hipot testing determines the electrical insulation adequacy in devices and assemblies for the overvoltage transient. In other words, you apply a high voltage to the devices for a certain period. With this, you check the dielectric integrity of a system. You can do this test either for new fittings or as a routine test for already installed devices.
How does a Hipot test work?
A Hipot test tests the electrical safety of a device with the help of a Hipot tester. You can conduct the test between mutually insulated sections of charged parts and electrical ground.
The Hipot tester checks for any breakdown or perforation under high voltage. Also, it ensures that insulation distances are correct. When there is adequate insulation, minimal current flows between the conductors. You can accept this small current. But make sure there is no breakdown due to air insulation and solid insulation. The tester can test different electrical products such as motors, transformers, or PC boards. You can measure 5K-400K volts with testers of various sizes.
Image: electrician examining current voltage with tools
Hipot tester setting
Based on the specific requirements of the equipment, the test duration, rate of application, and applied voltage may vary. A Voltage cable, consumer electronics, or switchgear have different test standards.
Time Duration of Hipot test
The duration of the test must match the safety standards of the device. For products that come under IEC60950, the standard test time limit is one minute.
A general rule of thumb is 110-120% of 2U+ 1000V. Apply this for 1-2 seconds.
The current setting for the Hipot test
Modern Hipot testers come with the option of setting a current limit. Here you need to set a time limit for testing. However, if you know the actual leakage current, you can predict the Hipot test current. You can expect this in two ways:
Firstly, test some samples of the products. Based on this testing, select an average Hipot current. Once you know this, set the trip level slightly higher than the average you got.
Secondly, you can use this formula to predict the current trip level:
E(Hipot)/E(Leakage)= I(Hipot) / 2XI(Leakage)
Ensure that the current trip level is a bit high to avoid any failures due to leakage current. However, keep it low enough so that you do not miss the breakdown in insulation.
The Output voltage
You can perform a Hipot test using a DC, AC, and very low-frequency AC test voltage. Let’s understand each dielectric withstand test in detail.
DC Dielectric Withstand Test
During this Hipot test, you apply a voltage to a specified value. However, you use the voltage gradually until it reaches the final voltage. During this process, there is a continuous leakage current. Generally, it takes 60-90 seconds to get the final voltage.
Once you reach the final voltage, hold for 5-15 minutes. During this time, if leakage current measurements are high, it will trip the test set, and if not, it means cable insulation is acceptable.
With DC Hipot testing, you measure the insulation resistance of cables by ohm’s law. The DC Hipot voltages value depends on the thickness and the type of the insulation, conductors’ size, and cable construction.
Firstly, there is an adjustable trip option for leakage current. You can set the leakage current trip level to a lower level compared to the AC test voltage. Thus, you can filter the products with marginal insulation.
Secondly, the voltage rise is gradual. As a result, you can monitor the current flow and detect potential insulation breakdown before it occurs.
DC voltage is free of capacitive division. Thus, it can detect mechanical damage and low resistance areas in the dielectric.
It is challenging to generate DC test voltages.
DC testers are slightly expensive than AC testers.
You do not get a thorough analysis of the cable condition with DC Hipot testing.
AC Dielectric Withstand Test
Sometimes, cables and accessories need power frequency or AC voltage for Hipot testing. However, it requires heavy and expensive equipment which are not readily available. In the AC Hipot test, capacitive and resistive currents are in parallel. The source frequency plays a significant role in charging the capacitance of the test specimen. Also, your test equipment must successfully capture the test specimen.
An AC Hipot test can check both voltage polarities
You do not require a long ramp time
Here, you need not discharge the device under test after completing the test.
It is not a very practical test for maintenance and acceptance tests on cables.
If the circuit has a high Y capacitor, an AC tester’s current trip setting can indicate a failure. To avoid this, you must disconnect Y capacitors before testing.
AC testing at high voltage can damage some insulation.
VLF Dielectric Withstand Test
You can use this test as withstand or diagnostic test, and it means you can conduct it as a proof text for the acceptance of cables. Or you can perform this as a maintenance test to assess the cable condition.
You can perform this test with an AC Hipot at a frequency ranging between 0.01Hz to 1 Hz. The most common frequency is 0.1Hz. The procedure of VLF testing is similar to DC Hipot testing. If the cable can handle the high voltage, you can consider it as a pass.
Unlike AC & DC Hipot voltages, very low-frequency testing does not damage good insulation. Also, it does not lead to premature failures.
Safety precautions during Hipot test
When you conduct a Hipot test, you must prevent injury due to electric shocks.
- Make sure that you receive a maximum of 45uC charge in a shock
- The total Hipot energy must not be more than 350mJ
- The total current must not exceed 5mA at peak
- If there is any fault current, it should not stay longer than 10mS
- If your tester does not meet these standards, it should have a safety interlock. With this lock, you will not come in contact with the cable during testing. Most of the advanced Hipot testers come with this safety interlock.
- Make sure that your Hipot tester has a calibration certificate.
Image: electrical safety testers, voltage testers, wire strippers, pliers
Along with the Hipot tester guidelines, there are some safety precautions for testing the cable.
- Verify that safety circuits are operating perfectly when you calibrate them.
- During testing, don’t touch the cable and continuity terminals
- Don’t remove the cable until the test is complete
- Always use insulating gloves for ultimate operator safety
- Keep the equipment and cable away from the children
- Don’t use the equipment if you have any electronic implants
- Make sure that your tester is working fine. Some product safety agencies verify the tester’s performance daily.
- Make sure that you are taking readings within + or – 5C calibration temperature.
Test Method of Hipot
Now, let’s understand the test procedure in detail. Make sure that only qualified electrical workers perform the test.
- Isolate the cable or circuit you want to test by opening circuit breakers or switches.
- Isolate the circuit under test from all other equipment and cables.
- In this Hi-pot test, the limited approach boundary at 1000 volts is 5ft. So, put some barriers around the boundary to stop other personnel from crossing it.
- Now, connect the ground lead of the Hipot tester to the ground electrode conductor. You can also use building ground to complete the Hipot test connection.
- Once you complete the Hipot test connection, switch the Hipot tester “ON”. Set the voltage to 1000V or any pre-decided DC voltage.
- Press the “test” button on the tester. Take the reading after one minute.
- Observe the resistance and record the reading for reference.
- When the one-minute test ends, switch the Hipot tester mode to voltage measuring mode with adjustable voltage output setting. Now make sure that the circuit phase conductor and Hipot tester voltage must read zero.
- Now, repeat the same test for all circuit phase conductors. Test each phase to the ground and each phase to each phase.
- Once you complete the test, disconnect the Hipot tester. Confirm that all the circuits are ready to re-connect and re-energize.
- The cable or circuit must undergo the stress of pre-decide voltage for 1 minute without any breakdown to pass the test.
- Equipment with less than 0.1m2 of an area must have insulation resistance not less than 400mega ohms.
So, now you know all about Hipot testing. You need to conduct this test regularly to ensure the Optional Insulation of the electrical equipment and cables. If you need high-quality cable assemblies and wiring harnesses to support your Electrical Safety, don’t hesitate to contact Cloom.