Introduction

A hipot test is shirt for “high potential” or “high voltage” test or dielectric strength testing, is a type of electrical test which is performed to assess the electrical insulation of a device or component. This is used to ensure the safety and reliability of electrical equipment.

Purpose of this test is to check if the electrical insulation in a device can withstand higher voltages than it would normally encounter during operation. It helps to identify potential insulation weaknesses or defects that could lead to electrical breakdown, short circuits, or electrical shock hazards. This test can indicate insulation degradation, such as pin holes, cracks and wear and tear.

Types of Hipot Tests

There are two primary types of hipot tests: DC hipot tests and AC hipot tests. Each serves a specific purpose, and the choice between them depends on the equipment being tested.

Hipot test procedure:

-Equipment to be tested needs to be first disconnected from power supply and its circuit breaker should be turned off and isolators if any needs to be opened. It means that equipment under test should be isolated. We do this in Tan-delta test also.

-Equipment to be tested needs to be discharged using discharge rods /connectors.

-A hipot tester, also known as a dielectric withstand tester, is used for this purpose. It generates a high voltage and applies it to the device under test (DUT). In other words it is a test of the dielectric strength of the insulation to ground. Tester is earthed for safety and one lead of HIPOT tester(HPT) is connected to insulation or chassis and high voltage lead of this tester is connected to the conductive part of the equipment under test as indicated in the drawing below, this testing process is nearly similar to measuring insulation resistance (I.R) values with respect to earth.

-Tester is then switched on and voltage is gradually increased or simply pressing test button can start injecting voltages .Record voltage and leakage current values.

-A high voltage more than the normal operating voltage of the device, is applied between the conductive parts (e.g., input, output, chassis) and the device’s insulation. Voltage is gradually increased to the required level signs of insulation breakdown is monitored. If the equipment passes the test, it can be considered safe for operation. As per IEC 60950, The Basic test Voltage for  Hipot test is the 2X (Operating Voltage) + 1000 V and the test time for most standards, including products covered under IEC 60950, is 1 minute.

(PC-Scope.com)

– The tester measures the current that flows through the insulation. This current is expected to be minimal, as the insulation should prevent the flow of electricity. Breakdown in the insulation if any will result in current flowing across the test points of the Hipot tester, this current flow is commonly known as leakage. If this leakage current is too high the item under test is deemed to be unsafe and the test will fail. In the video below ,How Hipot test is conducted has been explained:

A sample test format of hipot test of 33kv cable is like this:

If the insulation is in good condition, the current should be very low (leakage current). If the current exceeds acceptable levels, it may indicate a fault in the insulation. If insulation breakdown happens during testing, it will be reflected by abnormal noise and sudden high leakage current values.

DC and AC Hipots:

DC Hipot testing involves applying a high-voltage direct current to the equipment or cable under test as mentioned in example above.DC Hipot testing is effective in detecting certain types of insulation defects, such as impurities and voids. It is also a simpler test to perform compared to AC Hipot testing but it may not be as effective in identifying certain defects, such as partial discharge in cables, and it can be more damaging to the insulation in some cases.

AC Hipot testing involves applying a high-voltage alternating current to the equipment or cable under test.AC Hipot testers generate an alternating voltage, which is typically at a specific frequency, such as 50/60 Hz.AC Hipot testing is better at detecting certain types of insulation defects, including those related to partial discharge in cables and other types of weaknesses that may occur under AC operation.

Safety and precaution

All general precautions like proper body earthing connection, isolation of equipment under test, wearing of safety tools etc which are taken before conducting other HV tests should be taken here also.

Difference between a Hipot test and an insulation resistance test

Insulation resistance test -It measures the resistance of the insulation of an electrical device to the voltage applied and current flow. Higher the insulation resistance, better is the insulation. A good insulation resistance will prevent current from flowing through the insulation, which will help to protect people from electrical shock. On the other hand High Potential test checks for nonlinear breakdown by providing a known voltage and checking that the current is below a specified limit. This test is often done at a higher voltage than the device will normally be used to provide design margin unlike IR testing.