What is HIPOT test?

What is HIPOT test?

HIPOT Test is the test conducted to make sure the conductivity between two terminals. It can also be known as opposite of continuity test – Dielectric Strength Test. For more detailed learning please continue to the article about High pot test below.

Also read Three-phase power generation, transmission and distribution

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What is HIPOT test? or What is Dielectric strength test?

Hipot Test is the short form of high potential Test. High pot test is also known by name Dielectric strength test. High pot test ensures there is no current flow between two terminals when they are applied with high potential between the terminals.

Note: High voltage potential test can be written as High pot test or Hipot test. Both are same and correct.

In some ways a hipot test is the opposite of a continuity test. A continuity test checks for good connections, meaning current will flow from one point to its destination point. If current flows easily enough then the points are connected. Many people are less familiar with a hipot test.Hi pot test

Why we need Hipot test?

 

Importance of hipot test is, it guarantees the safety operating limits of device during rated electrical conditions, a way to check the effectiveness of its insulation. 

The objective sought during the high voltage testing will determine the type and amount of voltage applied and the acceptable current flow. If the high voltage testing target is getting the value of the insulation, we are speaking about Insulation Resistance Tests. 

When we are looking for the breakdown voltage, the point where the dielectric fails allowing much current to flow, it is a Dielectric Breakdown Test, and this high voltage testing is usually a destructive test.  

When the test checks that the leakage current is below a preset safety limit for an established voltage value (below the breakdown voltage), we are speaking about a Dielectric Withstand Test, which is a nondestructive high voltage testing, and through which we are also verifying the insulation integrity of the equipment.

 

Information we get from hipot test

 

We can find following properties of the device by hipot testing,

  • terminal spacing problems
  • conductive or corrosive contaminants around the conductors
  • nicked or crushed insulation
  • tolerance errors in IDC cables
  • stray wire strands or braided shielding

The production-line hipot test, however, is a test of the manufacturing process to determine whether the construction of a production unit is about the same as the construction of the unit that was subjected to type testing. 

Some of the process failures that can be detected by a production-line hipot test include, for example, a transformer wound in such a way that creepage and clearance have been reduced. Other examples include identifying a pinhole defect in insulation or finding an enlarged solder footprint.

Suggested reading: Three-phase electric power generation

 

IEC recommended test voltage for hipot test

As per IEC 60950, The Basic test Voltage for Hipot test is the 2X (Operating Voltage) + 1000 V. The reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-to-day transient over voltages. 

Experiments and research have shown that these over voltages can be as high as 1000 V.

 

Current setting for hipot test

 

Most modern hipot testers allow the user to set the current limit. However, if the actual leakage current of the product is known, then the hipot test current can be predicted.

The best way to identify the trip level is to test some product samples and establish an average hipot current. Once this has been achieved, then the leakage current trip level should be set to a slightly higher value than the average figure.

Another method of establishing the current trip level would be to use the following mathematical formula:

The hipot tester current trip level should be set high enough to avoid nuisance failure related to leakage current and, at the same time, low enough not to overlook a true breakdown in insulation.

Time duration for Hipot Test

The test duration must be in accordance with the safety standard being used. The test time for most standards, including products covered under IEC 60950, is 1 minute. A typical rule of thumb is 110 to 120% of 2U + 1000 V for 1–2 seconds.

HIPOT Test Procedure

 

Hipot test procedure is, usually one side of the supply is grounded and other side is connected to the conductor being tested. With the supply connected like this there are two places a given conductor can be connected: high voltage or ground.

When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect all other contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.

If the insulation between the two is adequate, then the application of a large voltage difference between the two conductors separated by the insulator would result in the flow of a very small current. 

Although this small current is acceptable, no breakdown of either the air insulation or the solid insulation should take place. Therefore, the current of interest is the current that is the result of a partial discharge or breakdown, rather than the current due to capacitive coupling.

What happens when you test something more complicated than just contacts? A series of contacts that are connected with wires, resistors, capacitors, diodes, and other components is called a “network” of connections (or “net”). 

To hipot test a net, you connect all the contacts in the net to high voltage and connect all other contacts in the device to ground. For example, if you have a wire that connects two pins, the high voltage will be simultaneously apply to both of those pins and the entire wire will be raised in voltage. 

All other wires and pins will be held at ground. If you have a resistor that connects two pins, both pins are raised in voltage and the voltage drop across the resistor is always zero. The entire resistor is raised in voltage. 

In short, all pins of a component see the same voltage at all times. Applying the voltage in this fashion makes sure the body of the component is isolated from the rest of the device.

How do Hipot test affect quality?

 

All these tests are tools used to understand how a cable will perform and to monitor any changes in the cable’s performance. Dielectric Breakdown testing is used in product design and qualification stages. 

It helps establish the maximum voltage of the design. It can also be used on a random sample basis to verify that the maximum voltage is not changing. Dielectric breakdown testing may be required during the development of assemblies used in critical applications.

Many test specifications require a Dielectric Withstand test on every cable produced. The test is usually performed at about 75% of the typical breakdown voltage and is done as a safety net. 

The test is sensitive to arcs or corona so it often finds terminal spacing problems, over-mold problems, tolerance errors in IDC cables, or any problem that might produce arcs. This test doesn’t significantly degrade the cable.

The Insulation Resistance test is typically done on every cable tested and usually done at 300 to 500 Vdc with 100 to 500 MegaOhms resistance. 

The test is very sensitive to contamination in the assembly process. Solder flux, oils, mold release agents, and skin oil can cause problems. 

This test excels at identifying insulation that will conduct in the presence of moisture. Doing this test on every cable allows you to detect contamination changes in the manufacturing process.

With all the high voltage being used, what about safety?

 

Products being designed today should comply with product safety regulations. Some of these regulations reduce the chance of receiving harmful electrical shock. 

During a hipot test you may be at some risk. The risk can be reduced by following the manufacturer’s instructions. When it comes to hipot charge, energy, and voltage, select the “safest” machine that will meet your cable testing requirements.

To minimize your risk of injury from electrical shock make sure your hipot equipment follows these guidelines:

  • The total charge you can receive in a shock should not exceed 45 uC.
  • The total hipot energy should not exceed 350 mJ.
  • The total current should not exceed 5 mA peak (3.5 mA rms)
  • The fault current should not stay on longer than 10 mS.

If the tester doesn’t meet these requirements, then make sure it has a safety interlock system that guarantees you cannot contact the cable while it is being hipot tested.

These guidelines come from the test standard EN61010-1, Safety requirements for electrical equipment for measurement, control and laboratory use, April 1993, CENELEC. Over the last decade many of the safety regulations have been harmonized (standardized) and EN61010-1 is like UL 61010A-1 (formerly UL3101-1).

While you are testing cables there are several things you can do to reduce the risk even more:

  • Verify the correct operation of the safety circuits in the equipment every time you calibrate it.
  • Follow all the manufacturer’s instructions and safety guidelines.
  • Don’t touch the cable during hipot testing.
  • Allow the hipot testing to complete before removing the cable.
  • Wear insulating gloves.
  • If you have any health condition that can be aggravated by being startled, then don’t use the equipment.
  • Don’t allow children to use the equipment.
  • If you have any electronic implants, then don’t use the equipment.

What causes current to flow through an insulator?

When you use enough voltage to overcome withstanding voltage, even the best of insulations will allow some current to flow. There are several reasons current will flow through insulation during a hipot test. Resistance, capacitance, arcs, electrochemical effects, and corona are all effects that describe current flow. All of these effects added together during a hipot test shape the results.

Difference between AC hipot and DC hipot testing

1. AC hipot testing

AC hipot test is done by applying a high AC voltage on the test conductor and measuring the leakage current. This test gives a higher leakage current reading because a cable can act as a capacitor. 

AC current flows easily on a capacitor than DC. The test voltage applied on the test conductor is lower compared to DC but more power is required because at 60 hertz, the capacitive reactance is low

2. DC hipot testing

DC hipot testing is done to measure the insulation resistance of cables by applying a high voltage on the test device. The leakage current is measured and computed to get the insulation resistance. 

The drawback of this test is that it can damage old cables during the test. This procedure is only advisable to be conducted on new cables. 

The voltage applied on the test cable is several times higher than AC hipot. This is why DC hipot test is more prone to flash overs and corona discharges.

Also read, Three-phase power generation and distribution

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