Hi-Pot Testing and Definition

   
 

Xmultiple's Engineering Department


Hipot is an abbreviation for high potential. Traditionally, Hipot is a term given to a class of electrical safety testing instruments used to verify electrical insulation in finished appliances, cables or other wired assemblies, printed circuit boards, electric motors, and transformers.

Under normal conditions, any electrical device will produce a minimal amount of leakage current due to the voltages and internal capacitance present within the product. Yet due to design flaws or other factors, the insulation in a product can break down, resulting in excessive leakage current flow. This failure condition can cause shock or death to anyone that comes into contact with the faulty product.

A Hipot test (also called a Dielectric Withstand test) verifies that the insulation of a product or component is sufficient to protect the operator from electrical shock. In a typical Hipot test, high voltage is applied between a product's current-carrying conductors and its metallic chassis. The resulting current that flows through the insulation, known as leakage current, is monitored by the hipot tester. The theory behind the test is that if a deliberate over-application of test voltage does not cause the insulation to break down, the product will be safe to use under normal operating conditions¡Xhence the name, Dielectric Withstand test.

In addition to over-stressing the insulation, the test can also be performed to detect material and workmanship defects, most importantly small gap spacings between current-carrying conductors and earth ground. When a product is operated under normal conditions, environmental factors such as humidity, dirt, vibration, shock and contaminants can close these small gaps and allow current to flow. This condition can create a shock hazard if the defects are not corrected at the factory. No other test can uncover this type of defect as well as the Dielectric Withstand test.

Three types of Hipot tests are commonly used. These three tests differ in the amount of voltage applied and the amount (or nature) of acceptable current flow:

Dielectric breakdown Test. The test voltage is increased until the dielectric fails, or breaks down, allowing too much current to flow. The dielectric is often destroyed by this test so this test is used on a random sample basis. This test allows designers to estimate the breakdown voltage of a product's design.

Dielectric Withstand Test. A standard test voltage is applied (below the established Breakdown Voltage) and the resulting leakage current is monitored. The leakage current must be below a preset limit or the test is considered to have failed. This test is non-destructive and is usually required by safety agencies to be performed as a 100% production line test on all products before they leave the factory.

Insulation Resistance Test. This test is used to provide a quantifiable resistance value for all of a product's insulation. The test voltage is applied in the same fashion as a standard Hipot test, but is specified to be Direct Current (DC). The voltage and measured current value are used to calculate the resistance of the insulation.

Hipot Tester

A hipot tester is an electronic device used to verify the electrical insulation in a cable, printed circuit board, electric motor, transformer or other wired assembly. A Hipot tester is used to perform a high potential test. Generally a hipot tester consists of:

A source of high voltage,
A current meter,
A switching matrix used to connect the high voltage source and the current meter to all of the contact points in a cable.


In addition to these parts a hipot tester may also have a microcontroller and a display to automate the testing process and display the testing results. A hipot tester can be very similar to a cable tester and often the two are combined into a single device. A hipot tester is used to verify that circuits that should be insulated are well isolated. It does this by applying a high voltage between the circuits and making sure no current flows.

In a typical wired assembly a hipot test should connect all circuits in common to ground. Then, one by one the tester will disconnect a given circuit from ground and connect that circuit to high voltage. The current that flows is monitored to verify that it is low enough.

Hipot Test Procedures

Products being designed today usually must comply with product safety regulations. Some of these regulations work to reduce the chance of you receiving a harmful electrical shock. Modern equipment is more likely to follow these regulations. When it comes to hipot charge, energy, and voltage you should select the "safest" machine that will still test your cables.

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 can not 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 similar to 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 of 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.

During Test Where To Apply High Voltage?

To understand a how hipot testing works you'll need to understand where to connect the high voltage supply. Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply is connected to the conductor being hipoted. 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.

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 of 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, 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.


Where is the current measured?

During the hipot test the current that flows out of the high voltage supply is measured.

Xmultiple HiPot test procedure includes testing the current at 2 mA amd 5 mA for connectors and parts requiring this testing.

There are 2 sets of coils (Transmit & Receive) wich are tested. We test both coils together.


What causes current to flow through an insulator?

Insulation "does not conduct." But if you use enough voltage even the best of insulations will allow some current to flow. You may wonder why the current flows? 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 add together during a hipot test to shape the outcome of the test.


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