Many people are familiar with a continuity test. A continuity test checks for "good connections." You do a continuity test by seeing if current will flow from one point to another point. If current flows easily enough then the points are connected. Many people aren't familiar with a hipot test. "Hipot" is short for high potential (high voltage). A hipot test checks for "good isolation." You do a hipot test by making sure no current will flow from one point to another point. In some ways a hipot test is the opposite of a continuity test.
Continuity Test: "Make sure current flows easily from one point to another point."
Hipot Test: "Make sure current won't flow from one point to another point (and turn up the voltage really high just make sure no current will flow)."
In the simple case a hipot test takes two conductors that should be isolated and applies a very high voltage between the conductors. The current that flows is watched carefully. Ideally not much current will flow. If too much current flows the points are not well isolated and they should fail the test.
Why high voltage test
You use a hipot test to make sure you have good isolation between the parts of a circuit. Having good isolation helps to guarantee the safety and quality of electrical circuits. Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in IDC cables. All of these conditions might cause a device to fail.
There are three common high voltage tests.
· Dielectric Breakdown Test
· Dielectric Withstanding Test
· Insulation Resistance Test
With dielectric breakdown testing you are trying to answer the question "How much voltage can I apply between the wires before the insulation fails?" You increase the voltage until the current suddenly increases. You are finding the highest voltage the cable can stand before it fails. Once the cable fails it is usually damaged or destroyed.
In dielectric withstand testing you are trying to answer the question "Will this cable withstand a required voltage for a required time?" You apply the voltage for the amount of time and watch the current that flows. Ideally no current flows and the cable is not harmed.
In insulation resistance testing you are trying to answer the question "Is the resistance of the insulation high enough?" You apply a voltage and very carefully measure the current. You then calculate the insulation resistance using Ohm's Law (R = V/I).
All of these tests are tools you can use to better 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 done at about 75% of the typical breakdown voltage. It 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. It is usually done at 300 to 500 Vdc with 100 to 500 Megahoms resistance. The test is a very sensitive to contamination in the assembly process. Solder flux, oils, mold release agents, and skin oil all 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.
Additional High Voltage Testing Resources:
During a hipot test you may be at some risk. The risk can be reduced by using a tester designed to be safe and by using that tester according the manufacturer's instructions.
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.
Where is the high voltage applied
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.
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.