Understanding HIPOT and Insulation Resistance

The term "hipot" is an acronym for "high potential." Hipot testing stresses the insulation and capacitors of a filter assembly by applying a voltage much higher than is usually experienced in normal operation. The purpose of hipot specifications is to a assure safety and reliability. All the major safety agencies require hipot testing for qualification of power line filters, and also require that each production unit undergo hipot testing to verify the integrity of the line-to-ground components and insulation. Every CORCOM filter is hipot tested twice; once during assembly and again after completion. Applying hipot testing as an incoming inspection procedure requires a thorough understanding of its uses and limitations. Hipot test voltages are applied from each line (both lines tied together for VDE) to ground and from line-to-line. The line-to-ground voltages are always higher. Test voltages may be specified as AC or DC, with the DC voltages at least 1.414 times the A C voltages. For incoming inspection testing, CORCOM recommends using the voltages given as "hipot rating" for each filter in the catalog. These DC voltages will always be equal to or higher than the peak AC voltage required by any safety agency whose approval the filter carries. A DC hipot test is generally used. A variety of hipot testers are available from a number of manufacturers. The tester shown should have at least a 500VA rating. The following precautions MUST be observed to ensure the safety of the operator and the validity of the test:

1. THESE VOLTAGES CAN BE LETHAL - use the utmost safety precautions to protect the test operator.
2. The possibility of high surge currents and oscillatory overvoltage during sudden application of the test voltage requires some method of limiting the applied current or increasing the voltage comparatively slowly. 
3. For AC hipot tests, use an oscillograph to monitor the applied voltage. The current limiting circuit may react with the filter circuit to distort the 60Hz waveform. This may produce a peak voltage that exceeds the expected peak value of a sinusoidal voltage having the specified rms value. The peak voltage should be 1.414 times the rms value. Higher voltages may cause unwarranted failures due to the peak currents exceeding the trip setting.
4. For line-to-line hipot testing, remember that most filters have a bleeder resistor (typical value 100kohms to 10Mohms) to discharge the line-to-line capacitors. Be sure to set the trip point of the hipot tester above the current level that will flow through the bleeder resistor: 10mA is usually a safe value

In a perfect world, all the electrical current sent along a conductive wire would reach its intended destination. However, in the real world some of it is lost along the way for various reasons. Wires are insulated with a resistant sheathing to contain the conductivity of the typically copper or aluminum core, but even with this insulation in place, some of the current still manages to escape.

Much like a leak in a water pipe, an imperfection in the insulation of a wire allows a steady flow of electricity to escape, which can be detrimental to electrical circuits and machinery. However, testing can help you determine whether the insulation is performing at an effective and safe level. Routine testing can identify problems before they result in injury or equipment failure.

Insulation is subject to many elements that can cause it to perform at a less-than-acceptable level. Excessive heat or cold, moisture, vibration, dirt, oil, and corrosive vapors can all contribute to deterioration. For this reason, routine insulation testing is necessary.

Total current in insulation testing.
Testing the integrity of insulation requires measuring its resistance to current flow across it. A high level of resistance means that very little current is escaping through the insulation. Conversely, a low level of resistance indicates a significant amount of current may be leaking through and along the insulation.
By pressurizing a conductor with a given voltage, it's possible to use Ohm's Law (R=V÷I) to apply a numerical value to resistance measurements. Divide the voltage by the current that escapes through the insulation and returns to the meter. This total current that flows through and along the insulation during a test is the result of capacitive current, absorption current, and leakage current.

Capacitive current.
The initial burst of current that occurs when voltage is first applied to a conductor is called capacitive current. Like the first rush of water flowing through a hose, it typically starts out high and then drops quickly once the conductor is fully charged.

Absorption current.
Like capactive current, absorption current also starts out high and then drops. However, it falls at a much slower rate. As the voltage builds up, the absorption level in the insulation decreases. This gradual change reflects the storage of potential energy in and along the insulation. Incidentally, absorption current is an important part of the time resistance method of insulation testing.

Leakage current.
Also commonly referred to as conduction current, the small, steady current present both through and over the insulation is called leakage current. Any increase in leakage current over time is usually an indication of deteriorating insulation. This would be noted on the insulation test meter as a decrease in resistance.

Types of insulation resistance tests.
With an understanding of the definition of insulation resistance and why it's important to measure it, it's possible to examine when and how to test.

When installing new electrical machinery or equipment, testing insulation resistance is important for two reasons. First, it ensures that the insulation is in adequate condition to begin operation. This type of initial test is usually referred to as a proof test. Second, it provides a baseline reading to use as a reference for future testing.
Due to fluctuating factors like moisture and temperature, insulation testing is mostly based on relative measurements. In other words, a reading of 1.5 megohms is more or less insignificant without a previous set of measurements against which to compare it. Measurements taken during routine maintenance tests can give valuable information about the quality of insulation, as conditions vary.
The proof test, short time/spot reading test, time resistance test, and step voltage test are four of the most prominent tests used today, and they encompass the steps necessary for keeping tabs on equipment from installation through day-to-day use.

Proof test
Proof testing is an important step in the installation of new machinery to protect against miswired and defective equipment. A proof test is often referred to as a go/no go test because it tests the system for errors or incorrect installation. The test is accomplished by applying DC voltage through the de-energized circuit using an insulation tester. If no failures occur during the measurement, the test is a success. Proof testing voltages are much higher than those used in routine maintenance test methods. The general guideline for choosing a test voltage is based on the equipment's nameplate rating. Follow the equation below to arrive at an acceptable test voltage.

Step 1:  2×nameplate rating)+1,000V=Factory AC Test
Step 2:  0.8×Factory AC Test×1.6=DC Proof Test Voltage

Short time/spot reading test
In a short time/spot reading test, the tester is connected across the insulation of the motor windings. A test voltage is then applied for a fixed period of time, usually 60 seconds. The most important aspect of this test is that it remains consistent in duration from test to test. Once the time period has elapsed, an insulation resistance measurement can be recorded.

As discussed earlier, a single maintenance test can act only as a rough guide for insulation quality. A more effective use of the short time/spot reading testing method is the establishment of a series of test results over several months so long-term trends may be examined. It's important to understand that a variety of factors like temperature and moisture can cause fluctuations in test readings. Typically, insulation will deteriorate at an extremely gradual, but consistent pace. A significant downward trend over the course of several measurements is usually a sign of insulation breakdown.

Time resistance test.
Unlike the short time/spot reading test, the time resistance method test can provide fairly conclusive results without the luxury of past test measurements. This test method is based on taking successive readings at fixed time intervals, and then plotting the readings. This is an especially effective method when moisture and other contaminants might be present.
As mentioned earlier, absorption current starts out high and gradually decreases over time as voltage is applied. In a machine with healthy insulation, this trend will continue for several minutes and show an increasing level of resistance. On the other hand, if the insulation is poor, the level of resistance will flatten out after an initial burst (Fig. 1).

The best way to quantify the results of a time resistance test is through a dielectric absorption ratio. The dielectric absorption ratio consists of two time resistance readings. A commonly used set of intervals is a 60-second reading divided by a 30-second reading. Another frequently used set is a 10-minute reading divided by a 1-minute reading. This resulting value is referred to as the polarization index. The information summarized in Table 1 above provides general guidelines for interpreting dielectric absorption ratios.

Step voltage test.
A step voltage test involves testing the insulation at two or more voltages and comparing the results. Good insulation will show a relatively consistent resistance reading regardless of the voltage applied. On the other hand, when the resistance level drops as the voltage level increases, it's usually an indication that the insulation is aging, contaminated, or brittle. This occurs because small imperfections like pinholes and cracks reveal themselves under increased electrical stress. When performing a step voltage test, it's important that you start with the lowest test voltage and then move to a higher voltage level. Test duration is typically 60 seconds.

Preparing for the actual test.
Correctly preparing the equipment and insulation tester is crucial to your safety and the well-being of your wiring and machinery. Adhere to the following four-step process before every test:

  • Take the equipment out of service

Shut down the apparatus, open all switches, and de-energize the unit. Disconnect the equipment under test from all other equipment and circuits, including neutral and protective ground connections. Make sure you follow proper lock-out/tag-out procedures during this step.

  • Check what will be included in the test

The more equipment included in a test, the lower the resistance reading. For this reason, it's very important to inspect the installation and understand exactly what you're including in the test. You don't want a true reading to be affected by additional equipment. However, if a complete installation with several pieces of equipment yields a high reading, it's safe to assume that each individual apparatus will yield an even higher reading. Consequently, sometimes separating components is unnecessary.

  • Discharge capacitance

It's important to discharge capacitance before and after making an insulation resistance test. You should discharge about four times as long as the test voltage was applied during the test.

  • Check current leakage at switches

Make sure readings won't be affected by leakage over and through switches, fuse boxes, or other connections. Such leakage can be detected by watching the level of resistance the moment the test leads are attached. Never perform an insulation test on an energized line or apparatus.

Interpreting test results.
Deciding what to do with the results of an insulation test can often be more complicated than actually conducting the test itself. Every piece of equipment has a general insulation "personality." In other words, no two pieces of equipment may operate exactly the same, but if a machine is behaving in accordance with its normal tendencies, there's usually no cause for concern. However, a safe rule of thumb is to judge results against a 1 megaohm per 1,000V ratio. Use the information shown in Table 2 as a guideline for what to do with the various conditions you may discover during your testing.
It's extremely important that you consult the motor manufacturer's operating handbook for specific information and guidance as to whether a particular value measured between two points should be considered acceptable or questionable. Insulation testing manufacturers can provide test equipment capable of providing you with accurate readings, but they have no way to determine if a particular measured value indicates that a piece of equipment meets its specifications for insulation integrity.

What is "hipot" testing?

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 to 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.

What kinds of high voltage tests are there?

There are three common high voltage tests.

Dielectric Breakdown Test
Dielectric Withstanding Test
Insulation Resistance Test

What is "dielectric breakdown testing?"

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.

What is "dielectric withstand testing" (DW)?

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.

What is "insulation resistance testing" (IR)?

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).

How do these "hipot" tests affect quality?

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:

AC Hipot Testing
Guidelines for using Voltage to Detect Insulation Defects
High Voltage arc distance.

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

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 shape the outcome of the test.

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