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I remember vividly standing at the Northeast Product Safety Show, overhearing two gentlemen discussing how high tech the new hipot testers are. One commented that all his “legacy” hipot testers worked with only a variable transformer and a voltmeter. Yes, hipot testers have come a long way.

Today’s testers provide more function than the single high voltage AC or DC test. The features within a new hipot tester have been added due to demand from the industry and most importantly safety. This article will discuss the use of the features that have brought the hipot tester into the 21st century, and which features to pay attention to when buying one.

Hipot tests are performed in production to meet safety standard requirements. Each product has a test standard whose specifications must be met. The first step in selecting a tester is to determine the primary as well as the supplementary test standards. Tests such as Ground Bond, Ground Continuity, Insulation Resistance, and Leakage Current are required for most product safety compliance. Some standards may require these tests for production also. If your company decides these are tests you want to perform, look for test equipment that can accomplish all of the tests in one instrument.

The Tests
Dielectric withstand, high potential voltage breakdown, or hipot tests stress the insulation of a product beyond what it may encounter in normal use. This test is performed using AC or DC voltage. The safety standard can call for AC or DC testing or both, depending on the specific product type. AC is often specified more than DC and can be more stressful. For example, power line consumer products are much more likely to experience AC voltage transients than DC transients. Typically a product powered by AC is tested with AC, and a product powered by DC is tested with DC.

AC testing has some advantages over DC testing as it stresses the insulation equally in both polarities, whereas DC testing stresses the insulation in single polarity only. There is not a charge time associated with AC and it is not necessary to discharge the product after testing. However, AC can have a downfall if the device under test is capacitive. The reactive current (caused by the capacitance) is typically much higher than the real current, resulting in the total leakage current to be comprised mostly of reactive current. An increase in real current may go undetected due to the magnitude of the reactive current. If an AC test is required and the DUT has capacitors, ensure that the tester you choose has high output current capacity.

Performing a DC test may be a better option than running the device at high unsafe currents. Performing a DC test may take longer because the device needs to charge, but once fully charged, the leakage is true leakage. This provides a more realistic indication of the quality of the insulation of the product. DC is also used in non-destructive testing of devices for predicting when a breakdown occurs. This is done by raising the test voltage in small increments and waiting for the charge current to reduce after each step. If the current suddenly rises, a breakdown may be on its way. By stopping the test at this point, destruction of the material can be prevented. However it does not tell where the breakdown would have occurred.

Ground Continuity, very common in production testing, is performed using a low DC current source, typically less than 1 Amp, between the ground blade on the power cord and any exposed metal on the product. Most hipot testers come standard with a ground continuity measuring circuit.

Ground Bond is not always required for production testing but some manufacturers choose this test over Ground Continuity because it may detect a problem in the ground circuit that the continuity test may not. The ground bond test applies high current, usually 25 – 40 Amps, to the same ground path as the continuity test. The resistance of the ground path, normally less than 100 mW is measured using a Kelvin connection. Not only does ground bond verify the continuity but it verifies the integrity of the circuit and its ability to carry high current.

Figure 1: Ground fault interrupt

There are a number of testers that combine hipot and ground bond. Many test manufacturers create a combo pack in which the hipot tester and ground bond tester are connected; one connection to the device under test is required, and the test is initiated at the ground bond tester. Once the ground bond test passes, the hipot will start automatically. Other testers combine both tests in one piece of test equipment. Whether you purchase a combo pack or an all-in-one unit, ensure that the tester can handle the current output and resistance measurement required.

An Insulation Resistance measurement is similar to a DC hipot test except that it measures the total resistance between any two points separated by electrical insulation rather than the leakage current between these points. Insulation resistance tests are typically performed at a lower voltage than DC hipot tests. When choosing a tester to perform Insulation resistance measurements, check the measurement range of the tester. Most hipot testers can measure up to 50GW, but sometimes standards may call for a higher resistance than the tester can handle. If this is the case then a megohmmeter may be necessary.

There are multiple leakage current tests that can be specified by product safety standards. The most common is the Line (Earth) Leakage Current test. The line leakage test measures current flow from the AC line source through the ground path of the product while under normal operating conditions. Variations of this test are Patient Applied Part Leakage, Patient Auxiliary Leakage, and Touch/Chassis (Enclosure) Leakage. All of these tests are performed by powering the product and measuring the leakage through a circuit that simulates the human body, known as a human body model. Line leakage tests are typically a design test on non-medical devices and a production test on medical devices.

A very similar test requirement is Power Consumption, common in the IT and appliance industries. Just as it sounds, power consumption measures the power being used while the product is running and while it is in standby mode. With all the personal (and commercial) electronic devices in use today, standby mode power consumption is a concern. Limits for standby power consumption of household electrical appliances are addressed in IEC 62301.

Higher-end hipot testers perform most or all of the line leakage tests as well as Ground Bond, Hipot and Insulation Resistance, and are often referred to as 5-in-1 or 6-in-1 testers. If purchasing a multi-functional tester, ensure the tester can handle the current needed to power the product. Some products such as fans, blenders and larger medical systems have a large in-rush current. Check with the manufacturer that their product can handle the load. Isolation transformers are frequently required by the safety standard, and by the test equipment manufacturer. Isolation transformers isolate the power to the product being tested so that an accurate leakage measurement can be made. Be certain the isolation transformer can also handle the power requirements of the product.

Features Within a Hipot Tester
Aside from the older analog testers, any Hipot tester on the market should have a digital display, not only for ease of use but to reduce operator error (by misreading an analog dial). One reason for the “high tech” Hipot is the rarity of skilled operators. The demand increases for the test equipment to incorporate safety features that prevent improper testing. Programmable test limits, keypad lockout and program memory are necessary features. New testers allow for multiple tests (with different limits) to be stored in memory. Once the test conditions have been stored, the panel can be locked out, another safeguard against incorrect parameter testing.

Safeguarding the product and test data is important, however safeguarding the operator is of the utmost importance. High voltage and current are output from these testers; therefore safety precautions should be put in place to prevent the risk of injury. A properly trained operator will reduce risk; however accidents do happen so Hipot manufacturers have implemented additional safety features. Look for features such as interlock, GFI, fast DUT discharge, and quick HV shutdown.

Interlocks are usually a mechanical method of stopping the tester if open. Interlocks may be connected to a light curtain located around the tester. If someone interrupts the light beam, the tester will shut down. Palm switches are another common form of interlock, which forces the user to keep his hands on the switches and away from the tester at all times. When the users hands are removed from the switches, the tester stops. Extreme forms of interlock have been used such as placing the tester in a room, once the door is shut the tester can run, if the door opens the tester will stop. In Europe these extremes are common and required per EN50950 if the tester can output more than 3mA of current. Some hipot testers limit their current to 3mA to meet this standards requirement. If using an interlock, the tester should have a quick shutdown. There is no purpose in installing an elaborate interlock system if the tester does not shut down quickly.

Additional safeguards are being implemented in the newer testers. GFI, Ground Fault Interrupt can now be found in most new testers. There are different variations of GFI on the market for hipot testers, but they generally serve the same function. If the current flowing out of the tester (iout) does not equate to the current flowing back into the tester (iin) then the tester shuts down. It is worth noting that this safety feature will work only if the device being tested is not grounded. If the device being tested is grounded, then the current will flow to ground and not through the user, for current flows through the path of least resistance.

Most testers will automatically discharge at the end of test. This is important for DC and IR testing. Automatic discharge time will vary from tester to tester; precautions should be implemented to ensure the device is discharged. If testing a highly capacitive device you may want to focus on the discharge times of the available testers. There may be significant time savings involved based on the tester you choose.

Once safety has been addressed, manufacturers are interested in reducing test time. Standards play a role in the duration of the test; however, the time taken connecting to the product, executing the test and recording the data adds additional overhead that can be eliminated. As mentioned earlier, a tester that combines multiple tests in one instrument is an advantage, not just because of only having one instrument but also one connection. For instance, let’s take the three most common tests: Ground Bond, Hipot, Line Leakage or Functional Run. A manufacturer can run these tests with three individual instruments or with one. Ground Bond requires connection to the ground plug and to the casing of the product. Hipot will then require connection to the ground plug and to line and neutral. Line leakage testing is performed with ground plug and line and neutral separated. Figure 2 shows connection to a 5 in 1 tester which will allow for Ground Bond, Hipot, Insulation Resistance, Functional and Line leakage testing. Item A and B in Figure 2 are used for Ground Bond. Item B connects to the ground connection of the adapter box. Hipot is performed using Item C and B. High voltage is output from both Line and Neutral of Figure 2 Item C, and the insulation between these points and ground is tested. Because Line and Neutral are separate connections, Functional Test and Line leakage are tested with the same connection.

Figure 2: Ground bond, hipot and line leakage connection

Combining different tests in one unit is a time savings; one must also consider the points which need to be tested within the device. When a device has several points to be hipot tested, a scanner speeds up the testing process and reduces human error. For example, a typical safety test for a power line filter requires a Hipot test of all line connections to ground in addition to Hipot from line to line. This test can become time consuming when moving test leads. A scanner will allow connections to be made once, and the hipot tester will scan through each connection for the proper voltage and time. Table 1 shows the hipot test sequence for a filter. If one test point takes 60 seconds, the whole test will take 6 minutes. Using a scanner allows the operator to multitask during the 6 minutes rather than having to change test leads every minute.

Figure 3: Electrical schematic of 3 phase filter

Table 1: Hipot Test Sequence for a Filter

Figure 4: Hipot connection for 3 phase filter using a scanner

Time can also be saved by digitizing data. More and more manufactures are moving away from the pass/fail hipot test to actual data that provides historical records to verify their shipping safe product and to enhance process control. Newer testers digitize data directly to a PC. The testers follow customized command protocols to allow full control over the tester and its results. Software or sample programs as well as Labview Drivers are generally available for most testers on the market today. When selecting a tester, review what software is available for the tester. There is no reason to recreate in your own shop what is available already.

Yes, Hipot testers have changed through the years. The move from analog to digital has allowed for safer testers with more accurate readings. Newer testers have incorporated a number of features to allow for easy execution of Hipot testing. To choose the correct Hipot tester, examine the standard and required tests. Review the products to be tested and consider future requirements to ensure the tester can grow with the requirements.

Shari Richardson is the product line manager of Electrical Safety Testers at QuadTech, Inc., and can be reached at srichardson@quadtech.com.