From Testing Equipment Searching for ‘Electrical safety tester,’ ‘dielectric withstand tester,’ or ‘Hipot tester’ on the Internet, you will quickly find a number of instruments. All of them perform the same task — applying voltage to a product or component and checking for current flow. However, their prices vary over a wide range – from $800 to $10,000. How do you choose which instrument best meets your testing needs? That involves a little more research. In this article, we discuss the features of electrical safety testers, and identify the conditions under which these various features are most beneficial.
AC, DC or both? Should you look for a tester which applies alternating current, direct current, or for one that can do both? A good place to start is to review the standard covering your product and see what it requires. Dielectric strength tests can be performed using AC or DC voltages. Test specifications can call for AC or DC testing or both, depending on the product. AC testing is specified more often 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, but there are pros and cons associated with each method.
AC testing has some advantages over DC testing as it stresses the insulation equally in both polarities whereas DC testing is only unipolar (unless repeated in opposite polarity). There is no charge time associated with AC testing, so it is not
necessary to discharge the product after testing. However, AC tests can have a problem if the device under test is capacitive. The reactive current (which passes through the capacitance) is typically much higher than the real (or resistive) current, with the result that the measured leakage can be primarily due to the reactive current. This may swamp the real current component, making changes in real current difficult to detect. If an AC test is required and the device under test (DUT) will be tested with capacitors in place, ensure that the tester you choose has high output current capacity.
If AC testing is not required, performing a DC test may be a better option from the perspective of operator safety because lower currents will be involved. Performing a DC test may take longer because any device capacitance needs to charge, but once it is fully charged, the current measured is true leakage. This provides a more realistic indication of the quality of a product’s insulation.
DC testing is also used in non-destructive testing of devices for breakdown prediction. This is done by raising the test voltage in small increments and waiting for any charging current to settle after each step. If the current suddenly rises, a breakdown may be on its way. By stopping the test at this point, destruction can be prevented. This method doesn’t measure the exact breakdown point, but lets you come close while preserving the device under test.
Digital or analog? An important product selection issue is the choice between a digital and an analog instrument. Each has its advantages and disadvantages. Digital testers lead in programmability, data logging, automated test set-up, reduction of operator error, and voltage regulation; analog testers are simpler to use, offer the virtues of manual “dial-in” operation, and good destructive “burn-in” test capability. Let’s explore these issues in more detail.
Digital instrumentation lends itself to many automated features which can improve productivity and operator safety. Digital displays usually have better resolution, repeatability, and accuracy than their analog counterparts. Automation makes it easy to incorporate programmable test limits, keypad lockouts, program memory, and test current limits (newer analog testers allow this as well). Multiple tests can be stored in memory, and the panel locked out once a desired test condition has been set. This improves accuracy by preventing the operator from inadvertently changing the test parameters.
While protection against operator error is one major advantage of digital testers over analog that is readily apparent, there is another that is less evident: line/load regulation. Line/load regulation ensures that the output of the tester remains constant regardless of the input voltage to the tester (line regulation), or the load attached to the output of the tester (load regulation). Figure 1 shows how the voltage being applied to the device under test (DUT) can vary based on the line voltage or test load. Most analog hipot testers have an unregulated output. In the example of Figure 1, if the tester is set to output 1000V at normal line voltage, the DUT may see a considerably different voltage in the presence of main power fluctuations. The second portion of the figure shows the effect of load resistance on the output voltage in the absence of load regulation.
Figure 1: Line Load Regulation Effect on the Output Signal
One of the greatest changes in electrical safety testing is the movement towards automation of data logging, as the need for historical records becomes more the norm than the exception. Electrical equipment manufacturers recognize their responsibility to ship safe products and the need to maintain records backing up their efforts. These records are valuable in minimizing legal liabilities, and in identifying performance changes to track down process and production problems.
The advantages of digital testers relative to analog testers could fill an article by themselves, but in some areas the advantage goes the other way. Most analog testers have burn capability. This feature can be useful, particularly in an R&D setting. Digital hipot testers will trip and fail if the leakage exceeds the programmed limit. Analog testers have the ability to maintain power following a failure, making it possible to perform destructive testing. Older analog testers allow for you to “dial” in the voltage. In an R&D environment turning the voltage up until the insulation breaks down is sometimes necessary to know exactly what voltage causes the product to fail.
The Virtues and Uses of Arc Detection Arc detection—do you need it? The best method of arc detection has been argued over by both proponents of both types of units. Some analog users like to hear the arc, and in fact, there are testers out there that come with a loudspeaker so the sound of the arc is amplified. Others are thankful they no longer need to listen and make the call on when breakdown occurs, and can instead use digital technology to detect it and make a go/no go decision.
Safety testers apply high voltage and monitor the current flowing through the insulation. This current is steady state current. When breakdown occurs, there will also be higher frequency current components. The zapping sound heard when breakdown occurs during high voltage testing is often referred to as ‘arc’, in reference to the obvious sparking that occurs in cases of overt breakdown. The current from an arc is not measured and displayed but a limit can be set on most digital hipot testers to indicate a failure if the arc exceeds a certain level. When an electrical arc occurs, there will be very rapid variation in load voltage and current (See Figure 2). The tester consistently monitors the flow and amplitude of current to the DUT. If there is a discrepancy in flow greater than, say, 10 microseconds at a specified level, the tester interprets the change as an arc and will shut down the tester.
Figure 2: Voltage wave during arc
Arc detection is not required by most standards; however it may be a good indication that the creepage or clearance distances are not sufficient for the product being tested. Arc detection is also useful when performing DC hipot testing in an R&D environment. With DC testing, the voltage is always at its maximum level. Under extreme arcing, the steady state current level will eventually reach the maximum capability of the tester, resulting in a test failure whether or not arc detection is used. However, without arc detection there will be no quantitative measurement of the steady state level at which the arc occurs. With arc detection the test will shut down just prior to the steady state failure, which will allow for you to read the steady state current just prior to failure. To benefit from arc detection the user should understand the product they are testing and the implications of an arc in that product.
Real and Reactive Current Real current is another useful item which is not specified in safety standards, but can provide the user with helpful information. As we mentioned in our discussion of AC versus DC testing, one of the potential downsides of AC testing is that the reactive current can mask the real current when testing a capacitive device. A significant change in real current may go undetected in the presence of a larger magntitude reactive current. Figure 3 shows a situation where a 100% increase in real current only causes a very small 1 % increase in total current when the product has a highly reactive component. Since the reactive and real components are 90 degrees out of phase, the total current is the “hypotenuse” of the two, as shown in the figure. As a concrete numerical example: If the real current is 10% of the reactive current, doubling it (to 20% of the reactive component) will only change the total current by 1.47%
Figure 3: Reactive Current Masking Real Current
Monitoring real current in addition to total current allows for tighter control over the process and may give early indication that something has changed. There are a number of the testers in the market which have the ability to measure and monitor both Total and Real current. Some testers refer to the real current measurement as “Capacitance Compensation.”
Some Specific Safety Standard Requirements and Their Impact on Instrument Selection Clearly, the tester chosen needs to be able to perform the required tests, and it needs to meet all requirements imposed by the test standard under which it will be used to assess compliance. For example, most Underwriters Laboratory standards define specific requirements for the test equipment used to perform Dielectric Voltage-Withstand tests, such as:
In some cases, a higher powered tester capable of 500 volt-amperes output is needed. These instruments, available from a number of manufacturers, are something of a specialty item. They are commonly referred to as 500VA units because they output up to 5000 VAC and have the ability to deliver and measure up to 100mA of current. As well as meeting the requirements of some specialized safety standards, these higher powered instruments can allow testing of more reactive loads than instruments of lower current capability.
Control of voltage ramp-up is mandated by some standards. UL 2601-1 and its successor standard IEC/UL 60601-1, which address medical equipment, not only specify the test voltage, but also the manner in which it is applied. “Initially, not more than half the prescribed voltage shall be applied, then it shall be gradually raised over a period of 10s to the full value, which shall be maintained for 1 min, after which it shall be gradually lowered over a period of 10s to less than half the full value.”[2] For tests motivated by these standards, one would want to ensure that the tester purchased has the ability to program or adjust the ramp and fall times as well as the duration of the applied test voltage.
In reviewing safety standards one may find supplementary electrical tests that need to be performed in addition to dielectric withstand. Tests such as Ground Bond, Ground Continuity, Insulation Resistance and Leakage Current tests are required for most products. Some standards may require these tests for production line testing in addition to initial product qualification. Consider whether your needs are best served by the purchase of individual or multi-function instruments.
Ground Continuity, very common in production testing, is performed using a low level DC current source, typically capable of less than 1 Ampere, 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 testing 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 pass. 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 (milliohms), is measured using a Kelvin (4-wire) connection. Not only does ground bond testing verify the continuity but it verifies the integrity of the circuit as well as its ability to carry high current. There are a number of testers which combine hipot and ground bond. Many test manufacturers create a combo pack where the hipot tester and ground bond tester are interconnected. Once the ground bond test passes, the hipot will start automatically. Other testers combine both tests into a single piece of test equipment.
Insulation resistance measurements are similar to DC hipot tests except that the measurement reports the 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. If you are choosing an insulation resistance tester, check the instrument’s measurement range. Most hipot testers can measure up to 50GW (gigaohms), but some standards may call for a higher resistance than that. If this is the case then a megohmmeter may be necessary to perform the measurement.
Product safety standards specify several types of leakage current tests. 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 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 which simulates the human body, known as a human body model. Line leakage tests are typically design qualification tests on non-medical devices and a production test requirement on medical devices.
Other Considerations: Multi-point Units and Safety Features In addition to multi-functional units described above, there are also there are multi-point testers. Multi-point testers have scanning capability to test multiple parts at once or multiple points on a product. For example, a typical safety test for power line filters requires a hipot test of all line connections to ground in addition to a hipot test line to line. On a simple filter this could add up to six different tests—where moving test leads to different connections will be very time consuming. A hipot with scanning capability allows the user to connect to the tester once and with one push of the start button perform all the necessary tests. Some testers have connection scanners built in whereas others have external scanners that attach to the hipot tester. There are scanners available for ground bond testing as well. Most scanners have eight channels and can be expanded further with additional sets of eight. If a high channel count is necessary then a cable tester may be another option. A cable tester is a commonly overlooked product when one thinks of hipot tests. However cable testers not only give the user flexibility with high point (channel) counts, they also have built in hipot capability.
Safety is of the utmost importance. High voltages and dangerous currents can be output from these testers, so safety precautions should be put in place to prevent the risk of injury to the user. Proper training will reduce risk, but accidents still happen. Consequently, electrical hipot equipment manufacturers have implemented additional safety features to minimize the likelihood of user injury. Look for features such as interlock, GFI (ground fault interrupt), fast DUT discharge, and quick HV shutdown.
Interlocks are a mechanical method of stopping the tester if it is opened. Interlocks may be connected to a light curtain located around the tester. If the user interrupts the light beam the tester will shut down. Palm switches are another common form of interlock, which force the user to keep their hands on the switch and away from the tester while it is active. If the operator’s hand leaves the switches then the tester is depowered.
In Europe special cautions are common and required per EN50950 if the tester can deliver more than 3mA of current. Some hipot testers limit their current to 3mA to meet this standards requirement. If an interlock is used, 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.
Sometimes an interlock is not practical. Additional safeguards are now being found in the newer digital 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. These work like the GFI interrupter outlets seen in many homes. If the current flowing out of the tester does not equate to the current flowing back into the tester then the tester shuts down. It is worth noting this safety feature will only work if the device being tested is not grounded.
Most testers will automatically discharge at the end of test. This is especially important for DC testing. Automatic discharge time will vary from tester to tester; precautions should be implemented to ensure the device under test is discharged. If testing a highly capacitive device you may want to look at the discharge times of the available testers, as a quick discharge may save you a significant amount of time.
Summary Hipot testers typically fall in four categories: low-end, midrange, high-end, and specialty testers. The low-end testers tend to be stand alone units with no computer interface capability. Midrange testers add in the capability to communicate with a PC via RS-232 or IEEE, many also have the ability to send the data to a printer connected directly to the tester. The high-end units are the 5 in one or 6 in testers. These incorporate all of the features of the midrange units. Specialty units meet unique application requirements where high voltage (>6kV) and or higher current is required.
After acquiring the instrument, you may need to integrate it into your data acquisition system. Test equipment manufacturers are beginning to produce software packages for their products as customer, although many companies still choose to roll their own. Canned programs allows for easy implementation, while customized software allows for flexibility. If you are going to write your own software, check to see if the manufacturer provides software drivers to make the job easier. National Instruments’ Labview is a popular test system integration language that a number of manufacturers provide drivers for. g
References 1. UL 1004; Fifth Edition; Standard for Electric Motors; section 28.6
2. UL 60601-1, First Edition, 2003, “Medical Electrical Equipment, Part 1: General Requirements for Safety”, section 20.4
About the Author Shari Richardson is the product line manager of Electrical Safety Testers at QuadTech Inc. (Maynard, MA). Previous to her position at QuadTech she was a Senior Process engineer at a high volume manufacturer. Shari can be reached at srichardson@quadtech.com. © Copyright 2007 Conformity |
