From Testing Equipment Before purchasing a hipot tester, consider the standard you are required to test to, which tests are required to ensure the safety of your product, and the test data necessary to prove compliance. Today’s hipot testers provide more functions than the single high voltage AC or DC test. Whether in production or laboratory applications, single and multi-function hipot testers can provide the solution to safety testing requirements. This article defines the tests associated with hipot, details the many features of a tester, and provides a check list to help the reader narrow down requirements.
A Note on Standards Many years ago, Underwriters Laboratories (UL) took a strong consumer position in adopting proposals for production line testing of appliances. A standardization of testers gradually evolved, so that two different testers would yield the same go/no-go results. Just about any appliance or electrical product is governed by one or more standards (from either UL or an international standards organization), requiring it to be tested before it can exit the production line.
The rule of thumb found in many of these agency requirements is to apply a test voltage two times the normal operating voltage, plus 1000V. This is usually a sinusoidal AC voltage, but in some cases a DC voltage test can be substituted. For finished appliances with hard-wired power cords, the test voltage is applied between the high (hot) and neutral conductors shorted together, and power ground or exposed metal parts.
There are many standards that govern electrical safety testing (EST). Most are specific to the device under test, some to the application of the device under test. For example, appliances usually refer to UL 982, 1010 and 1082. Information technology equipment (ITE) is usually tested to the specifications found in UL60950 and IEC 950. For electrical equipment for laboratory use, look to UL 61010 and CSA C22.2 No. 1010, while IEC 60601-1 is typically used for electrical medical devices.
The standard to which your product must be tested also depends upon where it is sold. Some countries recognize international harmonized standards like IEC 60601-1 that include national requirements, some do not. So the first order of business is to define the standard you are required to test to and then the tests you are required to perform within that standard.
The Tests Hipot – High Potential – Voltage Breakdown – Dielectric Withstand An electrical stress test is performed on a product’s insulation beyond what it might encounter in normal use. Hipot tests are usually required for 100% of electrical products in a production line.
AC Hipot In an AC hipot test, high AC voltage is applied between the operating circuits and chassis ground of the device under test (DUT) to determine if/when a breakdown will occur in the insulation of the DUT. The current is measured between the DUT insulation and ground. A high limit is required in an AC test. The high limit is the maximum allowed value for the test to be considered a PASS. If the measured value is higher than the high limit, the test is considered a FAIL.
DC Hipot In a DC hipot test, high DC voltage is applied between the DUT’s operating circuits and chassis ground to determine if/when a breakdown will occur in the insulation of the DUT. The current is measured between the DUT insulation and ground. A high limit is required in a DC test. The high limit is the maximum allowed value for the test to be considered a PASS. If the measured value is higher than the high limit, the test is considered a FAIL.
Insulation Resistance Insulation resistance (IR) is often included as a function of a hipot tester. The measurement of IR can pinpoint where a product’s insulation will breakdown. In an IR test, high DC voltage is applied to the DUT to determine strength of its insulation to the flow of current. The resistance is measured between the DUT insulation and ground. A low limit is required in an IR test. The low limit is the minimum allowable value for the test to be considered a PASS. If the measured value is lower than the low limit, the test is considered a FAIL.
Leakage Current The leakage test measures current flow from user accessible parts to ground when a product is operating at its normal line voltage. If this current is excessive, it can result in an electrical shock to the user. Due to this potential hazard, safety agencies have set standards for the maximum amount of current that may leak from a non-defective product. Line leakage tests are a design test on non-medical devices and a production test on medical devices.
IEC 60601-1 is the most widely recognized standard with detail regulations for the design of safe medical electronic equipment, however some parts of this standard do apply to production line tests. The test in Figure 1 simulates the effect of a person touching exposed metal parts of a product and detects whether the leakage current that would flow through the person’s body is below a safe level.
Figure 1: Setup for Leakage Current Test
An equivalent circuit of the human body consists of a 1000W resistor in parallel with the series combination of a 0.015mF capacitor and 10kW resistor. The leakage current is measured under various fault conditions, such as “no ground” or with line and neutral connections reversed. Voltage is applied first with normal line and neutral connections, followed by a test with the connections reversed (S1), and then with no ground (S2).
Ground Continuity and Ground Bond The ground continuity test verifies that a fault path exists between any exposed conductive metal surface and power line ground. Should a fault occur in the product where power-line voltage is connected to a user accessible surface, high current will flow through the connection to the power ground (tripping a fuse or breaker), rather than through the user. Verifying the presence of this connection is one thing, verifying the integrity is another, thus the term, “ground bond test.” The ground bond test verifies the current carrying capacity (usually 25 or 30A).
The ground continuity or bond test is usually a prerequisite to the hipot test to verify that the product is grounded before applying high voltage.
Power Consumption Knowing how much power the product consumes is a common test requirement in the appliance and ITE industries. In 2005, IEC published 62301, Standby Power Consumption of Household Electrical Appliances. Now, designers must be aware of how much power is being used when the product is operational and when it is in standby mode. Power consumption is the electrical energy supplied over time to the product to maintain its operation. Power consumption is comprised of two parts: the power the product uses to perform the function, and the excess power that is dissipated in the form of heat or light. Power is measured in watts (W). 1 watt (W) = 1 joule/second (J/s) = 1 volt-ampere (VA).
Typical Features in a Hipot Tester Believe it or not, there are still electrical safety testers in use which require the operator to turn a knob in order to adjust the output voltage, or use a screwdriver to set a current limit or voltage ramp time. Thankfully, this is not the norm today. The change from analog to digital hipot testers has been driven by new technology, automation needs, test integrity, and operator safety. Some of the more commonplace features being offered in today’s automatic testers include:
Test Voltage Regulation Inconsistent or non-reproducible results because of variations in input line voltage or output load are a thing of the past. An operator programmed test voltage can now be tightly maintained regardless of other changing conditions. Line and load regulation are standard equipment on today’s hipot testers.
Programmable Test Parameters with Setup Storage Since the advent of microprocessors, the programming of test conditions such as current limits, voltage level, and ramp time are easily placed in the hands of the test engineer. The ability to store these conditions in tester memory for later recall simplifies the testing of a product mix, meaning they can be tested the same way time after time by simply recalling the test setup from memory.
Front Panel Lockout Knowing that the test conditions cannot be tampered with or inadvertently changed is priceless in a production environment. Key Lock locks out the front panel except for the Recall and Test functions. Fail Lock does the same but in addition when a device fails a test, the instrument stops and the test does not resume until a password is entered. This is a great feature for process control “notification on failure”.
Digital Readout With the change from analog meters to digital readout, nothing has to be left to interpretation. Clear concise display of test conditions (programmed test voltage and current limit) and test results (actual measured leakage current) is now routine practice.
Current Detection Hipot testers impose a voltage on the DUT, sense the current, and compare this measured current to a user programmed maximum limit. Should this high limit be exceeded, the tester shuts down its high voltage and alerts the user of a device failure. Most agency standards specify that ‘there shall be no excessive breakdown current;’ however, limits of 5mA or 10mA are commonly used.
Maximum leakage current alone is no longer the only criterion for pass/fail results. Any device will pass the maximum limit of the hipot test if the tester never makes contact with the device. A small amount of current typically flows through the device and associated cables when properly connected to the tester. Using minimum current detection, it is possible to recognize the difference between this small current and no current (when the device is not connected properly, for example).
Figure 2: Maximum and Minimum Current
Arc Detection Although arc detection is not addressed in any safety standards presently, it is likely to be considered in the future since it can further minimize hazards or product failure. A tester with the ability to monitor short duration current transients (a few microseconds in duration) is a good way of identifying an impending fault that may appear later in a product’s life.
Real vs. Total Current When testing highly capacitive devices, it is often desirable to distinguish between real or total current. Total current is the vector sum of resistive and capacitive leakage current. If the tester monitors only the total current a substantial change in real current can go undetected. The ability to separate the real and capacitive currents is now becoming a real need in AC hipot testing. In fact, some test requirements clearly specify the measurement of real current rather than total.
Figure 3: Resistive and Capacitive Current
Sequential Testing During electrical safety testing, it is often useful to run a series of tests automatically in sequence. As mentioned previously, a continuity/bond test is typically run prior to a hipot test. Also, a base-line resistance measurement can be made by running an insulation test, a hipot test, and then a second insulation test. The hipot test then stresses the device, and the second IR test determines the effect of the hipot test on the product’s insulation. If the results of the second IR test match the first, the integrity of the insulation has not been compromised. If the insulation resistance of the second test is much lower than the first, the insulation resistance has been altered and the device may experience early failure. Sequence testing can be a very valuable tool in detecting latent product failures before the customer is exposed to them.
Operator Safety Features It is the intention of an electrical safety tester to ensure the safety of a product. Yet the inherent problem is the potential for electrical shock to the person using it. In addition to operator training and dedicated safe work areas, significant safeguards should be built into hipot testers, such as interlocks, voltage shutdown, alarms and indicators. Many testers provide some kind of interlock terminals whereby high voltage turn-on is inhibited until the operator closes a cover around the test device or is forced to use both hands in order to start a test. Further, current monitoring techniques in conjunction with fast voltage shutdown are now being employed to enhance operator safety. One new technique involves simultaneous monitoring of the current exiting a hipot tester’s output and the current through the DUT; any imbalance could imply that the operator might have come in contact with the output voltage, thus signaling the unit to shut down. Employing a high voltage transformer for quick discharge of the DUT and GFI (ground fault interrupt) circuitry are two additional safety features often used in hipot testers.
Multi-Function Units A tester can range from an instrument intended for one specific test only, or an instrument with several test functions in one box. For example, in a high volume product environment where only one measurement (AC hipot) needs to be performed, the single function, less expensive unit should be the one of choice. However, an alternative to the single function unit is becoming very popular in areas such as product development or low-volume production where requirements can vary based on product mix. Testers which offer the whole range of production testing capability of AC hipot, DC hipot, ground bond, insulation resistance measurements and current leakage measurements are common today. By anticipating future test requirements in addition to present ones, you can avoid purchasing a tester which has limitations in the long run.
Multi-Point Testing So you have a tester that can run multiple tests, but do you have a tester that can test more than one device at a time? Yes. With a scanner, it is possible to connect to more than one device at a time or to test multiple points on the same device. Some hipot testers have a built-in scanner for just this reason. Others have external scanner options for hipot, ground bond and leakage current testing. Multi-point testing saves time and reduces the amount of operator interaction necessary to change the test leads.
Automation In past applications, standard procedure was to observe the pass/fail light or listen for the fail buzzer to draw the conclusion of a test. Present applications sometimes require the transfer of test results to a computer for traceability purposes, further analysis or archiving. IEEE-488 or RS-232 interfaces are commonplace interfaces for parallel or serial communication. Evolving product standards and product liability issues place increased demands on how a product is tested, and usually require the records to prove it.
Automation Software How will the test results be used? Test records are necessary to prove compliance with a standard. Take it a step further with automation software and amass an electronic database of test records with unlimited potential for process control, data analysis, archiving and more.
A Hipot Tester Checklist To choose the right hipot tester, run through this checklist (pdf) and put a check in the box next to each function you require. Examine the required electrical safety test standard, the test requirements, and data necessary for compliance. Keep an eye on future test requirements to avoid the purchase of a hipot tester with limited use. Product standards may be the basis for shipping safe product but the ultimate responsibility for safety rests with the manufacturer. g
About the Author Tracey Gilpin is a Technical Writer at QuadTech, Inc., Maynard MA, and can be reached at tgilpin@quadtech.com. © Copyright 2007 Conformity |
