Electrical safety is of utmost importance for all the electrical and electronic products. Today’s HIPOT (short for high-potential) testers—key test equipment for electrical safety—have come a long way in terms of features, programmability and safety involved in testing itself
SHWETA DHADIWAL BAID
JULY 2010: Someone has rightly said that “Safety doesn’t happen by accident.” Today, most consumer electronics devices operate in close vicinity of the human body, be it a mobile phone, laptop, home appliance or medical device. Any product designed without electrical safety parameters in mind may result in a mishap. The finished product needs to be tested thoroughly before it is put in use. Several tests have to be conducted from design to production stage in order to ensure electrical safety of the component and the finished product.
No designer wants to design an unsafe product, but many a times the product may fail in operation due to some design flaw or faulty components. Electrical safety tests are performed on components such as transformers and switches, cables, motors and finished goods. Several standards organisations publish the permissible limit for different parameters. Organisations like CSA in Canada, UL/ETL in the United States, GS in Germany, EK in Korea, CCC in China and CNS in Taiwan provide electrical safety/international standards, shares Helena Wang, sales manager, GW Instek. In India, Indian Standards Institute (ISI) specifies electrical safety parameters for many products. Usually, manufacturers hire certified test labs to perform safety tests to comply with these standards and get their products certified.
Insulation plays an important role in electrical safety. During production, most manufacturers include tests around insulation in order to meet safety requirements as per specific standards. As opposed to continuity tests that ensure current flow, there are tests performed using high potential (HIPOT) to ensure that the current does not flow from one point to another at a given voltage for a given time.
Need for electrical safety tests
Electrical safety tests may be mandated by the standards’ organisations, but as a manufacturer, it is your social responsibility. Tests do consume some time in the production line, but these significantly reduce the risk of fires, accidents, electrical shocks, etc from the equipment. It is very important to test insulation as 80 per cent of electrical maintenance and testing involves evaluating insulation integrity.
Electrical insulation starts deteriorating in its performance as an insulator (extremely bad conductor) with time. The most important reason for testing insulation is to ensure public and personal safety. “Insulation testing is used to determine the integrity of windings or cables in motors, transformers switchgear and electrical installations,” shares Prachi Gupta, director, Avante Global. “By performing a high-potential test between de-energised current-carrying, grounded and grounding conductors, you can eliminate the possibility of having a life-threatening short-circuit or short-to-ground faults,” she adds.
Electrical and electronic systems when exposed to environmental factors such as dirt, grease, temperature, stress and vibration affect the dielectric material used in insulation. This may lead to insulation failure, resulting in loss of production or even fire. “It is important to identify the deterioration as early as possible to take corrective measures,” suggests Ms Wang.
When a high potential is applied between isolated parts of a circuit or a product, the behaviour of electrical parameters helps identify broken or poor insulation, stray wire strands or braided shielding, spacing problem between terminals, tolerance errors, etc. This helps in making informed decision on the safety and quality of electrical circuits.
“The value/range of HIPOT depends on the device under test (DUT),” shares Rohit Manwani, chief executive officer of Sunil Associates. “Range of the instrument varies from industry to industry. Some industries can accept the sample if insulation resistance is more than 1000 mega-ohms, while some look for values in giga-ohms,” he adds.
High-potential tests can be divided into three distinct tests with different goals. To understand the high-voltage capacity of an insulating material, dielectric breakdown test is used. The voltage between the test points is increased continuously until the current increases suddenly. This is the point where insulation gets into conduction. Normally, this test is done during the product design and qualification stage.
During the insulation test, a high voltage is generated that causes a small current (in micro-amperes) to flow through the conductor and insulation. The amount of current depends on the amount of voltage applied, the system’s capacitance, total resistance and temperature of the material. The value of insulation resistance read on the meter will be a function of following three independent sub-currents:
Conductive leakage current. Conductive current is a small current that normally flows through insulation, between conductors or from a conductor to ground. This current increases as insulation deteriorates and becomes predominant after the absorption current vanishes.
Capacitive charging leakage current. When two or more conductors are run together in a raceway, these act as a capacitor. Due to this capacitive effect, a leakage current flows through conductor insulation. This current lasts only a few seconds as the DC voltage is applied and drops out after the insulation has been charged to its full test voltage. In low-capacitance equipment, capacitive current is higher than conductive leakage current, but usually disappears by the time you start recording the data. Because of this, it is important to let the reading settle out before recording it. On the other hand, when testing high-capacitance equipment, the capacitive charging leakage current can last a very long time before settling out.
Polarisation absorption leakage current. Absorption current is caused by polarisation of molecules within the dielectric material. In low-capacitance equipment, current is high for the first few seconds and decreases slowly to nearly zero. When dealing with high-capacitance equipment or wet and contaminated insulation, there will be no decrease in the absorption current for a long time.
—Prachi Gupta, director, Avante Global
2. Arrange the test bench and test procedure in orderly manner
3. Do not modify the test procedures without any knowledge
4. Make sure that custom test fixtures carry proper ratings and test equipment are correctly rated
5. Do not touch the DUT while the test is running
6. In case of a problem, first turn off/stop the test
7. Discharge the DUT completely in the DC test
“Of course, the sample DUT that fails the test is rejected as it is completely damaged now,” shares Manwani. The dielectric withstand test is normally done at 75 per cent of the breakdown voltage. The test is performed to understand for how long the dielectric can withstand high voltage.
Insulation resistance (IR) test measures resistance of the insulating material. You apply a very high voltage across the dielectric, carefully measure the current and calculate the resistance using Ohms law. “For most of the products, the test is carried at 500V DC and the resistance should be at least 2 mega-ohms,” shares Ms Wang. “Usually IR is checked first, before putting the device for HIPOT test,” she adds.
Hipot testing can be carried by an AC or DC voltage. However, there are different considerations when using AC or DC voltages. The difference in the reading becomes significant when the DUT is highly capacitive. “AC voltage cannot charge a capacitive load, as reactive current flows instantly and remains consistent regardless of the time the voltage is applied. DC voltage will charge the capacitance of the DUT so you may see an inrush of current upon initial application of the voltage, but gradually the readings will dissipate as the DUT becomes charged. Normally, the relationship between AC and DC HIPOT tests is AC=1.414DC. For example, if you perform AC HIPOT test at a test voltage of 2 kV, then DC HiPOT test should be performed at 2.83 kV,” explains Ms Wang.
Expressing her views on the test methodology, Ms Gupta believes that the test method also depends on the type of the DUT. “For instance, when a low-capacitance equipment like electrical cabling or switchgear is tested, the time-dependent capacitive leakage and absorption leakage current become insignificant and decrease to zero almost instantly. This provides a perfect condition for accurate spot reading. On the other hand, when high-capacitance equipment such as long runs of cables,
large motors or generators are to be tested, the time-dependent current will last hours. This current will cause the meter reading to change constantly, making it impossible to obtain an accurate steady reading. This condition can be overcome by using a test that establishes a trend between readings, such as the step voltages or dielectric-absorption test. These tests do not depend on a single reading but on a collection of relative readings.”
New features in HIPOT testers
To serve the rising demands for features and, most importantly, safety, there have been a lot of developments in the design of HIPOT testers and features. Most companies are promoting their all-in-one kinds of equipment that can perform dielectric breakdown, dielectric-withstand and insulation resistance tests from a single box. Many equipment are designed to perform both AC as well as DC HIPOT test.
Handheld portable testers are now the need of every industry. The need for insulation testing is increasing in the field and traditional testers are large and bulky. “Nowadays, battery-operated insulation testers are there to work in the field. Good-quality probes are available in the market to do the test efficiently,” informs Manwani.
Some T&M manufacturers have launched a thoughtful combination of insulation tester and digital multimeter in a lightweight tool that fits in your palm. This has significantly helped electricians and technicians on the field in troubleshooting and preventive maintenance, addressing their basic multimeter needs, and gives them an insulation tester that generates up to 1 kV.
“Humidity is a major factor affecting insulation measurements in India. Nowadays, insulation testers are designed to be humidity-proof,” shares Thirumalaisami R., head of T&M, Yokogawa, India. “During the DC HIPOT test, the DUT is fully charged to provide accurate leakage current. Normally, to discharge the testers, the probes are shorted. This reduces the battery and instrument life. Today, testers have a built-in discharge key with which the battery and instrument life are optimised,” he adds.
As per ISI requirement, insulation is to be tested for 500V DC, but today industries require various higher and wider voltage ranges. Manwani shares, “Insulation testers or million mega-ohm meters with analogue meter are preferred because of the non-linear scale of insulation. These instruments cover most of the higher-range requirements and are indigenously developed and available. Digital insulation testers are also available but these have limited range.”
“Today HIPOT testers come with a digital LCD. This is not only for ease of use but also to reduce the operator error,” Ms Wang believes. “The market demands fully-automatic all-in-one testers, so the manufacturers have come up with 4-in-1 and 5-in-1 kinds of electrical safety test solutions,” she adds. “Thoughtful safety, reliable test result, user-friendliness and fully automated interface make these equipment an advanced safety tester that can perform essential electrical safety tests and deliver fast and reliable test results from a single test connection.”
The output of the HIPOT tester is normally high voltage and current. If mishandled, this can prove dangerous to the person performing the test. Automation of the tests has in a way helped the operator to be away or have minimal contact for handling of the device. However, safeguarding the operator is still utmost important. Equipment today come with interlock features, ground fault interrupts, faster DUT discharge and quick high-voltage shutdown. These also include audio-visual warnings like blinking LEDs and alarms in case of any unforeseen situation.
Choosing HIPOT tester
Understanding when to use AC test or DC test is important. Some standards may require either of the two or both. AC test scores over DC as it tests the insulation for both polarities. However, the downfall is that in the case of a highly capacitive DUT, the discharge current will be higher, misleading the real current reading. In such cases, choose a tester with high output current capacity.
As there are several combinations of multiple test equipment available, choosing the right combination is important. If the tester combines HIPOT tester and ground-bond tester, ensure that the tester can handle the output current and resistance measurement required.
Some tests are performed during designing before finalising the material, while many tests are performed during production. It makes a lot of sense to combine the tests during production into one equipment.
Ms Wang adds, “The leakage current test is performed by most of the DUTs at 10% sampling rate except in the case of medical equipment (this means leakage current is tested only on ten samples if 100 samples are provided). Hence manufacturers don’t perform leakage current test on the production line.”
She suggests that since most of the standards require insulation resistance to be tested at 500V and 1000V, test voltage for IR is fixed at these values.
Time to test is another important consideration. Standards play an important role in the duration of the test. As all-in-one kinds of boxes have a single connection, these save a lot of time because of simultaneous operation. Functioning and line leakage are tested with the same connection.
With so many technologies working together, and developments in electronics, HIPOT test is becoming a necessity and a routine in production line testing. Today, consumers look for products that are not only feature-rich but also safe to use. With a little understanding of the basic test procedure, and use of the right type of test and equipment, you can perform the tests safely. This will ensure compliance to regulations and guarantee that your product is safe to use.
The author is a senior technology journalist at EFY