Substations are increasingly being designed, adapted or expanded with gas-insulated switchgear (GIS). There are various reasons for this trend towards GIS, such as better protection of the system components against aging processes, contamination or corrosion. The associated reduced need for maintenance of the components and the longer service life can be translated into lower operating costs in the long term. Very often, however, the often most important reason is the significantly lower space requirement. A GIS can get by with a fraction of the space required for an air insulated system. Wherever space is either very expensive or simply not available in sufficient quantities, such as in cities, on platforms or underground, GIS are often the only option.
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In the applicable standard, the IEC -203, the test is also called power-frequency voltage test. The GIS must be installed completely and gas-filled at rated filling density during the test. If the GIS is extended with new parts, these shall again be subjected to a dielectric test on site. The test is passed if the insulation can withstand the withstand voltage for 60 seconds, with test frequencies ranging from 10 to 300 Hz.
Introduction
Electrical safety testers – often referred to as “hipot” testers – are an integral part of electrical and electronic equipment manufacturing. Hipot testers get their name from the high-potential (high voltage) they produce to perform dielectric withstand and insulation resistance tests. In addition to these tests, many hipot testers provide accurate low-resistance measurements and low-resistance/high-current outputs to test ground resistance and ground bond integrity.
Hipot testing has long been a standard procedure for assuring the electrical safety of electronic equipment. Early commercial hipot testers were actually not much more than a step-up transformer to adjust to applied voltage in stepped increases over prescribed time segments to test for leakage or component breakdown. This method could easily lead to incorrect results when leakage current caused the voltage output from a high impedance transformer source to droop. Modern hipot testers utilize electronic source technology to assure compliance with IEC- that explicitly requires that “the voltage test equipment shall be able to maintain the required voltage for the specified period of time.”
Product Safety Certification
Electrical safety testing and certification is a requirement for virtually every electronic device and electrical apparatus. The details of what constitutes a certified product is dependent upon a daunting number (hundreds) of safety standards and the region of the world where the device will be sold and used. Standards setting organizations include:
Manufacturers must submit samples of their products to recognized certification agencies. Nationally Recognized Certification Laboratories (NRTLs) include UL, VDE, FM, ETL and others. The agency certification process is conducted to confirm compliance with the relevant standard(s). This compliance evaluation investigates two key areas:
Work is being done to harmonize standards from global agencies. For example:
To further help manufactures address this often-bewildering array of international (and sometimes conflicting) standards, the Power Sources Manufacturers Association (PSMA) has established a standing committee and forum on its website.
Production Electric Safety Testing
Electrical safety testing is an important final step in the production process for most electrical and electronic equipment to:
Once in production, products must be 100% tested to confirm compliance with the related agency certifications and safety standards. Production tests are less stringent than initial certification but will generally include basic dielectric withstand and shock hazard (leakage) tests. Plug-connected devices will also be subjected to ground resistance and (if the standard requires) ground bond tests. Electrical motors, transformers and other such devices will likely include insulation resistance tests.
Periodic inspection and
calibration of test equipment is a standard requirement to maintain NRTL
certification. Agency inspection will include check of hipot instrument
calibration certification. This “cal cert” is typically required on an annual
basis. (UL and other NRTLs require compliance certification with ISO.) Another common requirement prescribed by most
NRTLs is a daily functional test of the hipot equipment.
Dielectric Withstand – Hipot
The basic hipot test applies a high voltage from the conductors to the chassis of the device-under-test (DUT). This test is often referred to as “dielectric” or “voltage” withstand. Its purpose is to confirm that the insulation and isolation of the non-conducting surfaces from the operating voltage is sufficient to avoid a shock hazard. The typical specification for this test is: V + 2 x normal operating voltage. Refer to Figure 1, above.
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Both AC and DC hipot tests are possible and, in general, the test should use the same type of voltage as it would be during normal operation. However, if a DC hipot test is used on an AC circuit, the hipot voltage should be two times the peak (2 x 1.4 x RMS)+ V.
Depending on the applicable standard, units pass this test if either:
For double-insulated products, higher voltages will often be specified in the test standard. In addition, this class of device typically requires special fixturing to connect the non-conductive outer shell to a conductive element.
Defects that are often detected with the hipot test include contamination (dirt, debris) and lack of proper spacing (creepage and clearance) of components. Creepage is measured across surfaces, clearance is the air gap between components. Contamination would likely cause an unacceptable level of leakage current. Clearance problems could result in breakdown.
Desirable Dielectric Withstand Test Features
Insulation Resistance
Insulation resistance testing is likely to be required in motor winding, transformer winding and other applications involving cabling or insulated wire. Insulation resistance testing typically involves confirming that the resistance exceeds a defined high resistance value.
In many instances, insulation resistance needs to be measured between several conductors. Examples include cable/connector assemblies, multiconductor cables and relays. To make this measurement, all the conductors except one are shorted together and the test voltage is applied from the remaining conductor across the bundled ones. Each wire is then, in turn, tested in this fashion (Figure 3).
Desirable Insulation Resistance Test Features
Ground Continuity
Ground continuity testing is performed to confirm that the conductive chassis of a device is safely connected to the earth ground pin on the power plug. This assures protection against shock hazards even if the equipment suffers an internal short to the chassis. The current would be shunted via the ground wire and would likely trip the breaker or blow the fuse.
Ground continuity is performed by measuring by applying a low current (e.g., 50 mA) and calculating the resistance from the ground pin on the power plug to selected locations on the exposed surfaces of the DUT. Desirable ground continuity features include:
Vitrek’s TL-UP1 accessory (Figure 5) is an example of an accessory device the simplifies ground continuity test setup. With its 4-foot leads, the accessory offers easy hipot and continuity test connection of corded products.
Ground Bond
Where ground continuity measures the resistance of the safety ground connection, the ground bond test assures the integrity of the connection. Using the same test setup, a high current is passed through the circuit. If the ground bond is solid, the current passes without a change in resistance. If weak, the resistive heating of the current would induce a failure of the bond.
Desirable Ground Bond Test Features
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