O F E X P L O S I O N - P R O T E C T I O N T E C H N I Q U E S
4.1 NON-DESTRUCTIVE TEST (NDT)
Non-destructive testing techniques may be used by the repair facility to:
(a) Identify volume defects (voids or porosity) in metal.
(b) Determine wall thickness of metal and bonded dissimilar metal materials (e.g.
bearings).
(c) Locate and size cracks, both surface breaking and sub-surface.
Techniques available include:
(i) Magnetic particle testing, which uses magnetic fields to identify and size surface and near-surface defects.
(ii) Dye-penetrant testing, which uses coloured dyes to identify surface breaking defects, usually on non-magnetic materials.
(iii) Ultrasonic testing, which uses non-audible high frequency sound to locate and size cracks and voids in bulk material, and can be used to determine the thickness of materials.
(iv) Radiography, which uses gamma or X-rays to identify and size voids and larger cracks in bulk materials.
(v) Eddy-current testing, which uses magnetic fields to identify surface-breaking cracks in magnetic materials.
General testing techniques are specified in Australian Standards, but specific details and acceptance criteria will be job dependent. The selection of the most appropriate technique depends onof the following factors:
(A) The configuration and materials of construction used in the equipment for overhaul.
(B) The presence of surface coating materials (paint) and surface finish used on the equipment for overhaul.
(C) The anticipated type and size of the expected defect.
Care should be exercised to ensure that the technique selected is appropriate for the testing task, and to establish that should defects be identified, they are evaluated for significance to the integrity of the item under test and/or to the integrity of the explosion-protection properties of the equipment.
Non-destructive testing is a group of highly specialised techniques and overhaul facilities using NDT should be familiar with the relevant Standards, techniques and competence systems, or use an appropriately qualified subcontract supplier.
4.2 DIELECTRIC WITHSTAND (HIGH POTENTIAL OR HI-POT) TESTING 4.2.1 General
Dielectric Withstand or high-potential testing is commonly abbreviated as hi-pot testing.
Hi-pot is the term given to a class of electrical testing instruments that uses a power frequency test voltage to verify electrical insulation in finished equipment cables or other wired assemblies, printed circuit boards, electric motors and transformers.
A hi-pot test verifies the insulation of a piece of equipment or component to identify excessive electrical leakage and deterioration in the insulation scheme.
Under normal conditions, any electrical device will produce a minimal amount of leakage current due to the voltages and internal capacitance present within the device. Yet due to design flaws or other factors, the insulation in a piece of equipment can break down, resulting in excessive leakage current flow. This failure condition can cause shock or death to anyone who comes in contact with the faulty equipment.
A hi-pot test applies a high voltage between a device’s current-carrying conductors and its metallic chassis or screen. The resulting current that flows through the insulation, known as leakage current, is monitored by the hi-pot tester (detailed in the next Clause). The theory behind the test is providing a controlled over-stressing of the insulation scheme to verify its Dielectric Withstand performance for future service.
In addition to over-stressing the insulation scheme, the test can also be performed to detect material and workmanship defects, most importantly small gap spacing between current-carrying conductors and earth ground. When a piece of equipment is operated under normal conditions, environmental factors such as humidity, vibration, contaminants, and dirt can close these small gaps and allow current to flow. This condition can create a shock hazard if the defects are not corrected. No other test can uncover this type of defect as well as the Dielectric Withstand (hi-pot) test.
4.2.2 Hi-pot (high voltage, line frequency or Dielectric Withstand) test
A hi-pot tester is a test instrument used to stress test the electrical insulation in a device or other wired assembly that could fail and cause someone to receive an electric shock. It generally consists of a—
(a) variable high (a.c.) voltage source;
(b) leakage current indication meter; and
(c) switching matrix to connect the high voltage source and leakage current indicator to all of the contact points in a cable or device.
Hi-pot testers may also have a microprocessor and a display to automate the testing process and display the results.
A hi-pot tester can be very similar to a cable tester and often the two are combined in a single unit.
In a commonly wired assembly, a hi-pot tester connects all circuits in common to ground.
Each circuit is then individually disconnected from ground and connected to the high voltage. The current that flows is monitored to ensure that it is low enough.
Typical test levels are twice the system phase voltage (U) plus 2500 V for high voltage equipment or twice the system phase voltage (U) plus 1000 V for low voltage equipment.
These tests are generally done on each phase in turn, with other phase and auxiliary equipment bonded to ground. Where the testing configuration does not require a lengthy charging period to achieve the test voltage, the test duration may be just a few seconds. A testing configuration with lengthy cables may require substantial amounts of charging time to achieve the required test voltage.
NOTE: The above test voltages are for new equipment only. De-rating factors need to be applied to repaired and overhauled equipment (see AS/NZS 4871.1, Table H3). For further guidance on hi-pot testing, refer to AS/NZS 4871.1, Appendix H.
WARNING: HI-POT TEST EQUIPMENT UTILIZES HIGH AC VOLTAGES OF 25KV AND GREATER. IT IS CRITICAL THAT SUITABLE SAFE WORK PROCEDURES ARE DOCUMENTED AND UTILIZED FOR THIS TESTING.
The hi-pot tester is intended for use by experienced electrical personnel only and requires the use of established safety procedures and proper personal protective equipment (PPE).
The test is performed only on de-energized, out-of-service isolated equipment. The ramp-up voltage is usually about 2 kV/s and the hi-pot tester should be wound fully down at the end of the task to avoid injecting extremely high voltage spikes into the tested equipment. Care should be taken to ensure the equipment under test is completely discharged (noting that some equipment has the tendency to recover charge), at the end of the test prior to disconnecting the hi-pot tester.
4.3 INSULATION RESISTANCE
An insulation resistance test is necessary to ensure that the insulation resistance between all live conductors and earth or, as the case may be, all live parts and earth is adequate to ensure the integrity of the insulation.
Refer to AS/NZS 4871.1, Appendix H for the insulation resistance test procedure and acceptability of results.
4.4 COMPONENT TESTING 4.4.1 Circuit breakers
The testing of circuit breakers for use in explosion-protected equipment can be performed at the service facility. There are a number of different methods of achieving this. The most common method is by current injection where a test unit injects a set current and measures the time taken for the circuit breaker to trip. The results of multiple tests at different currents may be plotted and are compared with specified current and time curves that are usually supplied by the circuit breaker manufacturer. Test units should be accurate to within a few percent. The results of circuit breaker tests are supplied as part of the overhaul report.
Together with these tests the report should note the unit’s general physical condition, and include comments on the condition of contact tips, under voltage trip units and no volt trip units, if applicable. The test unit’s details and a calibration date (if relevant) should also be noted.
4.4.2 Overloads
Overload testing is similar to testing circuit breakers, using the same test rig and the appropriate manufacturer’s test curve. The maximum current, the condition of the overload, condition of any heaters, external condition of the unit under test and the results at various stages of the test should be recorded.
4.4.3 Current transformers 4.4.3.1 Primary injection testing
This test is used to test the overall operation of a current transformer. In this type of test, a high current is injected in the current transformer (CT) primary winding and the resulting secondary current is measured in each secondary CT. This test is mainly conducted during a major circuit modification or rewire. This is part of the overhaul process. The polarity of the current may also be critical and other equipment, such as a phase angle meter, may be used in conjunction with the high-current test source.
NOTE: Care should be taken to ensure that the current transformer is always connected. Open circuit secondary windings can produce very high voltages.
4.4.3.2 Secondary injection testing
This test is performed on the individual devices, such as relays and meters, to verify the accuracy and proper operation of the equipment. These devices receive their input current from the CT secondary winding so these tests are at a much lower level of current than that used for primary injection. The correct operation of the current-sensing protective equipment can be verified by comparing the device operating characteristics with the manufacturer’s published time-current characteristic curves.
Ensure that the equipment nameplate data complies with drawings and specifications.
Every CT test sheet should include the information included in AS 1675.
4.5 TEMPERATURE MEASUREMENT
Monitoring of temperature is undertaken during numerous test processes, including the following:
(a) Motor and transformer load and no-load testing.
(b) Winding removal.
Temperature sensing can take the form of contact, infra-red or embedded devices. Other less frequent forms of temperature measurement may also be employed.
S E C T I O N 5 O V E R H A U L O F