Product category:
Compliance Engineering
News Release from: Clare Instruments | Subject: Flash testing
Edited by the Electronicstalk Editorial
Team on 30 June 2006
All you need to know about Flash/Hipot
testing
John Barnett of Clare Instruments, provides an in-depth explanation of flash/hipot/dielectric withstand testing.
As well as supplying and developing turn-key testing solutions throughout the manufacturing and rental sectors, Clare has been instrumental in the formation of certain BS EN standards This article covers the requirements for the tests, the testing process, and offers practical tips and advice gained from Clare's extensive 50 years experience in the design and manufacture of test equipment
This article was originally published on Electronicstalk on 6 Nov 2006 at 8.00am (UK)
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What is the difference between a Flash, Hipot and Dielectric Withstand test? None.
These three terms relate to the same fundamental test.
The term Flash testing is commonplace within the UK market place, the term Hipot and Dielectric Withstand within the rest of Europe and most other geographic regions.
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What is Flash/Hipot/Dielectric Withstand testing? This test is not a measurement, but a process for ensuring that a product remains electrically safe when subjected to a high voltage.
A high voltage can occur when transients are present on the power supply.
When subjected to a high voltage the product must not expose the user to a hazardous voltage.
This test is designed to stress the product's insulation, which will ensure that there are no defects in the construction.
This test will ensure that internal wiring and conductor spacing is correct and electrically safe.
If this test is omitted, then a product with poor insulation or internal conductor spacing, may fail under the influence of environmental factors such as ingress of moisture, dirt, or the effect of vibration.
These defects cannot be consistently detected by any other electrical safety test.
Why is the test necessary? In conjunction with the ground bond test, this test is designed to ensure the basic safety of the product.
Legislation such as the LVD, Machinery and Medical Directives demand that 100% of manufactured products are subjected to this test.
In addition a number of independent and government product approvals agencies require that this test is performed, and that records are kept for each product design, and for each product manufactured.
These organisations include: * Underwriters Laboratories (UL) * Canadian Standards Association (CSA) * International Electrotechnical Commission (IEC) * British Standards Institute (BSI) * German Electrical Engineers Association (VDE) * Technische Uberwachungs Verein (TUV) * Japanese Standards Institute (JIS) Within the European marketplace these directives must be obeyed for a product to comply with the CE mark regulation.
Relevant agency approval must also be attained before a product can display an agency safety marking logo.
What are the test requirements? Before testing it is important to differentiate between the two distinct test requirements - type testing and routine (production line) testing.
Type testing levels vary according to the relevant product specific standard.
The test voltage ranges from 1000 to 1500V (Class I metal clad products earthed/grounded products), and 2500 to 4200V (Class II double insulated products - no earth/ground connection).
Usual practice is to use an AC voltage, as it is representative of the supply, but DC can be substituted where high leakage is experienced due to the inclusion of line filtering capacitors within the design.
If a DC test is to be applied then the test voltage will be the AC value multiplied by 1.414.
As type testing is used to potentially destructively test the product to the design standard, a leakage capability of 100mA is common place.
Due to potential high levels of leakage, these tests must be conducted in a safe environment (laboratory) and by competent test operators who are aware of the hazards associated with test equipment capable of delivering a potentially lethal 100mA of current from a high voltage source.
Production line (routine testing) needs to consider the requirements of the production environment.
These generally comprise the overall test time burden and safety implications of installing a high voltage test area within the production environment.
Here the need to have faster but equally rigorous tests is achieved by applying a 10% higher voltage to the type test level, but with a reduced test time of a few seconds.
It is recommended that the leakage current limit is reduced to 5 mA to maximise operator safety.
Some typical issues that must be addressed within the production line environment are: Is the product properly connected to the tester? Is the product switched on? Even for experienced test operators these issues can be a stumbling block, due to the overall desire to reduce the test time factor and to increase throughput.
Some techniques that Clare employ to eliminate these issues are: Precede the flash test with a phase to neutral continuity test to ensure that the product is switched on.
Employ a lower leakage trip level that must be satisfied during the test.
This can be set just below the normal level measured for a batch of good product, when tested with the supply switch on.
The test will then record a fail condition if the product is tested with the supply not switched on.
Periodic (regular) test fault simulation at the test connection point.
What are the pass or fail criteria? The test itself can be used to take quantative measurements, for process control, but the pass and fail criteria are that a breakdown must not occur, or that typically 5mA of leakage must not flow in the test circuit.
In certain exceptions this leakage can be increased where it can be demonstrated that a good product has high leakage due to the inclusion of devices such as line filtering capacitors within the design.
Why conduct Flash/Hipot/Dielectric Withstand testing, followed by an Insulation Test? Firstly although both these tests involve a high voltage, their purpose is distinctly different.
The flash/hipot/dielectric withstand test is designed to detect gaps or clearances between conductive parts and earth/chassis, pin holes in the insulation and any degradation in the construction of the product caused by wear and tear in the production process.
Insulation resistance testing is designed to provide a quantitative measurement of the quality of the product insulation, measured in MOhms.
Consider a bare conductor positioned 0.5mm from exposed earthed/grounded chassis; an insulation test may provide a pass reading, however a flash test will detect this dangerous condition.
By comparison, if insulation is somehow contaminated a flash test may produce a pass, but the insulation test would highlight the deficiency in the insulation.
How do I Flash/Hipot/Dielectric Withstand test safely on a production line? Type testing is not suitable for use in a production line environment, due to the hazard of electrocution from high levels of leakage.
Type testing must be performed in a controlled, safe, laboratory environment, by skilled trained operators.
Production line testing calls for high test throughput, by low skilled test operators.
For this reason Clare pioneered the approach of using a test voltage that is 10% higher than the type test, with a reduced test current trip of 5 mA - to protect the operator from electrocution.
This has been adopted by the majority of product routine testing standards.
With the advent of the EN 50191 standard - A Guide for the Erection and Operation of Electrical Test Installations, Clare recommends that defined test areas are created in accordance with this standard.
Clare has produced a short form guide to help with the task of implementing test areas to the requirements of EN 50191.
How do I Flash/Hipot/Dielectric Withstand test Class II Double Insulated Products? In Class II double insulated products the absence of the protective ground/earth connection requires protection via primary and secondary insulation.
Due to this a much higher voltage is required, typically between 2500 and 4200V.
A common problem, particularly due to the manufacturing process involved, is that you can have a failure with the primary insulation, that is undetectable by a test on the outer surface to phase and neutral connections, hence the higher test voltage.
Due to the practicalities of testing the primary insulation, this can only be done during the manufacturing process, before the secondary insulation has been added.
It is common place to apply the hot terminal of the tester to the phase and neutral product connections, and the cold tester terminal to the outer surface of the product.
The outer surface is usually encapsulated in conductive foam or foil, to ensure that all accessible parts of the case are tested.
Typical faults can be screw heads trapped just below the surface of the case or wire strands from screw terminals suspended in the secondary insulation.
To satisfactorily encapsulate the product, it is usual to test the product within a safety test enclosure that has a test nest to house the product under test.
In this way operator safety can be ensured and throughput kept to a maximum.
Clare has pioneered this approach and although not detailed in product standards, is commonly recommended by test houses and approvals agencies.
Does Flash/Hipot/Dielectric Withstand testing degrade insulation? The view that this type of test is essentially destructive is often an area for discussion.
This view originates from the use of this test in laboratory type testing, where the long applied test time may degrade the insulation.
However in terms of production line testing the short (less than two second) applied test time and 5 mA trip setting, effectively reduces the risk.
Clare pioneered the technique of employing a DC insulation test immediately after this test, to ensure that the insulation has not been degraded.
How do I Flash/Hipot/Dielectric test sensitive equipment? By employing arc detection and leakage trips, sensitive equipment can be safely tested in a non destructive way.
Low levels of leakage can be safely detected with the new digital era of test equipment.
Clare test equipment features arc detection circuitry, which allows the formation of a potential breakdown to be detected, prior to the actual breakdown occurring - which is usually destructive to the insulation.
This allows sensitive electronic components and PCBs to be tested, and reworked through the production process.
How do I Flash/Hipot/Dielectric test equipment with high levels of AC leakage, fitted with line filters or suppression capacitance? Designers are now faced with strict design rules concerning EMC/RFI conformance.
Typically large networks of capacitors are employed within the design - these provide large amounts of AC leakage to ground/earth, when testing.
This can make testing with AC impractical, where the product leakage is only a fraction of that caused by the capacitor network.
In this case we would recommend that a DC test is used, which when applied to the product will give the true leakage of the product design, not the conducted AC leakage through the capacitors.
As the capacitive element within the product will remain charged, we recommend that this type of testing is not deployed onto the production line, unless the product is tested within an isolated test enclosure.
The test enclosure includes safety interlocks, and a discharge network to remove any stored DC charge on the product post test.
How do I test a product that has circuits that are not electrically connected to the supply, unless powered? Clare has pioneered a technique, commonly known as 'Quadrature Testing', which performs the test whilst the product is energised from an isolated supply.
In this way all secondary circuits and conductors are connected to the product supply terminals, during the application of the test voltage.
This technique is becoming very common due to the inclusion of microcontrollers in the new era of 'smart' consumer products (ovens/washers/dryers/audio visual/IT products).
Advantages/Disadvantages of AC or DC Flash/Hipot/Dielectric testing Highly capacitive products will require a high AC output current capacity from the test equipment - which can present unnecessary risk to the operator.
With AC testing the majority of the leakage measured is typically associated with the line filtering capacitor networks on the product input and not with the insulation.
For this reason DC testing is an option.
With DC testing, once the capacitive elements are charged (from a low level of current output) by the ramp up of the DC test voltage, the effective leakage measured will be the resistance of the product insulation only.
A draw back with DC testing is that at the end of test, or if the test cycle is interrupted, a hazardous voltage/charge may remain on the product supply terminals - causing a shock hazard to the operator when disconnecting the product.
For this reason it is usual to stipulate that discharge circuits are employed, and that the product is located within an isolated test enclosure that includes a locking door to ensure that the product cannot be touched until a full discharge cycle has been invoked.
Another drawback is that products with a very capacitive input can cause instability in the control circuit used within the ramping drive circuit of the DC tester, which renders the DC test impossible.
However DC is currently the only option for testing the working voltage rating on capacitors, and similarly the PIV rating on diodes.
Although a perceived advantage of AC testing is that short ramp up and down times will be much shorter than for DC, since with DC high charge currents will occur is the ramp up time is too short.
This can be the most significant factor in production line testing.
In general AC testing is preferred by most agency approvals - some will still not allow DC, and since AC testing is more representative of the supply waveform, can be viewed as a more representative test that DC. Request a free brochure from Clare Instruments ...
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