Ensuring a product remains electrically safe - John Barnett explains how technical advances ensure the effective production line use of electrical flash/hi pot/dielectric withstand testing.

The terms flash test, hipot and dielectric withstand test all relate to the same fundamental test and describe the process for ensuring that a product remains electrically safe when subjected to a high voltage, or to check for any signs of damage or poor quality of the basic insulation of a product. A high voltage can occur when transients are present on the power supply. In such circumstances the product must not expose the user to a hazardous voltage.

Poor or damaged insulation can also occur during construction, particularly if wires are trapped within the chassis or case of a product.

This test is therefore designed to stress the product's insulation and ensure that there are no defects in the construction and that internal wiring and conductor spacing is correct.

If the 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.

* Test standards - legislation such as the Low voltage directive, machinery and medical devices directives demand that all manufactured products are subjected to this fundamental electrical safety 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, before their approvals mark can be displayed on the product.

Within the European marketplace these directives must be obeyed for a product to comply with the CE mark regulation.

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, resulting in ionisation and potentially destructive breakdown.

However in terms of production line testing the short (less than two second) applied test time and 5mA trip setting effectively reduces this risk.

Applying a DC insulation test immediately after this test will provides a means of ensuring that the insulation has not been degraded.

The flash/hipot 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.

* Testing times - routine testing on the production line invariably demands high product throughput and is often carried out by relatively low skilled test operators.

Factors such as these bring specific demands in terms of reducing the overall test time burden and the safety implications of installing a high voltage test area within the production environment.

Modern flash/hipot testing systems have been developed that utilise a 10% higher voltage than would be applied during a type test, but for a much reduced test time of a few seconds.

It is also recommended that the leakage current limit is reduced to 5mA to maximise operator safety and the majority of product routine testing standards has adopted this approach, and to ensure that if a product fails, that destructive damage is avoided.

In addition other new techniques have been introduced to address other issues over production line throughput.

For example, it is usual to precede the flash test with a phase to neutral continuity test to ensure that the product is switched on.

This eliminates the possibility of incorrect testing - since all circuits must be connected to the supply during the test.

For the same reason, lower leakage trip levels can also be that are set just below the normal level measured for a batch of good product, when tested with the supply switched on.

The test will then record a fail condition if the product is tested with the supply not switched on.

* Operator safety - in terms of operator safety, the introduction of the EN50191 standard - A Guide for the Erection and Operation of Electrical Test Installations - sets out requirements for defined test areas in accordance with various safety measures - particularly where high voltage testing is to be used.

By employing arc detection and leakage trips, sensitive electronic equipment can also now be safely tested in a non-destructive way.

For example, low levels of leakage can be safely detected with the new digital era of test equipment, allowing sensitive electronic components and PCBs to be tested, and reworked if necessary as part of the production line process.

Other special techniques are also available for the safe testing of Class II double insulated products and for products that have high levels of AC leakage which may be fitted with line filters or suppression capacitors.

This can be achieved by using a higher DC test voltage, but incorporating a bespoke design of test enclosure system with user interlock or special 'test nests' to house the product under test.

Another technical advance has been the development of test systems for newly emerging smart electronic and electrical products that utilise circuits that are not electrically connected to the supply, unless powered.

Mostly found in microprocessor controlled intelligent products.

To overcome such problems a technique known as quadrature testing has been introduced which performs the test whilst the product is fully energised and operational 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 modern consumer products such as ovens, washing machines, tumble dryers and AV and IT products.

Technical developments such as these have ensured that modern flash/hipot testing systems continue to enable manufacturers to meet their product compliance standards and that the tests can be applied safely without any disruption to fast moving efficient production lines.

* About the author - John Barnett is with Clare Instruments.