Wednesday, September 10, 2008

Insulation Testing

Insulation testing is often seen as the third core test and is usually preceded by an earth bond and a flash test.

The insulation test measures the resistance of a product's insulation protection by applying a DC voltage between phase and neutral to the earth conductor for Class I equipment and between phase and neutral to the outer case for Class II equipment. This will show a reading of resistance in Mohms.

The test ensures protective insulation is good enough to form a barrier to make sure people aren't electrocuted and to ensure other manufacturing systems and machinery are not affected.

Legislation such as the low voltage directive (LVD) requires evidence of due diligence and the results of this and the in-service test can be used as well.

The test can be applied to both Class I and Class II equipment and should be carried out using probes or insulated clips and without the equipment being connected to a power supply.

Test voltages vary between standards - although 500V DC is the most common application - and the voltage is applied for a maximum of three seconds.

However, electrical safety testing specialists Clare Instruments has designed test equipment for supplying up to 1000V where greater protection is called for (for example in automotive industry ignitions).

Equipment has also been supplied with voltage as low as 100V where higher voltages may cause damage (for example in motor industry switches).

In general, pass/fault limits for Class I equipment is a resistance greater than 2Mohms and for Class II equipment is a resistance greater than 7Mohms.

There are differences in the flash/hipot tests - flash testing detects gaps or clearance between conductive parts and earth, pin holes in insulation and other degradation that may be the result of production processes and/or wear and tear, while insulation resistance testing is designed to provide a quantitative measurement of the quality of insulation.

If a wire was positioned 0.5mm from exposed metal, an insulation test conducted in dry air could provide a pass reading.

However, a flash test is more likely to detect if this situation was dangerous.

Similarly, if insulation is somehow contaminated, a flash test would produce a pass, but an insulation test would highlight the deficiency.

For example, the normal minimum insulation resistance value for Class I appliances is 2Mohm, but with a 1,500V AC flash test, the current would be 0.75mA and would not be detected by the 5mA trip, which has to accommodate the capacitive losses that occur.

A DC flash test with a leakage meter can provide insulation resistance monitoring, as the capacitive component is leveled out after the initial switch-on surge.

The test time of two to three seconds does not generally pose a problem on the production line, but some practicalities need to be addressed.

For example, the use of a conductive foam nest can aid the testing of Class II products, and this nest can be integrated with test enclosures and probes.

Typical causes of insulation test failure include poor quality insulating materials, material that has been over-stressed either mechanically or electrically, poor maintenance and cleaning procedures, ingress of fluids and dust, and assembly faults.

For more information, request a free brochure from Clare Instruments, where this article is from.


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