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Guide to insulation testing

Stewart Haile, business manager at electrical safety testing specialists Clare Instruments, provides a guide
to insulation testing, which is an essential part of most UK, European and International Standards. Insulation testing is often seen as the third ‘core’ test and would usually be preceded by an earth bond and a flash test. But what exactly is an insulation test?

The insulation test performs a measurement of 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. The test results in a reading of resistance measured in M ohms.

So why is it necessary to carry out the test? The test is designed to ensure that protective insulation is sufficient to form a barrier to make sure that electricity does not come into contact with a user causing harm or to ensure that other manufacturing systems and machinery are not adversely affected. In the manufacturing environment, the advent of legalisation such as the Low Voltage Directive (LVD) requires evidence of due diligence and the results of this and other tests can be used in this respect. The same can be said of in-service testing too.

The test can be applied to both Class I and Class II equipment. The test should be carried out using probes or insulted 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, there are exceptions. Clare has designed test equipment for supplying up to 1000V (e.g. automotive industry ignitions), where greater protection is called for. Equipment has also been supplied with voltage as low as 100V (e.g. motor industry switches), where higher voltages would cause potential damage. In general, pass/fault limits for Class I equipment is a resistance greater than 2M ohms and for Class II equipment is a resistance greater than 7 M ohms.

On first examination, both insulation tests and flash/hipot tests are often seen as very similar. However, there are fundamental differences - flash testing is designed to detect 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 1/2mm from exposed metal, an insulation test – conducted in dry air – could well provide a pass reading. However, a flash test is more likely to detect this situation as 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 2 M ohm but with a 1500V 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.

Obviously a DC flash test with a leakage metre can provide insulation resistance monitoring, as the capacitive component is levelled out after the initial ‘switch-on’ surge.

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, dust etc and assembly faults.

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