The alternatives for insulation testing

While it is accepted that electrical currents are a necessary part of medical electrical devices, faulty or excessive currents can cause a serious hazard to the patient, operator, or medical device. The electrical industry and international community have therefore implemented stringent procedures and requirements to ensure the safe and effective operation of medical devices, embodied in the IEC 60601 standard for medical electronic devices. The risk of unacceptably high electrical fault currents can be minimised through design criteria, i.e. through effective levels of electrical insulation/isolation between the operator or patient and live parts, or potentially live parts, in a fault condition. Such insulation can be achieved through physical spacing (air gaps), dielectric materials, or component choice, in order to achieve the highest possible level of insulation while ensuring that the device operates properly. The effectiveness of electrical insulation is tested through electric leakage measurements (results in mA or ?A), while the level of isolation is often tested using a dielectric or insulation test. During a dielectric, or hipot test, a potentially high voltage (of up to 4,000 V AC) is applied across different parts of the electronic design in order to stress the dielectrics. The results are displayed in mA or ?A – similar to the method used in leakage current measurements. An insulation resistance test applies a lower DC voltage, typically between 250-500 V DC, across different parts of the electronic design. The results are displayed in Mega ohms (M?). Unlike the dielectric testing at high voltage, conventional insulation resistance measurement (the insulation test) has been the traditional method of completing preventative inspections of the insulation levels in medical devices. While a 500 V DC insulation test is not specified within the IEC 60601 standard (type testing), such a test is now an optional part of the recently published standard for routine testing of medical devices, IEC 62353.

Potential damage

Despite the traditional merits of a 500 V DC insulation test to verify the level of insulation, it has also been recognised that this method can be problematic in some circumstances, causing damage to the equipment under test, and also not indicating the true state of the insulation when presented with an alternating voltage. Therefore there is a new alternative leakage test within IEC 62353 that applies a typical line voltage (~230 V) and frequency (50 Hz) as the insulation test source rather than DC. Both have their relative merits and place in periodic testing, provided that the different limitations of each test method are understood. Insulation resistance Insulation resistance is normally checked by applying 500 V DC between: