Medical equipment must meet the design requirements as set out by the IEC 60601 (a harmonized standard), which has been adopted by all IEC member states. This sets out all the design criteria for producing equipment that is electrically and mechanically safe, as well as placing the onus on the manufacturer to understand how to reduce the risk of harm when patients and operators are exposed to their medical devices. All tests relating to the electrical safety of medical equipment and devices can be categorized into two categories:
- MEANS OF OPERATOR PROTECTION (MOOP) - Means of protection for reducing the risk of electric shock to persons other than the patient.
- MEANS OF PATIENT PROTECTION (MOPP) - Means of protection for reducing the risk of electric shock to the patient.
To ensure that medical equipment does not pose an electrical hazard to the patient, or any other person, it is designed with sufficient levels of isolation (dielectrics) to reduce the amount of electrical leakage current to an acceptable and safe level - as low as 10µA.
This is achieved by separating high electrical potentials from any conductive parts, accessible to the operator or patient. Dielectric strength is proven by applying a high voltage between high and low electrical potentials. However, this could lead to a breakdown of the isolation and would therefore be referred to as a destructive test.
A safer way to test the effectiveness of dielectrics is to perform a number of electrical leakage tests, such as leakage originating from the power supply to the enclosure (MOOP) or protective ground wire (MOOP & MOPP) or even to the patient connected parts (MOPP).
In IEC 60601, the test requirements for electrical leakage must be carried out under the worst possible conditions to ensure absolute safety. This is achieved using an elevated mains at 110% of the highest expected voltage (i.e. at 240V mains this would mean testing at 264V). Preconditioning of the medical equipment is required prior to testing. Tests are done under normal condition (no fault conditions), and including any one of the specific and relevant fault conditions.
Testing the protective ground circuit design for sufficient current carrying capabilities is achieved by stressing the design, passing a minimum test current of 25 ampere RMS through the circuit for a minimum of 10 seconds. At these current levels, time duration and resistance values (<0.1Ohm internal equipment resistance), enough energy will be created to convert current into thermal heat. By observing the thermal profile of a design, one can establish parts of the design that might need to alter in order to reduce the electrical resistance and thus the converted energy;
E = I² × R × t
Conducting such tests during the development and approval stages of a products life cycle, provides sufficient levels of confidence that the medical equipment meets the design requirements of IEC 60601. Once a design is approved for manufacturing and marketing, a subset of tests will suffice to ensure the product has been built and assembled to the required product quality and safety requirements. This subset of tests is commonly referred to as routine tests and are not clearly defined in IEC 60601 thus can vary between manufacturers and product designs. It’s for this reason that the new IEC 62353:2014 makes a recommendation that IEC 62353 can be used during final testing and before putting a piece of medical equipment into service.
In-Service Test Requirements
IEC 60601-1 does not provide any guidance on routine test requirements. This has led to different interpretations across the world on how to apply IEC 60601 to routine test scenarios. Once a medical device leaves the factory, a number of potential test scenarios arise, including:
Acceptance testing - also referred to as an initial or reference test. This test is carried out prior to a new medical device being authorised for use and is undertaken to ensure correct and complete delivery. Acceptance testing is often not limited to an electrical safety test, with some basic function tests also being applied to verify correct performance.
Routine testing - also referred to as planned preventative maintenance (PPM). This form of testing is often conducted at fixed time intervals, which vary between types of equipment, manufacturers’ recommendations and risk assessment procedures undertaken by individual BME or medical physics departments. Routine testing is not limited to safety testing and often includes the verification of correct functionality.
After service & repair testing – this is carried- out following a repair or product upgrade and is often part of a service carried out by in-hospital mechanical or clinical engineering teams. In many cases, more rigorous electrical safety testing is needed after the replacement of components or reconfiguration of medical devices.