Evolving Standards Reshape Medical Power Supplies

Apr 1, 2007 12:00 PM
By Peter Resca, Director of Engineering, and Dave Murray, Senior Product Engineer, Astrodyne, Mansfi

Proposed Standards

The third edition of the IEC 60601-1 standard will force a philosophical shift in designing for compliance. The proposed medical standard is a response to the rapidly changing medical device market. The market has taken advantage of technology and material science advances to develop more sophisticated and complex products. Current medical standards are challenged to keep pace with these changes while maintaining the purpose of their existence: providing safe products for use in the medical industry.

To address current and future technology advances, the proposed standard formally introduces several new concepts to the medical approval process. First and foremost is the inclusion of risk management. A second new and important term is that of “essential performance.” To support these approaches, new testing and design processes are required. The net result is that the certification process will be less about type tests with defined limits — although these will still be important — and more about the manufacturer identifying potential hazards and documenting how they are addressed.

IEC 60601 Third Edition

The new standard, as mentioned previously, still includes type tests for leakage, isolation and creepage/clearance. The difference is that the determination of the correct category and values is guided by the concept of “essential performance.” Essential performance identifies operating characteristics that can impact the safety of operators or patients. This will tie into the risk analysis performed under the risk-management system employed with the new standard. The purpose is to allow the manufacturer to identify the appropriate levels to ensure safe medical devices. In some cases, this may be a reduction in limit from the current standard, but in many cases it will require additional protection or analysis.

A second concept introduced in the new standard is means of protection (MOP), which describes the isolation protection between the electrically charged circuitry and any equipment that may come in contact with the device (Fig. 2). The isolation protection includes the creepage/clearance distances, insulation and protective earth connections. The means of protection is further separated into two categories: means of operator protection (MOOP) and means of patient protection (MOPP).

As the terms suggest, the classifications provide greater protection for the patient w ho may be more vulnerable to the medical device in use. MOOP is more closely aligned to the traditional IEC 60950, which is the standard of protection for ITE, while MOPP maintains the more stringent requirements similar to the current IEC 60601 standard (Table 2).

The compliance measures for leakage current have been altered to ensure patient and operator protection in contact with medical devices. First, the present standard definition of “enclosure leakage current” is now described as “touch current.” Touch currents are the leakage paths from an enclosure that may be in contact with a patient or operator. The levels within the third edition of IEC 60601 are 100 A for normal operation and 500 A for a single fault condition, which is the same as the enclosure leakage limits for the current IEC 60601 standards.

In addition to the terminology change, a new leakage test has been added called total patient leakage current. The basis for the total patient leakage current test is to measure the leakage current when all applied parts required for the operation of the medical device are in contact with the patient (Fig. 3). This new test further protects the patient in light of devices that may have multiple connections and leakage paths.

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