DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

TESTING FOR COMPLIANCE 127designed for applications where defibrillation protection is desired, withstands pulses of upto 5 kV for 10 s. At prices in the range $29 to $34 each (one patient connection), theadded protection provided by these fault interruptors is certainly affordable for evaluationprototypes, and sometimes even for the final design.TESTING FOR COMPLIANCEAlthough the presentation of medical device electrical safety standards above is by nomeans intended to replace the actual standards in scope or in content, it hopefully introducedmany of the most important design requirements covered by the major standards.With medical equipment, however, designing solely for compliance is not sufficient. Theconsequences of a malfunctioning device can be so serious that testing to ensure properperformance is of utmost importance. As such, construction standards are only one aspectcovered by applicable safety standards. The other major aspect defined by the standardscomprises performance requirements.Within this second category, standards specify the multiple tests which are applicableto diverse types of equipment and to identify in detail the criteria for compliance. In fact,almost every construction requirement is linked to a certain performance requirementwhich defines the testing that is performed to verify the acceptability of the construction.In essence, the electrical tests described by the standards are designed to probe insulation,components, and constructional features which could lead to a safety hazard under eithernormal or single-fault conditions.Ground IntegritySince the enclosure of the medical device is the first barrier of protection against the risksof electrical shock, the first test to be conducted is one to assess the integrity of the protectiveground which guards a metallic enclosure and any other grounded exposed part. ULstandard 2601-1 establishes that the impedance between the protective ground pin in thepower plug and each accessible part which could become live in case the basic insulationfailed should be less than 0.1 . The standard also requires that the test be conducted byapplying a 50- or 60-Hz ac current with an RMS value of 10 to 25 A for 5 seconds. Despitethis, however, a reasonable approximation of this measurement can be obtained by usingthe 1-A dc current supplied by the circuit of Figure 3.21. Resistance is then assessed bymeasuring the voltage across the grounding path.Here, op-amp IC2 and power FET Q1 form a voltage-to-current converter that is drivenby a reference voltage set by R6 to maintain a 1-A constant current on a conductor connectedbetween the ground terminal of J5 and connector J4. Power for the circuit is derivedfrom three alkaline D cells, providing a maximum voltage compliance of approximately4.5 V. Because full-range operation of the circuit is accomplished by driving the gate ofQ1 well above its source-to-drain voltage, IC2 is operated from 12 V generated by chargepump IC1.You may notice that J2 and J3 are labeled to be connected to a Kelvin probe. This typeof test probe separates the point through which current is introduced from that throughwhich the voltage across the unknown resistance is measured. The use of such a techniqueis required for low-resistance measurements because it effectively excludes the resistanceof the test leads and avoids the voltage measurement errors that are often introducedby high-current-density concentrations on the current-injection terminals. As shown inFigure 3.22, a large alligator clip (e.g., Radio Shack 270-344) can easily be converted intoa Kelvin probe by replacing the standard metallic axis by a nylon bolt with nylon spacersto isolate the jaws from each other and by covering the ends of the inner spring with a