Clinical engineering and the entire healthcare technology management (HTM) community has been faithfully and obediently measuring medical device leakage currents since its beginnings over 50 years ago. Arguably, the very birth of our profession was fueled by the fear, ignorance and misinformation surrounding these shadowy microamps. So much so, that leakage current testing has become not only synonymous with “electrical safety” and an integral task within device periodic maintenance inspections, that to suggest they be eliminated approaches the blasphemous. Nonetheless, the elimination of routine leakage current testing is precisely what is being proposed here; the collective labor hours spent making these useless measurements can be better utilized elsewhere. Here’s why.
As illustrated in Figure 1, all AC line-operated devices with an electrically conductive case, or conductive surface(s) capable of being energized from within, will likely have a measurable 60 Hz leakage current from this case to ground. The origin of this leakage current, of course, is the incoming power line voltage (nominally 120 Vrms) with the conductive pathway, being represented by an equivalent series-connected resistor and capacitor. Here, this capacitor may be physically present (as seen within incoming power line RF filters) or, and more often, due to parasitic or stray capacitance that exists anytime two conductors are separated by an insulator; any conductive case or surface effectively becoming one of the plates of this “stray” or virtual capacitor.
Experimentally, the effective values of this resistor-capacitor (RC) circuit can be determined by the simultaneous oscilloscope measurements of the AC line voltage and leakage current (as a voltage across a 1K ohm resistor). If the leakage current waveform “leads” the line voltage waveform, this then implies that the impedance (ZLeakage) of this conductive pathway is capacitive in nature. From an actual electrosurgical unit possessing 81 microamps of leakage current, this equivalent R-C network is determined from the equivalent circuit, therefore, from the “hot” side of the power line to the equipment’s case being a 155K ohm resistor in series with this 1.8nF capacitor.
While this equivalent R-C network does not represent discrete components, it does effectively determine the magnitude of the measured leakage current. It is this R-C equivalent pathway and coupling to the device’s conductive case (or surfaces) that also dictates and limits measurable leakage currents.
For this now available and maximum leakage current to deliver either a macro- (from outside of the body) or microshock (to within), there are five independent and necessary conditions – all of which must be present – for a shock to occur. Namely, and as illustrated in Figure 2, these include:
Since all five of these conditions must exist before a shock can be delivered, eliminating any one them also eliminates the shock risk. For example, if the device case is non-conductive or has no conductive surfaces capable of being energized from within, there is no shock hazard; even with a conductive case, if it has a reliable low-impedance connection to earth ground, there is no shock hazard.
Even if any event cannot be eliminated, the chance or compound probability of getting a shock from the presence of these five independent events is so ridiculously low as to approach the near impossible. As such, leakage current measurements, aka “safety testing,” do nothing to prevent a shock or ensure the electrical safety of a device.
Verifying and continuing to routinely inspect the device’s line cord, strain relief, plug cap, however, remain most important and necessary. Similarly, ground cord-to-case continuity and resistance checks and outlet tension testing remain useful as well. Since device leakage currents will always follow a low-resistance pathway to earth ground, maintaining and ensuring ground cord integrity makes leakage currents – of any magnitude – a moot issue; further supporting the elimination of their routine and time-consuming measurement.
Clearly, none of us within the HTM community would ever condone, support or encourage any action that puts our patients or staff at risk. The more real risk, perhaps, is continuing to perpetuate the illusion that electrical safety is somehow being assessed and ensured by measurements that have no ability to do so. We can and should be using our limited time and resources more wisely.
– Larry Fennigkoh, Ph.D., PE, CCE, is an adjunct professor of biomedical engineering at Milwaukee School of Engineering.
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