FEATURE POWER
A shock to the system Standards for medical devices
The specification for power supplies in medical devices must match the end use for a safe and cost-efficient solution. John Quinlan, senior product marketing manager at Murata, considers some power supply characteristics, with insights into how they affect patients
T
he market for medical devices is being driven by technological advances and an ageing population suffering from a growing list of chronic conditions. Therefore, designers are aware that there are particular requirements for power supplies in the healthcare environment. But, the relevant standards can be confusing, with multiple options to achieve a safe result. The international standard for the basic safety and performance of electrical equipment is IEC/EN 60601-1, with the largely equivalent American ANSI/ AAMI ES 60601-1. Countries around the world have adopted the IEC standard, but with declared local deviations. The complex standard covers all application possibilities, from heart surgeries to non-patient connect equipment only accessible to qualified ‘operators’. For power supply equipment, the main requirement is to prevent electric shock to patients and personnel, although other risks have to be considered. According to the standards, to prevent electric shock, power supplies must incorporate measures of protection (MOPs), which can comprise physical separation, barriers and arrangements of protective earthing. Two MOPs are necessary and are of two types – measures of operator protection (MOOPs) or measures of patient protection (MOPPs) – with MOOPs being allowed in environments where patient contact is unlikely, such as analysis labs, and the more onerous MOPPs required where patient contact is normal or likely. In the patient environment, there are further categories of equipment. There’s ‘B’, or ‘body’, where the potential patient connection is grounded. There’s ‘BF’, where the connection is ‘floating’.
30 DECEMBER/JANUARY 2020 | ELECTRONICS
And there’s ‘CF’, or ‘cardiac floating’, with connection to a patient’s internal organs. B, BF and CF connections affect the allowed leakage current that can pass through the human body, with different maximum levels specified under normal and single fault conditions.
POWER SUPPLY SPECIFICATIONS Power supplies marked ‘medically approved’ will typically have 2 x MOPPs for use in operator or patient environments. The requirements for internal clearance distances do increase depending on altitude, starting at 2000m for MOOPs and 3000m for MOPPs, so a power supply will have an altitude limit to which the MOP rating applies. There are eight capital cities at over 2000m elevation, for example, including Mexico City with its 12 million-plus inhabitants, so a power supply meeting just the minimum altitude clearance distances, claiming to be ‘medically approved’, would actually be deemed unsafe for operator protection here. Physical separation of the patient from dangerous voltages is only part of the story – there will be stray capacitance from live parts to patient connections, and intentional capacitance from live parts to ground in power supplies, causing leakage currents at mains frequencies that can be harmful or even fatal under fault conditions. A potentially dangerous condition is when a power supply is Class I – that is, in a metallic earthed case, with interference suppression (RFI) capacitors fitted from live and neutral to the
One of many use cases - the hospital bed, now embedded with various sensors for vital signs and motors for position adjustment. The integrated power supply must conform with established standards to protect the patient using it
earthed case. If a single fault occurs on an open supply ground wire, the case then ‘floats’ with a connection through the RFI capacitors to AC mains. If a grounded patient touches the case, current will flow through their body, limited by the value of the capacitance. To prevent injury, the capacitors must not exceed a value that allows more than a specified leakage current with the mains voltage at its highest tolerance. Under normal conditions, the leakage current from this power supply is limited by stray capacitance, which typically meets the limit of 10 µA for the most severe CF application. If the ground line breaks, the limit is now 500 µA for B and BF applications, and 50 µA for CF, putting an upper limit on total capacitance value. This considers leakage from one power supply unit with a single failure. The figure of 500 µA (B and BF) doubles to 1 mA for the ‘total leakage current’ caused by multiple applied parts with any signal input/output leakage paths included.
When the power supply is permanently wired in and maintained, such that the likelihood of the ground connection failing is very small, leakage current limits are increased and internal capacitors from line and neutral to ground can sit at higher values for good RFI suppression. Class II power supplies do not have a leakage path to earth, but must comply with ‘touch current’ limits to accessible, metallic parts.
Murata
www.murata.com / ELECTRONICS
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