Why isolating medical devices and maintaining a stable power supply is key to accurate diagnosis
16 June 2015
The increased complexity of medical device setups is causing concern in the medical industry. As more and more healthcare professionals use a variety of electrical equipment as part of their diagnosis and measurement regime, there is a knock-on effect on the stability of the mains power supply. Steve Hughes explains the importance of electrical isolation and good product design.
Notepads have been replaced with iPads, mechanical beds with electrically actuated adjustable beds and traditional film imaging has been substituted with the latest digital picture archiving and communication system (PACS). Add to this other innovations, such as endoscopy cameras and 3D printed dental systems, and it’s clear how much the entire healthcare system depends on access to mains power and backup.
As well as this, the advent of wearable devices manufactured by consumer facing businesses is increasingly pushing consumer original equipment manufacturers (OEMs) to develop devices that provide diagnostic and monitoring features. Functions that, until now, were reserved for professional medical devices are easily accessible to patients using smartphone apps and wearable devices such as the Samsung Gear and the newly released Apple Watch.
For instance, the US Food and Drug Administration (FDA) recently approved the first medical app that will allow remote monitoring of patient glucose data. With this precedent set, it will not be long before more powerful medical devices are directly accessible to patients.
Voltage distortion, electrostatic discharge and electromagnetic interference
However, the increasing electrification of devices creates problems on the mains power supply. AC to DC power conversion introduces voltage distortion, current harmonics, electrostatic discharge, power surges and electromagnetic interference (EMI). These power quality problems can affect the calibration and sensitivity of diagnostic devices used by doctors and healthcare professionals. Erroneous test results can result in misdiagnosis and potentially harmful treatment plans for patients.
The increased use of smaller and more compact power supplies in electronic devices and controlled motors has largely been made possible through the use of switch-mode power supplies (SMPS) in personal usage and variable speed drives (VSDs) in commercial and industrial usage. This is achieved by manipulating the mains power supply, using components in rectifier and chopper circuits, in a process of high frequency switching, or pulse width modulation (PWM).
Although PWM achieves very low power losses, the process introduces harmonic currents into the supply. Harmonics are essentially multiples of the fundamental 50Hz frequency and are responsible for numerous problems, particularly in industrial environments.
The most immediate problem is increased energy consumption. The distortion caused by harmonics results in a non-linear, non-sinusoidal waveform. For example, if the fundamental current drawn is 70A, this capacity may consist of 20A of harmonic frequencies, meaning that the total current the system has to supply is actually 72.8A
This increased consumption is often taken for granted and goes unchallenged. It’s only when harmonic currents lead to more noticeable component damage that concerns are raised.
Collectively, these problems can begin to affect both the local system internally and the wider supply system externally. Internally, electromagnetic interference (EMI) can begin to affect telecommunication equipment and metering apparatus. Significant distortion of the current waveform locally can begin to distort the voltage supply externally, creating symptoms such as flicker on public low voltage networks.
And it’s not just at the material level. The prevalence of harmonics can cause operational problems too. System design and equipment purchasing decisions can become more difficult for managers. When the time comes to expand the system it’s quite possible that the power capacity of equipment will have been reached prematurely, meaning that equipment may need to be overspecified to accommodate the harmonics.
Endoscope using REOMED transformer
Despite the many cost saving measures introduced over the last decade, healthcare providers are still some of the most energy intensive users of electricity in the UK. The NHS, for example, spends more than £750m (about €1 billion) annually, spread over 2,300 hospitals and 10,500 general practices, as well as numerous additional trusts. It’s not surprising then that the carbon footprint of the NHS is about 25m tonnes per year. To make a bad situation worse, energy costs in the UK are set to rise faster than inflation.
Government-funded measures in the past have included automatic LED lighting, efficient heating and ventilation, biomass boilers, pipes and insulation. At an estimated cost of £1.5bn capital outlay, these measures are not cheap. We’ve calculated that an endoscope using a REOMED transformer uses approximately 50 per cent less energy, an equivalent saving of £900 per year for a product that is continuously powered.
European legislation has focused on tackling these electrical issues by introducing minimum requirements for the electrical isolation of medical equipment. For instance, IEC 60601-1, the regulatory standard that governs the general requirements for basic safety and essential performance of medical electrical equipment, has recently been updated to the third edition.
The directive previously classified devices into three areas based on how closely they are used to the patient’s body. Type B devices operate within a six foot vicinity of the patient without bodily contact. Type BF makes physical contact with the body and Type CF makes physical contact with the heart.
MOPP, MOOP, impedance and creepage distances
Part of the transition from the second to third edition involved introducing clearer guidance on protection circuitry in relation to AC-DC power conversion, distribution and protection. New devices can use various means of patient and operator protection (MOPP and MOOP) to meet the stringent requirements related to isolation, impedance, insulation, creepage distances, clearance and leakage currents.
Most large equipment used in a clinical setting, such as magnetic resonance imaging (MRI) machines, anaesthesia units and ventilators, contain an embedded transformer or are disconnected entirely from the mains by using battery packs. However, devices in patient environments do not usually contain a transformer and it is not always cost effective, practical or necessary to use battery packs. It is for this reason that we developed the REOMED isolation transformer.
Toroidal-core insulation transformer
Intended to provide isolation of medical devices in patient environments, including doctors’ surgeries, hospitals, dental practices and care facilities, the REOMED transformer provides safe galvanic separation between the primary and secondary circuits. This double isolation results in a very low leakage current of less than 500µA and, as well as this, we have designed a toroidal-core insulation transformer to conform to the strict air and leakage clearances.
The REOMED offers an easy way to meet European electrical isolation standards by simply retrofitting the transformer into an existing setup. However, it is important to remember that the legislation also goes further in stipulating requirements for ergonomic design.
In the United States, the FDA recalled nearly 200 products that had failed due to electrical problems, mechanical faults due to poor design, bacterial contamination during manufacturing and device software issues.
At REO we have found that sustainable design is only achieved through a consultative process between doctors, regulatory bodies, OEMs and patients. By undertaking thorough design testing, manufacturing controls, and post-installation servicing, OEMs can hope to provide the successful wave of next generation medical devices and cope with the pace of technology in the healthcare environment.
Steve Hughes is managing director of REO UK http://www.reo.co.uk/home