By Felix Wedel, product manager, Schaffner EMV AG
Bringing a new medical device to the market is a complex task. Technical innovation or a better solution to a problem is not enough in itself. With lives on the line, flawless reliability and safety is a must. Regulatory bodies impose high standards to protect lives and ensure safe product designs. Meeting these strict requirements in the medical field requires extensive experience and expertise.
EMC (Electromagnetic compatibility) regulatory requirements are only a part of the complex requirements matrix but should not be taken lightly. Solving EMI (Electromagnetic Interference) issues and passing subsequent EMC testing can become a roadblock between product design, development, and bringing a product to market.
EMC regulatory requirements define the acceptable limits of electromagnetic interference between electrical systems. Such a system shall not propagate electromagnetic energy in conducted or radiated/spatial form above a defined level. On the other hand, electrical systems need to tolerate defined electromagnetic levels and always be able to function safely. If both conditions are met, an electric system is deemed “electromagnetically compliant.”
The limits to the described emission and tolerance levels can be found in IEC’s CISPR 11.
Ideally, EMC performance is monitored continuously during a development cycle. If an EMI issue can be identified early in the design phase, solving it has a low impact on project timing and costs (“rule-of-ten”). Troubleshooting at the end of a product development cycle may even result in a re-design. That being said, an engineering team sometimes need to operate in shortened design cycles and face tough competition situations. Time to market can be of essence. Having a good EMC filter in combination with an EMC friendly design, is a good place to start.
Another important global standard for medical products is the IEC 60601-1. It describes requirements for basic safety and essential performance of medical electrical equipment (terminology IEC/EN 60601-1).
Leakage currents – Complying to the IEC/EN 60601-1 for Medical Electrical Equipment
For patient safety as well as EMI mitigation, leakage currents play an important role. These currents are an effective method for most applications to mitigate EMI, but counter-productive for high patient safety. For medical applications leakage currents need to be limited.
In electric systems, voltage naturally exists between the live/phase connector and the voltage potential of protected earth. In normal operation small leakage currents flow from phase to protected earth. In the case of medical electrical equipment with physical contact to patients, the requirements for products need to be stricter. In the hypothetical case of an earth fault, the current could flow through a patient touching the device. For sensitive applications as syringe pumps, medical ventilators, lasers or patient monitoring devices, the maximum permissible leakage current is therefore limited to 0.1 mA AC in normal operation. In the single fault condition, where, for example, one line of the mains connector is disconnected, 0.5 mA AC is permissible. This limit is considered as safe level for the greatest risk, which is ventricular fibrillation in the heart of the patient.
To reduce this risk, EMC engineers often apply “medical” EMC filters which have no capacitors from live to protected earth. (e.g., Schaffner FN9262xB..). These filters do not add leakage current towards protected earth to the equipment and therefore facilitate compliance to IEC EN 60601-1. The simplest version has only one capacitor between live (P) and neutral (N).
In non-medical EMC filters, capacitors (not in the picture) would mitigate EMI between live (P)/ neutral (N) towards protected earth. In a medical grade EMC filter without a Y-capacitor, this so-called common mode functionality is provided by the common mode inductor, marked “2x L1” in the picture above.
The Means of Protection (MOP) safety classification in IEC60601-1
Safety requirements in IEC60601-1 define two categories of product safety. Means of Patient Protection (MOPP) and Means of Operator Protection (MOOP). Each of these categories are subdivided in values for basic or double insulation.
|Classifications||Isolation||Creepage / Clearance||Insulation|
|1 x MOOP||1.500 VAC||2.5 mm / 2 mm||Basic|
|2 x MOOP||3.000 VAC||5 mm / 4 mm||Double|
|1 x MOPP||1.500 VAC||4 mm / 2.5 mm||Basic|
|2 x MOPP||4.000 Vac||8 mm / 5 mm||Double|
Choosing between MOPP and MOOP seems straightforward. Medical electrical equipment which does not come in contact with patients can be rated with the lower requirements of operator safety: MOOP. This saves time in costs in the certification, but needs certainty, that no patient can come in contact unintentionally.
The standard also specifies situations, where more than one MOP is required, in case either one fails. Now, double insulation needs to be applied.
All parts of the medical electrical equipment need to be reviewed for their MOP level. “Medical” EMC filters typically provide the value in the datasheet. Some filters even comply to the highest rating (2 x MOPP) without any additional insulation measures (e.g., Schaffner FN9274).