Medical devices

Why do medical devices need 2xMOPP?

Also see: Does 2xMOOP / 2xMOPP require two separate converters / layers of isolation?

For medical devices the IEC60601 norm specifies more stringent requirements for electrical safety like the 2xMOPP requirement (MOPP = Means of Patient Protection). Many developers wonder why the requirements are different to other types of devices like consumer devices.

  • It is assumed that patients might already have impaired health and hence any additional damage caused by e.g. a malfunctioning medical device might have greater consequences than in healthy patients.
    • For example, assume that a patient with a severe infection receives a light  electric shock from a malfunctioning infrared thermometer. While the electric shock is unrelated to the original injury of the patient (i.e. the infection), having to treat both issues might be much harder than just treating the infection: For example, the infection might spread to the body part where the electric shock occured, or the additional inflammation due to the electric shock might contribute to the deteriorating health of the patient
  • Additionally, if a patient receives e.g. an electric shock from a medical device, many patients will not trust medical devices – and potentially even the entire medical system – any more, resulting in less capability of treating those patients.
    • Imagine if you would receive a small, unpleasant (but not in any way dangerous) shock every time you use a thermometer to measure fever. Most people would refrain from measuring their temperature when they are ill in order to avoid the impleasant shock. This will lead to many patients being diagnosed at a later stage of their disease which would in turn impede the treatment.
  • Furthermore, it is assumed that the medical device will be used under circumstances like pandemics, where any additional injury will unneccessarily consume medical resources such as hospital beds and keep medical personnel from treating potentially more serious injuries.
Posted by Uli Köhler in Compliance, Electronics, Medical devices

Does 2xMOOP / 2xMOPP require two separate converters / layers of isolation?

Also see: Why do medical devices need 2xMOPP? 

Developers working on medical devices are frequently faced with the challenge of designing according to the stringent electrical safety requirements of IEC60601-1.

One question is whether you need two separate levels of isolation in order to fulfil the 2xMOOP and/or 2xMOPP requirements.

For example, you could design your device like this in order to achieve 2xMOPP isolation:

This is expensive and often unneccessary!

First, you have to understand that IEC60601 does not specify how many converters you have to use, it mandates that you have a minimum isolation voltage! 2xMOPP does not neccessarily mean that you have 1xMOPP plus an additional 1xMOPP converter, it just means that you have to fulfil more stringent isolation requirements than for many non-medical products.

The isolation requirements are:

  • 2xMOOP: 3kV isolation
  • 2xMOPP: 4kV isolation

IEC60601-1 allows two methods of achieving MOOP and MOPP:

  • Either you use two separate layers of isolation, like the two converters shown above
  • Alternatively, you can use reinforced isolation, i.e. a single converter that has additional isolation.

In most cases, IEC60601 products use reinforced isolation instead of having two separate converters.

Some arguments for preferring reinforced isolation as opposed to using two separate converters are:

  • Two converters are typically more expensive than using a single, reinforced isolation converter
  • You have additional risk of one of the components not being available for purchase any more, which could lead to expensive re-certification of your device
  • If you have two converters, you will have approximately two times the risk of one of them being defective (for example, because of aging capacitors after operating your device for a couple of years.)
  • Your device will have lower efficiency since more power is wasted in the two converters than would be wasted in a single converter.

Source & more reading: Johner Institute

Posted by Uli Köhler in Electronics, Medical devices