Did you know that even 10 uA of current is hazardous for a human

If you want to know more about the basics of electrical safety of Medical Devices, I encourage you to read this article.

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I will very shortly describe what areas of interest must be covered in order to make electrical Medical Devices safe and some most basic ways to achieve this. I strongly recommend this article not only to electrical and systems engineers in Medical Industry but to any person wanting to increase their horizons on how devices should be designed and constructed. Who knows? Maybe one day you will save someone because during designing e.g., the 110-230V AC-connected to the IoT device you followed just one more safety rule, as you remembered what you read in this article?

You can use this knowledge also as good design practice, to ensure higher reliability and safety of your designs.

I will refer often to IEC -1:+A1:+A2: standard, as it describes general requirements for Basic Safety and Essential Performance of medical electrical equipment. This document provides more details, my target is only to make you realize the importance of electrical safety and where to look for more data.

You can also find this article in Polish.

Please note: I’m using all CAPITAL LETTERS words on purpose: whether they are used, the term is a direct reference to the standard.

(*At least according to standard IEC -1:+A1+A2 (par. 8.7, table 3). This value refers to Normal Condition DC current).

What can I start with?

The first questions you should ask yourself are: how can a person (patient or operator) have contact? Where are any hazards in this area?

It is beneficial to start with a device block diagram that depicts all required isolations (so-called “Isolation diagram”). It can be very simplified, but allows you to notice Operator and patient leakage currents paths, e.g.:

The next step is to define the required levels of MOPP (Means of Patient Protection) and MOOP (Means of Operator Protection). Both of these MOPs (Means of Protection) are used to reduce RISK due to electric shock acc. According to the IEC -1., MOOPs are required in every spot where an operator can have a contact (like housing, a.k.a. ACCESSIBLE PARTS). For MOPPs you need to specify the kind of every APPLIED PART.

If you don’t know whether an operator (e.g., Nurse, or caregiver) can touch any hazardous part, the IEC -1 specifies even a test finger to evaluate for ACCESSIBLE PARTS! Please, note that this is not necessarily the same test finger as for IP testing, although generally IP3X-rated (and most IP2X) devices do not need any additional checks.

APPLIED PART type is important as relevant limits and minimum number of MOPPs are based on this qualification. Generally, there can be Type B, BF, and CF APPLIED PARTs requiring 1xMOPP or 2xMOPP. For example, Type CF requires reinforced insulation (2xMOPP) between such an Applied Part and other Patient connections of accessible parts. For details, please refer to IEC -1 in its latest revision. Clause 8 gives thorough information on how to achieve protection against electrical hazards. Let’s get back to 10uA stated in the title. See Table 1 based on standard IEC -1, presenting the limits below:

Some of the readers might have some kind of “certificate of qualifications” for working with High Voltage. They might be shocked by such low limits, as “general” safety limits of currents need to start at 0.5 mA (500 uA) to even “feel” the current. But remember: patients very often have some other medical conditions, cardiac problems, implants, etc., and can have contact with many Medical Devices at the same time. The IEC -1 exactly defines how to measure these values, so I will refrain from going into detail on this.

What are NC and SFC?

At this point there might be some unexplained terminology:

• NC means NORMAL CONDITION
• SFC means SINGLE FAULT CONDITION

NC might sound self-explanatory, but must be taken into account together with SFC. In short: SFC is any fault detected before making any unacceptable risk or before the next component is faulty.

While making a design, you should ask yourself: if this component breaks in any way, will I detect it in time? If not, then it’s a Normal Condition – you should always assume it’s broken, unless… Well, there are some ways to deal with that