Electrical safety in medical technology according to 60601-1

The safety of electrical medical systems is a major issue that can affect us all. Recalls and notices for medical devices are published on the website of the Federal Institute for Drugs and Medical Devices (BfArM) almost every day. It is therefore not surprising that regulatory authorities impose special safety requirements on medical devices that are typically used on already weakened patients. These state-of-the-art requirements are documented in various standards. This article deals with the requirements of IEC EN 60601-1 (VDE0750-1) entitled "Medical electrical equipment - Part 1: General requirements for essential safety including essential performance." This standard describes the safety of medical electrical equipment in patient areas, i.e., equipment used on the patient or within a range of approximately 1.5 m around the patient.

Electrical safety according to 60601-1

The electrical safety of medical devices consists of the following components:

1. Limiting leakage currents
2. Protection against electric shock
3. Insulation of the application parts
4. Potential equalization
5. Safety in medical electrical systems

The importance of these points for the development of electrical medical devices will be described below.

Limiting leakage currents

Leakage currents are currents that usually occur unintentionally and flow along undesired current paths. However, they cannot be avoided; they must be carefully considered during development to keep them under control. Leakage currents can be caused by, for example,

• Capacitive current paths that form between different circuit components due to the alternating voltage.
• Coupling of interference into ground loops.
• Filter capacitors against the protective conductor.

Different leakage currents are distinguished in the 60601-1:

• Patient leakage current

The patient leakage current flows from the applied part, which is the part of the circuit that is directly connected to the patient.

• Touch leakage current (formerly housing leakage current)

The maximum permissible touch currents are defined for normal use and the first fault. They are significantly lower for patients than for normal users, as they are considered vulnerable individuals and are permanently connected to the device. The old name "casing leakage current" already suggests one path through which these leakage currents can flow.

In the 3rd edition of 60601-1, the term "casing leakage current" is replaced by "touch current." There are additional leakage currents via the protective conductor terminal and the patient connection.

The above values are effective values for touch leakage current. Patient leakage and patient auxiliary currents may be significantly lower, depending on the classification of the applied part.

• Earth leakage current

This is the current flowing from the power supply to the protective conductor, e.g., via filter capacitors. This current may not exceed 5 mA under normal conditions, which is lower than the limits specified in other safety standards.

Leakage currents are typically measured using a circuit defined in the standard, the so-called measuring device. The measuring device is a low-pass filter, so that higher frequencies are attenuated according to their reduced physiological effect. However, no leakage current may exceed an effective value of 10 mA when measured frequency-independently.

Protection against electric shock

This is ensured by the correct classification of the device (depending on the supply) and is guaranteed by the insulation distances. Here, too, the device must be safe in the event of a first fault. Therefore, all insulation distances must consist of at least two protective measures, both of which can withstand the rated load. In the event of a fault, it is assumed that only one protective measure fails, and the user or patient is still protected from electric shock by another measure.

Protection against electric shock is documented in the insulation diagram.

Insulation of the application parts

The third component is the classification of the device according to its application area, also known as type classification. Here, devices are divided into different classes according to their application area and must meet different requirements depending on the class. For example, an external ECG monitor requires a Type BF applied part. This means that the leakage currents must not exceed a certain value, as the device is permanently connected to the patient, and the applied part must be galvanically isolated.

In this context, the concept of first-fault safety is also important. It states that the device remains free from unacceptable risks in the event of a first failure. This is ensured by ensuring that the essential performance characteristics are met even in the event of a first failure, since their absence would, by definition (60601-1 paragraph 3.27), lead to an unacceptable risk. Of course, a residual risk always remains, and this is assessed in a risk analysis according to DIN ISO 14971.

The measures for isolating the applied parts are best documented in the insulation diagram.

Potential equalization

There is no explicit requirement for a separate connection for an equipotential bonding conductor, but if one is provided, then 60601-1 also sets out some requirements, e.g. that the conductor must be able to be removed from the connection without tools and that the connection must not be used as a protective conductor connection.

The function of potential equalization is to provide an additional measure to prevent minimal contact voltages that can occur between devices and flow through the patient. Currents as low as 10 µA can trigger physiological effects and can be prevented by additional potential equalization. Voltages of several volts can occur between different wall sockets, even within a room, which can lead to significant patient currents between devices.

The operator of medically used rooms is responsible for equipotential bonding.

Safety of medical electrical systems

Before 2006, medical electrical systems (ME systems for short) were covered by a separate standard, namely DIN EN 60601-1-1. With the 3rd edition of DIN EN 60601-1, this standard was incorporated into the basic standard, so that ME systems are now covered in Section 16.

The definition of medical electrical systems in 60601-1 is:

Combination of individual devices, as defined by the MANUFACTURER, of which at least one must be ME EQUIPMENT, and which are connected together by a FUNCTIONAL CONNECTION or by the use of a MULTI-SOCKET.

Therefore, the use of multiple sockets in medical rooms is already considered an ME system! This means, for example, that the sum of the earth leakage currents of all devices connected to the multiple socket must not exceed 5 mA.

Even devices connected via radio links (e.g. Bluetooth) are ME systems as soon as an ME device is involved!

Other important aspects for ME systems are:

• Devices in the ME system must meet the requirements of the IEC or ISO standards applicable to these devices.
• Leakage currents according to DIN EN 60601-1 must be observed.
• There are special requirements of 60601-1 for multiple socket outlets.
• Conductors connecting different parts of an ME system must be protected against mechanical damage.

So much for the electrical safety requirements according to DIN EN 60601-1. This article provides only a rough overview of the basic philosophy and can never replace studying the standard and consulting the notified body for specific questions.

If you have any ideas, questions, or requests, please send me an email. I look forward to discussing medical electrical devices.

Best regards

Martin Bosch

Further reading and links:

Of course, the basic standard for the electrical safety of medical devices: 60601-1, optionally in the edition of IEC, EN, DIN or the other countries, for Europe the current edition is EN 60601-1:2006+A1:2013.

Armin Gärtner: Medical Device Safety, Practical Knowledge on Medical Technology Series, Volume 2, TÜV-Verlag

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