Active medical devices – Protective earthing as a means of protection

Protective earthing of accessible metal parts as a means of protection against electrical hazards: What is behind this? In this article we would like to show you why protective earthing can be lifesaving in the truest sense of the word.

In our last article „Q&A – means of protection” we discussed the various possibilities of protection against electrical hazards.

As a recap: the standard IEC 60601-1 considers the following four types of means of protection:

  1. Protective earthing of accessible metal parts
  2. Solid insulation
  3. Creepage distances and air clearances
  4. Protective impedance

Let us look more closely on the first one: protective earthing of accessible metal parts – what is important here?

In addition to the L (live) and N (neutral) wires in home installations, there is also a PE (protective earth) wire. The job of this PE line is to carry the current in a fault situation (for example: a short circuit) in a safe way until a protection measure (circuit breaker or fuse) responds to the fault.

But what does this have to do with medical devices? Well, so far, nothing – but it should contribute to the understanding of the protective earthing concept…

Let us consider a medical device is designed with a metal enclosure. In the case of a single fault (failure of a protective means) such as a short circuit between an active wire in the device and the enclosure, touching the enclosure could have fatal consequences. But now the protective earthing enters the picture. Connecting the accessible metal enclosure to the terminal of the protective earth of the device will lead to the effect described earlier in this article: the safe conduction of the fault current. Even in single fault condition we can safely touch the enclosure without risking an electrical shock. The medical device will be single fault safe for patients, operators, and bystanders!

Devices designed according to this concept of protection are called class I devices. These devices must be equipped with a mains power supply cord with three conductors (L, N, and PE).

The terminal for the protective earth connection on such devices shall be marked with the symbol shown in picture 2 (IEC 60417-5019).

(Picture 2)

So, we have learned that a protective earth connection can be lifesaving in the truest sense of the word. That is the reason why the standard IEC 60601-1 defines specific requirements for these protective earthing. Let’s have a look at the most important points:

  • The protective earth connection shall be made before and interrupted after the supply connections are made or interrupted. These requirements will be fulfilled by having a longer earth pin than supply pins in the mains plug or the appliance inlet.

  • Protective earth connections shall be able to carry fault currents reliably and without excessive voltage drop. Therefore, they must be of low impedance.

    Picture 1
    As shown in picture 1 the impedance between the earth pin in the mains plug and the accessible metal part shall not exceed 0.2 ohm. The impedance between the earth pin in the appliance inlet and the accessible metal parts shall not exceed 0.1 ohm. These requirements are tested with a current of 25 A.

  • Furthermore, it is important that the protective earth connections can not be loosened from the outside of the device – either with or without tools.

  • If protective earth connections are made by wires or cables, the cross-sectional area of these shall not be less than those of the supply wires or cables.

In summary, we can say, that protective earthing is a commonly used solution. And in addition, such a connection to the protective earth has the advantage of supporting the fulfillment of the requirements of EMC (Electromagnetic compatibility).

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