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Earthing
Requirements
Conductive parts of a Class 1 medical device, which are separated from
live circuitry by basic insulation, must be reliably connected to earth.
The suitability of the earthing is tested as follows:
A current of 25A or 1.5 times the rated current of the device (whichever is the greater) is caused to flow between parts which are
earthed and the earth terminal or earth contact of the appliance inlet (if used). The resistance between these points under these conditions must be less than 0.1 ohm. If the device has a
non-detachable power cord, then the resistance between parts that are earthed and the earth pin on the plug must be less than 0.2 ohm under these conditions.
The resistance is measured under high current conditions, in order to simulate a fault to earth.
Design Tips
In many situations the earth conductor from the power supply cord or the appliance inlet would be connected to an earthing stud.
The wire is terminated with a closed loop connector which is fixed to the earthing stud or screw with a star or lock washer
and a nut. Other parts of the product that need to be earthed are connected by closed loop connectors to the same stud and locked with an additional nut. It is important that the earth
wire from the power supply cord is located at the bottom of the stud and locked with its own nut. The earthing stud must not be used for any purpose other than earthing. It cannot be
used, for example, for the mechanical fixing of parts other than the earth conductors. Its mechanical structure must also be such that it cannot be loosened from outside the device.
For example, it cannot be a post fixed with a screw from outside the product.
Reference
Complete requirements are given in Clause 8.6 of IEC 60601-1.
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