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
The resistance is measured under high current conditions, in order to simulate a fault to earth.
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.
Complete requirements are given in Clause 18 of IEC 60601-1.