Care must be taken when interconnecting medical devices if electrical safety is not to be compromised. The diagram shows a simple arrangement, in which a connection is made between two locations by means of a data link. Each location is earthed back to the earth bus in the substation, and so it would seem that there would be no problem in running a data connection between the two locations.
However, every conductor will have some resistance, so that there will inevitably be a difference in voltage between the ends of the conductor, determined by the current passing through the conductor and its resistance. If we assume that the total earth leakage current flowing down the earth conductor from Location B to the substation earth bus is 1 A, and the resistance of the conductor is 100 mΩ. The voltage across the ends of the conductor will be 100 mV.
If we now assume that Location A is connected back to the main earth bus via a conductor having a resistance of 50 mΩ and that the total current following down the earth conductor is 100 mA, the voltage across the ends of this earth conductor will be 5 mV.
If a data connection is made between Location A and Location B, current will flow from B to A and back to the substation, the magnitude being determined by the resistance of the data connection. In most cases, the current will actually flow down the screen of the data connection, which is likely to have a comparatively high resistance. Although the current flowing down this path is unlikely to be sufficient to overload the cable, it is likely to induce noise into the data lines, corrupting the data.
There is also the possibility that the current flowing down the data connection will raise the touch voltage on any device connected to the data line. If the device is making contact with the patient either directly or indirectly, the increased touch voltage could do harm to the patient.
In some cases, this problem can be eliminated by using wireless connection between devices. Where this is not possible, then isolators can be inserted into the data connection. Suitable devices are available for insertion into video, serial or parallel data cables, networking cables, and USB connections. Tests should be made before and after fitting the isolators, in order to ensure that the touch voltages are within the recommended limits.
In any location where interventional procedures will be performed, it is important to establish just one earth point, to which all earths are connected. An example of a 30 A earth reference bar (ERB) is given in the MEIGaN document. The touch voltage should be measured between the ERB and all accessible and extraneous conductive surfaces within the location. Isolators should also be fitted when interconnecting medical devices that are supplied from an IPS supply and non-medical devices supplied from the normal mains supply.
