Many operating theatres and angiography X-ray rooms (group 2 medical locations) include pendant mounted mains sockets. These provide a convenient means of powering the many mains powered medical devices now in use. This guidance is offered in order to assist in achieving the high standards of electrical safety that are required in such locations.
Since the pendant sockets will often be used to supply life critical equipment, the supply to the sockets will in most cases be provided by an isolated power supply (IPS). An IPS is required in order to increase the supply resilience by reducing the impact of first fault conditions downstream of the IPS. The IPS is usually combined with an uninterruptable power supply (UPS) which prevents disruption of the supply due to problems upstream of the UPS.
Sufficient sockets should be provided to discourage the use of extension mains leads and multi-socket outlets, since these are much more prone to failure than standard, fixed mains sockets. Any wall mounted mains sockets that supply medical devices used in the same location should also be fed from the IPS supply. Patient connected devices should not be supplied from a mixture of IPS and TN-S supplies.
An IPS has no line and neutral; both sides of the supply are referenced to earth only via the stray capacity of the secondary of the transformer and its associated wiring. Therefore the mains sockets installed in the pendant should be either un-switched or double pole switched. In order to identify IPS sockets, the sockets should be blue in colour, engraved with the words ‘Medical equipment only’ in white. Where this is not practical, white sockets should be mounted on a blue background, and engraved ‘Medical equipment only’ in blue.
Equipment that is not CE marked as medical electrical device (conforming to IEC 60601) can have high earth leakage currents and should not be used within the patient environment. If they are connected to an IPS fed mains socket they may trigger the insulation monitoring circuits of the IPS.
Users should always be advised to check that every device is turned off before plugging it into a mains socket. Most modern equipment is powered by means of a switch-mode power supply, which at switch-on has a high inrush current. This is caused by having an input rectifier connected across a fully discharged capacitor inside the power supply. Inserting the mains plug can make a ‘dirty’ contact, which may result in excessive inrush currents, high enough to trip out the overload circuit of the IPS.
In group 2 medical locations only 10 µA of current may flow between any two earthed points (10 mV across a human body model). In order to ensure that this limit is maintained, all earthed equipment needs to be at the same (earth) potential. This can most easily be achieved by fitting so called ‘clean earth’ sockets on the pendant and for any other sockets used to supply any other medical electrical equipment within the patient environment.
A clean earth socket has two separate and isolated earth connections – one which goes to the earth pin receptacles of the sockets, and the other which goes to the fixing screws (back box). The connection to the earth pin receptacle should run via a separate cable, back to the earth reference bar (ERB). The fixing screws may make contact with the structural earth via the body of the pendant (or back box) and so be at a different potential to the ERB. Therefore a separate earth is connected to this ‘dirty’ connection and also returned to the appropriate point on the ERB.
Note: there is a difference between clean earth sockets and double earth sockets. Double earth sockets simply have two receptacles for the earth wires, connected to both the fixing screws and the earth pins. Some of the companies supplying cardiac X-ray equipment will call for the metal body of the pendant to be insulated from the structural earth and be earthed independently, back to the ERB. This is the best way to ensure the required limit of touch voltage (10 mV) is maintained, and is part of the installation requirements when mounting their X-ray equipment within the patient environment. The same general rules also apply when installing other devices within the patient environment (theatre lamps, etc.).
In other cases, the body of the pendant will be bolted to the structural steelwork of the room, but should still be earthed back to the ERB by means of a dedicated earth conductor. Any mains sockets on the pendant must still be of the clean earth type and earthed separately, back to the ERB. In pendants that depend on slip-rings to maintain the earth connection, the bonding resistance must be measured throughout a full 360º rotation. The resistance must remain below 100 mΩ throughout the test. The test must be repeated annually.
Note: In principle, earthing cannot be by means of bearings and sliding joints. If the mounting structure is made up of a number of parts, for example a chassis and a cover, these should be bonded together.
If an equipotential earthing jack is provided on the pendant (POAG-ID6 receptacle), this should be isolated from the pendant earth and returned to the ERB via a separate cable. A slip-ring connection must not be used for the equipotential earth.
The resistance between the ERB and the earth pin on the sockets, or any earthed metalwork (such as the body of the pendant) within the patient environment should be less than 100 mΩ. In order to achieve this, a conductor of at least 6 mm2 will often be needed.
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