7.18.2 - Recommendations for medical locations

Patient safety
Since the hazard to people will depend on the treatment being administered, hospital locations are divided into groups as follows:

1. - Group Zero: where no treatment or diagnosis using medical electrical equipment is administered, e.g. consulting rooms;

2. - Group One: where medical electrical equipment is in use, but not for treatment of heart (intracardiac) conditions;

3. - Group Two: where medical electrical equipment is in use for heart (intracardiac) conditions.

TN-C systems (where earth and neutral are combined - see {5.2.5}) is not allowed in medical locations because load currents in PEN conductors and parallel paths can cause electromagnetic interference to medical equipment. Protection against direct contact must be by insulation of live parts and by barriers or enclosures. SELV and PELV systems (see {3.4.4}) may be used in Group One and in Group Two locations, but is limited to 25 V r.m.s. a.c. or 60 V ripple free d.c. Even at these voltages, protection of live parts by insulation or by barriers or enclosures is essential. Exposed conductive parts of PELV systems must be connected to the local  equipotential bonding conductor. FELV Systems (see {3.4.4}) may be used in Group Zero locations only.

Protection against indirect contact may be by automatic disconnection of the supply (see {3.4.6}), by electrical separation (see {5.8.4}), or by the use of Class II equipment (see {5.8.4}) except as indicated below:

For TN Systems (see {5.2.3} and {5.2.4}) any medical equipment in Group One or in Group Two locations which is situated within 2.5 m above the floor or within 1.5 m horizontally from the edge of the bed, operating table, etc., on which the patient is lying, called the patient environment (see {Fig 7.27}), must be RCD protected, the rating of the device being 30 mA or less if the circuit protection does not exceed 32 A, or 300 mA or less if the circuit protection exceeds 32 A. Note, however, that in Group Two locations, RCDs may only be used for X-ray units or for equipment with a rated power greater than 5 kvA,

Fig 7.27 - The patient environment

For IT systems (see {5.2.2}) used in Group One and Group Two situations, the same requirements apply as in TN systems with earth fault protection provided by RCDs.

For IT Systems (see {5.2.6)) it is recommended that an IT system is used for circuits supplying medical equipment that is intended to be used for life-support of patients. This is because the total absence of an earthing system makes indirect contact an impossibility. Any medical equipment in Group Two which is situated in the patient environment (see {Fig 7.27}) must have a safety isolating transformer incorporated into its IT system, and must have an insulation monitoring device. This device must have:

1. - acoustic and visual alarms that are triggered at the first earth fault, and

2. - an a.c. internal resistance of at least 100 kOhms, and

3. - a test voltage not exceeding 25 V, and

4. - a test current not exceeding 1 mA even under fault conditions, and

5. - activation when the insulation resistance falls to less than 50 kOhms, and

6. - activation when any wiring or earth is disconnected.

If a socket outlet on a patient bedhead location in a Group Two situation is supplied from an IT system, it must be fed from at least two separate circuits. Socket outlets in a Group Two situation fed from other systems (TN-S or TT) must be clearly marked to distinguish them from IT system socket outlets. Any wiring system within a Group Two location must supply only the equipment and fittings in that group (see {Fig 7.28}.

Fig 7.28 Distribution network with insulation monitoring

Supplementary equipotential bonding
This bonding is required within Groups One and Two locations, the resistance of the bonding conductors not exceeding 0.2 Ohms. In Group Two locations circuits supplying electrical equipment situated within the patient environment (see {Fig 7.27}), the voltage between extraneous conductive parts and the protective conductor system must never exceed 20 mV, even under fault conditions. This figure is based on the leakage current through the patient never exceeding 50 mA. Assuming a body impedance of 1 kOhm, this gives a maximum voltage of 50 mV, 30 mV of which is assumed to be the volt drop from the socket outlet to the equipment.

Explosion risk
The gases used as anaesthetics in operating theatres are flammable if present in high concentrations. Provided that there is adequate ventilation (20 air changes per hour) no special precautions are necessary for the electrical installation.

Standby power supplies
The failure of the power supply may well have fatal consequences, for example if the lighting fails in an operating theatre, or the feed to a life support system is lost in an intensive care unit. Safety power supplies are split into three categories depending on how quickly the supply is restored after failure. The time periods are 0.5 s, which applies to operating theatre lighting and where the standby system will be an uninterruptible power supply unit (UPS), 15 s for most essential hospital supplies, where a standby generator is likely, and more than 15 s, which is likely to apply to offices, staff accommodation and so on. Emergency lighting fed from a safety source is required at escape routes, exit signs and locations of essential services in Groups One and Two locations.

Department of Health recommendations
The Department provides guidance through its Health Technical Memoranda (HTM) series of publications. These extremely detailed volumes are available from HMSO bookshops.