Selection of ME equipment

The first thing a user must do is make sure the equipment they are using satisfies the fundamental safety and performance standards for ME equipment.

Medical electrical equipment Particular requirements: Basic safety and essential performance

• ISO 80601-2-12 – Critical care ventilators
• ISO 80601-2-84 – Ventilators for the emergency medical services environment
• ISO 80601-2-79 – Ventilatory support equipment for ventilatory impairment
• ISO 80601-2-87 – High-frequency ventilators
• ISO 80601-2-84 – Ventilators for the emergency medical services environment
• ISO 80601-2-80 – Ventilatory support equipment for ventilatory insufficiency
• ISO 80601-2-72 – Home healthcare environment ventilators for ventilator-dependent patients

Note: There are hundreds of ISO standards that exist on ME equipment and system, which is not the focus of this article.

Installation of ME equipment

• The location where the ME equipment is installed shall satisfy special requirements such as:
• Additional protective bonding and protective devices to restrict continuous voltage differences.
• Restriction of voltage differences by supplementary equipotential bonding (e.g. a fault voltage of < 25 volts in group 2 locations). During the application of equipment with direct contact with the patient, at least a potential equalized zone around the patient shall be provided with a patient centre bonding bar to which the protective and functional earth conductors of the equipment are connected. All accessible extraneous conductive parts in the zone shall be connected to this potential equalization bar.
• Restriction of the potential equalization zone to the zone around one patient, meaning practically around one operation table or one bed in an intensive care room.
• If more than one patient is present in an area, the connection of the various potential equalization centres to a central potential equalization busbar should preferably be connected to the protective earth system of the power supply. In its completed form the equipotential bonding network may consist partly of fixed and permanently installed bonding and part of several separate bonding which are made when the equipment is set up near the patient.
• The necessary terminals for these bonding connections should be present on the equipment and in the installation.
• Restriction of the duration of transient voltage difference by the application of SPDs.
• Continuity of power supply to certain equipment in the case of a first insulation fault to earth and restriction by electrical separation.
• Monitoring of the first insulation fault to earth in a medical IT system.
• Prevention of ignitions and fire in rooms where flammable anaesthetics or flammable cleaning or disinfection agents are used by ventilation, anti-static precautions and careful layout of the installation.
• Safety supply system for major parts of the hospital, usually a DG and or a UPS and essential circuits to be connected to it.
• Special and uninterrupted safety supply system for critical equipment such as life-supporting equipment and operating room lamps.
• Suppression of electromagnetic interference by the layout of the building and wiring and provision of screening arrangements. Limits for magnetic fields of main frequency are necessary for several sensor measurements.

Electrical installation in general locations of a hospital

• TN-S power supply.
• An efficient “protective equipotential bonding and automatic disconnection of supply” ensures fault voltage of less than 50 VAC and disconnection time < 0.4 seconds at the final circuits up to 63 amps.
• Overvoltage control for the safety of the equipment.
• Electrical installation of Overvoltage category III and IV at a fixed part of the installation, Category II for all connected equipment.
• Electrical equipment of overvoltage category I shall not be connected to the power supply from the transformer.
• Protective conductor currents are limited to a few mA.
• Effective shielding and cable routing measures.
• All circuits’ efficiency of automatic disconnection and continuity of conductors were tested to avoid loose contacts and overheating.
• Insulation resistance of the complete installation, satisfying the safety standards.

Equipment in Hospitals requires special care for the power supply impedance (e.g X-ray machine)

A conventional X-ray unit uses high power during Radiography. The power used during this time will be multiple times the power consumed during standby mode; however, it is for a short time of a few mS. Some applications also used a burst of exposure which may exist for a few seconds. The selection of a protective device (such as Fuse / MCB) and the input impedance of the supply system shall meet the requirements of safety and short-time requirements of high power. E.g. for a high-power 3-phase X-ray machine, the input impedance between phases shall be up to 100 mΩ. These requirements shall be fulfilled by testing as the best practice and if possible, by periodic testing.

Fundamental mistakes in Hospitals leading to accidents

L.V. electrical supply falls under the TN-S, TN-C-S, TT, IT, etc. categories. In these networks, safety is achieved by automatically cutting off the supply (except for IT), but how this is done varies between networks. Unfortunately, system earthing is commonly disregarded or incorrectly performed in hospitals.

Code of practices of the Bureau of Indian Standards such as IS 3043 and IS 732 clearly explains the methods for disconnecting the supply during earth fault:

• The characteristics of protective devices and the cross-sectional area of the conductor shall be so chosen that if a fault of negligible impedance occurs anywhere between the phase conductor or exposed conductive part and protective conductor, automatic disconnection of the supply will occur within a possible minimum time.
• The fault loop impedance has to be low enough to allow adequate fault current to flow to cause an overcurrent protective device (for example, a fuse or circuit breaker) in the faulty circuit to operate in a sufficiently short time.
• In general, every circuit is provided with a means of overcurrent protection. If the fault loop impedance is low enough to cause these devices to operate within the specified times (that is, sufficient current can flow under fault conditions), such devices may be relied upon to give the requisite automatic disconnection of supply.

Hence, Fault Loop Impedance is the deciding factor. Low fault loop impedance ensures a higher fault current which, in turn, trips the protective device within the specific time during a fault.

Fault loop impedance and its measurements are an unknown subject in India. Due to that, wrong protective devices are used, resulting in the non-functioning of the protective device during a short circuit, resulting in fire.

Using power from DG is common in India. Fault loop impedance testing while running the facility from different sources shows that the protective device is not tripping while running in DG. This means when the building runs in DG, Line to Neutral short circuits can cause a fire. Conventional Protective devices such as RCD are ineffective.