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A "PLAIN ENGLISH"
EXPLANATION OF THE REQUIREMENTS OF |
| A number of standards cover
the requirements for the electrical supplies to critical areas in
hospitals. These include a new IEC standard covering "Medical Locations"
as Part 7, Section 710 of IEC60364 and NHS Estates H.T.M.2007. I have
attempted to explain what the current requirements are in "Plain English"
so that no -Electrical Engineers can understand them. Medical locations are divided into three groups; the most critical of which is group 2. A group 2 area is deemed to be a medical location where applied parts are intended to be used in applications such as intracardiac procedures, operating theatres and areas of vital treatment where failure of the supply can cause danger to life. this covers areas such as operating theatres, operating theatre preparation rooms, heart catheterization rooms, intensive care and angiographic examination rooms. In these areas the design of the power supply systems can contribute to improve patient safety in three ways. Risk of exposing the patient to even small electrical shocks. In critical areas patients may be connected to electrodes, their skin may be punctured and they may have conductive catheters passed directly into the blood stream. This increases the risk of passing potentially fatal electrical currents directly through the body. Even very low currents of the order of 10mA passed through the body can result in muscular paralysis followed by respiratory paralysis. Loss of normal heart muscle rhythm can occur at around 25mA ( See IEC report 479-1:1999 ) and may prove fatal. this does not compare favourably with the 30mA trip level for a normal RCD ( earth leakage trip ) ! The risk of exposure is reduced by ensuring that all the conductime "touch voltage". This is normally achieved through the earth wire in the equipment plugs, via the power socket and back to a main earthing point. However it is essential that all earths in a critical area are connected back to the same "equipotential earth" to ensure that differing earth voltages do not produce a touch voltage. To ensure patient safety in the event of failure of the standard earthing system ( E.g. a broken earth wire in a plug ) it is recommended that the separate Equipotential Earth Sockets are installed. Each piece of equipment can then be separately and externally earthed to minimise touch voltages. An Isolating Power System ( IPS ) improves protection further by using an isolating transformer to provide a very high resistance path to earth ( > 50M Ohms ). As a result, any person accidentally touching the Live or Neutral line would not experience any harmful leakage current. typically a current of only 40 microamps will flow through the body, or less than 0.1% of the current required to stop an earth leakage device on a traditional system. Risk of power supply being tripped by faulty equipment. Should a piece of equipment with an earth fault be plugged into a normal power supply system this may result in the local MCB tripping. this would remove all the power supply from all of the sockets on that circuit. this could have disastrous consequences if this included patient monitoring or life preserving equipment in an operating theatre. The use of an IPS as described above, isolates the earth from both live and neutral and prevents a short circuit to earth from tripping the MCB. this ensures the continuity of supply to critical equipment. Continuous monitoring of the system means that a fault is alarmed to the users via an audio-visual panel before it can cause a problem. these conditions are monitored and reported to the user. 1. If a faulty piece of equipment is plugged in, the system will detect the insulation fault and sound an alarm alerting the users to unplug the equipment and have it replaced. 2. Should excessive load be placed on the system, it will display the increasing load and sound an alarm as the load approached 100%. 3. Should an overload or fault cause the transformer temperature to rise above normal levels an alarm and flashing indicator will warn of the problem. Risk of Power Supply Failure. Critical care areas require backup systems that will provide continuity of supply if there is a power cut. Most hospitals have a generator system that will provide an alternative supply. The requirement is that it must do this within 15 seconds. However a delay of several seconds is too long for critical areas. Uninterruptible Power Supplies ( UPS ) protect against the failure of the hospital distribution system or of the supply to the site itself. The UPS can provide supply for the few seconds during the generator run up period or if it does not start. Modern UPS systems are used to protect delicate electronic and computer equipment against voltage spikes on the main supply as a result of local thunderstorms or "brown out" caused as the local supply voltage drops below acceptable levels. Most UPS systems are designed to supply power for a short period of time while the generators are brought on-line. Operating theatres often have a battery change over system to power the operating theatre lamps during a power cut. These systems use a relay to connect the operating lamps to a battery supply in fractions of a second. They are used in preference to UPS systems for this particular application as they are required for at least 1 hour ( 3 hours if no generator is present ) and have to supply quite a large electrical load. They are generally more cost effective than UPS for this particular application. In Summary. It is clear that, as the equipment that we use in our hospitals becomes more sophisticated, we need to pay more attention to our electrical power supply systems. New critical care facilities should include 1. Equipotential Earthing, 2. Monitored Isolated Power Systems, 3. Backup systems in the form of generators, and UPs and battery changeover systems. Nigel Davill, Technical Director ( Brandon Medical ) & advisor to the British Standard Institute on Surgical Lamps, and BSI representative on the International Standards commission. |
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