Chris Thompson 2003 ELECTRICITY AND ELECTRICAL HAZARDS 1) Basic Electrical Concepts Voltage: the 'push' on electrons to move through a conductive medium 2) Effect of 50 Hz electricity on muscle Microshock Fibrillation 0.1 mA Sensation depends on current density. Small currents over large areas are not felt as well as the same current over a smaller area. The myocardium is most sensitive to 30-100Hz electricity, so mains at 50Hz is ideal for inducing fibrillation. Higher frequencies (ie, diathermy), DC electrical current, and AC which does not pass through the heart do not cause fibrillation but rather heat up and burn the muscle they flow through, sparing skin and fat. Microshock is a term describing the induction of ventricular fibrillation by small electrical currents (below the threshold of skin sensation) when applied to very small areas of ventricular muscle, usually by vascular catheters or wires. It requires a small area of contact with heart muscle so that the current density is high despite low current. 3) Safety featrures of the mains electrical supply Typical problems:
Class Z areas: Characteristics:
The Earth Wire: The earth wire should always be in good contact with all metal parts of any appliance the user may touch. Should the equipment become faulty by the active wire touching the case, then current will rush to earth, smoke will rise from the equipment, and if the current is very high the fuse in the active line will blow, disconnecting mains power from the faulty appliance. Without the earth wire, this fault would go undetected, allowing high current to flow through the patient whenever the patient formed a circuit to ground. Standards limiting current in any circuit are vital to prevent excessive heating of the power supply wires leading to fires. Should the leak to the metal parts of the appliance not be enough to blow the fuse, then the earth wire will drain the electricity away to earth by a low-resistance pathway. The user will be unaware of the potential dangerous nature of the device. If the earth is poor and a patient circuit to ground occurs, then the user may feel a "tingle" or a more substantial shock. Obviously the presence of a good earth wire at all power points is the main safety feature of Class Z areas. It may become faulty due to corrosion of the sockets, leads, or extension cables, bending of the pins within the wall sockets, and corrosion or poor installation of the electrical supply. The high resistance of normal dry skin helps protect patients from electrocution by many of the failures which may not be sufficient to blow the fuse. Danger is particularly high when the user is working in wet surroundings (low-resistance pathway to earth) or using power tools or extension leads. Small domestic Earth Leakage Core Balanced relays are available for use in extension leads and in hazardous (ie wet) areas, and can in fact be installed as well as or instead of fuses in the home fuse box. These provide very greatly reduced risk of electrocution, by quickly disconnecting the electricity in the event of even a minor failure (more later). 2) Class B areas. Characteristics:
The sort of minor equipment failures where there is a leakage of power through the earth circuit (but not enough to blow the fuse) may be lethal once the patients protective skin resistance is reduced. All patient care areas where the usual skin resistance is reduced owing to the presence of low-resistance patient circuits, ie on ECG machines, diathermy devices, etc, must therefore have special precautions to reduce the risk of electric shock. Residual Current Devices (RCD's) RCD's's are the cheaper way to do this. Since normally all the current leaving a piece of electrical equipment goes back to ground by the Neutral wire, then the current flowing through the Active and Neutral wires should at all times be equal. Should a fault exist in which electricity is leaking to ground by any other route, then the currents in the Active and Neutral wires will no longer be equal, and under these circumstances the RCD will be "tripped" and both the Active and Neutral supply will be disconnected from the socket providing power to the faulty device. The RCD will disconnect power within 10 to 20 milliseconds when a leak current of 5 to 10 mA is detected. Thus it will protect against most cases of macroshock. An audible alarm will sound and a loud thump will be heard when the relay is tripped, and power can only be restored manually, by pushing the switch back to the "on" position. Usually a test button is available to check the correct operation of the RCD. The RCD will not detect faults in which electricity passes through the body and back through the Neutral wire, however these are very infrequent. It will not protect against microshock as up to 10 times the required current for fibrillation may pass through the patient without tripping the relay. RCD's are generally wired to several power outlets, and if tripped will disconnect power from them all, and keep on doing so until the faulty device is removed. This is not desirable in some patient care areas, where a different type of system may be used. Isolating Transformers and Line Isolation Monitors. These are the more expensive alternative to RCD's and are widely used in operating theatres because they do not disconnect the power when a fault is detected, yet provide safety should such a fault exist. The first component is a large transformer (the Isolating Transformer) mounted in the wall cavity which converts the earth-referenced mains supply to a "floating" supply. The floating supply provides 240V between two active wires, but because the supply is not earth-referenced, the presence of an earth circuit through the patient or anyone else is perfectly safe and no current will flow. All the circuit to earth does is to reference the floating supply to earth; no current actually flows through the earth connection. The Line Isolation Monitor continually checks that the floating supply is not earth-referenced, and indicates on a dial how much current could flow to earth if there was an earth connection. If the potential earth current would be more than 5mA an alarm will sound, alerting the anaesthetist to the presence of a loss of the "floating" nature of the supply. It does this by intermittently connecting one of the two active wires to ground through a very large resistance. If the other wire is connected to ground a circuit will be formed and current will flow, and this indicates how much current would flow through the circuit if either of the two active wires are connected to ground. As with an RCD the device will not alarm under 5mA, so microshock may still occur unnoticed, however macroshock is very unlikely; only current flowing through the patient from between the active wires will no be detected. Class A Areas Characteristics:
Only these areas should be used when intracardiac conductors are present. All potential sources of leak current must be equipotentially earthed by special low-impedance green cables, and this includes the anaesthetic machine, IV poles with IMEDs on them, etc. 4) Saftety features of electrical devices a) General Construction The device should be manufactured so that the risk of the casing of device or other conductive part becoming active or partially active without the user noticing is small. If the case became active and the earth fail, a person who completes a circuit to ground may be electrocuted. Correct construction of the device, ie proper attachment of power wires inside the casing, the use of double-insulated design techniques, etc, minimise this risk. There are three classes of equipment based on these general construction considerations Class 1: Earthed All these devices must be earthed. All electrical devices leak electricity to their cases and this will seek a return path to ground. This is normally provided by the earth wire. There is a limit to the allowable leakage down the earth wire, so that should the earth fail, the patient would not be at risk of electrocution from the case (unless there was another fault as well). Most people would feel a faint tingle from 500uA, and may get the feeling that something is wrong. Type of patient circuit Earth Leak (max)
Class 2: Double-insulated This equipment need not be earthed as there are no conductive parts on the exterior capable of carrying current. Often the internals are earthed, and an earth leak current will then occur. Class 3: Low-voltage This is less than 40V DC. If AC is used then a special isolated transformer must be used. Low voltage sources (particularly DC) are very safe. b) Patient Circuits Normally skin resistance is quite high and this is a safety barrier to electrocution. This barrier is lost by many patient monitoring circuits and by having a wet patient. Small currents, ie the normal leakage current of the device, may be hazardous when they flow through low-resistance pathways to ground. This potential problem is reduced in all devices with patient circuits by including a very high resistance in the device, to limit the maximum current that could possibly flow through the patient to extremely low levels.
Class BF devices will conduct up to 5mA through their patient circuit in worst-case conditions, and while this would be quite painful it would not be fatal if applied to the skin. When intracardiac conductors are used as little as 100uA may be fatal so the Class CF devices will not allow any more than 50uA to flow under worst case conditions. Both Class BF and Class B devices leak amounts of electricity easily capable of causing microshock from their patient circuits and must never be connected to any patient with an intracardiac conductor. Devices with cardiac protected patient circuits are the only type to be used on the patient when an intracardiac conductor is present. Great care must be taken to ensure that the intracardiac conductor never contacts earth or the chassis of any nearby electronic device. Note that should the patient be connected to 240V via a skin connected patient circuit, the patient will suffer:
Comments? please use this form. |
|||||||||||||||||
 |
|
|
|||||||||||||||