Electrical safety By Dr. Ahmed Mostafa Assist. Prof. of anesthesia and I.C.U. Electrical safety The hazards associated with the use of electrical equipment are: • Electric shock – macroshock. • Electric shock – microshock. • Diathermy hazards. • Burns. • Fire and explosion. Electrical safety Electric shock (macro-shock) • Occurs with the external application of a voltage to the skin, causing an electric current to pass through the body tissues. • Commonly electric shock occurs from the AC mains supply. • Occurs from the mains supply, when the body forms a circuit between the live mains line and a local earth connection or the neutral mains line. Electrical safety Electric shock (macro-shock) Earthed circuit: Electrical safety Electric shock (macro-shock) Isolated circuit: Electrical safety Electric shock (macro-shock) • It produces VF when a current of 100 – 200 mA is applied to the skin and passes to the heart (VF threshold). • Its risk is increased by: 1- The size of the current: - Small (e.g. 1 mA) ---» tingling. - If current increases ---» Pain, muscle spasm. - Large (e.g. 100 -200 mA) ---» VF. Electrical safety Electric shock (macro-shock) 2- The impedance: If large ---» the current will be small. Source on impedance: - Skin: 10 kΩ. - Antistatic footwear: About 100 kΩ. - Antistatic floor: About 25 – 50 kΩ Electrical safety Electric shock (macro-shock) 3- The current density: A current passing through a small area is more dangerous. 4- the form of electric current: 50 Hz AC is more dangerous than 1000 Hz DC. 5- Duration of exposure. 6- Myocardial disease. Electrical safety Electric shock (micro-shock) • Current is delivered internally to the myocardium, causing arrhythmias. The conducting pathway may be through an intravenous catheter or pulmonary artery catheter and its contained fluid. • The magnitude of currents required to produce ventricular fibrillation in micro-shock is in the order of 100–150 μA. This is much smaller than in the case of macro-shock. • Its risk is increased by the same factors like macroshock Electrical safety Electric shock (micro-shock) Electrical safety Prevention of electrocution General measures: - Adequate maintenance and regular testing of electrical equipment. - Ensuring the patient is not in contact with earthed objects. - the wearing of antistatic shoes, whose high impedance will reduce any current flowing through the body. Electrical safety Prevention of electrocution Equipment design: Different classes of equipment are specified for the medical working environment. The safety specifications may refer to the risk of electric shock, as follows: Class I – Earthed Class II – Double-insulated but not earthed Class III – Low voltage (<24 V), battery-powered Electrical safety Prevention of electrocution Earth circuits: These are designed into equipment in order to reduce risk of electric shock and reduce interference. Earth connections reduce the risk of electric shock by maintaining exposed metalwork at zero potential. Such metalwork cannot then deliver an electric shock current if inadvertently contacted. Electrical safety Prevention of electrocution Isolated patient circuit (Floating circuit): Floating circuit is a circuit, isolated from the mains because it has no physical point of contact between it and a circuit of higher voltage. i.e. it gets its power via a transformer. Electrical safety Prevention of electrocution Isolated patient circuit (Floating circuit): Electrical safety Prevention of electrocution Leakage currents: • Small electric currents (<500 μA) which arise unintentionally. • Such currents can either pass or ‘leak’ down to earth through a designed safety circuit (e.g. earth connection, antistatic shoes) or may flow through an unintentional pathway, creating a risk of microshock. Electrical safety Prevention of electrocution Leakage currents: • based on maximal permissible leakage currents • Type B: may be class 1, 2 or 3 but maximum leakage must not exceed 100microamps (thus must not be directly connected to heart). (B indicates that the equipment is safe to be contact with the body surface) • Type BF: as for type B but uses a isolated (or floating circuit). Electrical safety Prevention of electrocution Leakage currents: • Type CF: these provide the highest degree of protection using isolating circuits and having a maximum current leakage of if leaves entire power susceptible to failure (C indicates that the equipment is safe to be contact with the cardiac muscle) Electrical safety Prevention of electrocution Isolating transformer: This system supplies all equipment attached to the patient via a transformer so that no equipment associated with the patient is directly connected to the mains. Circuit breaker: This is a sensitive mains switch which operates to disconnect the mains whenever abnormal currents are detected Electrical safety Prevention of electrocution Suitable footwear: • Footwear impedance should be designed to isolate the wearer from earth. • The impedance should be high enough to prevent large current passing to earth (avoiding electric shock), but low enough to allow a leakage current to earth to prevent the wearer and clothing from accumulating a static charge. Such shoes normally have an impedance of between 100 kΩ and 1 MΩ. Electrical safety Prevention of electrocution Ingress protection (IP) rating: • Electrical equipment has safety information affixed to it to provide the user with information about which environments it is safe to use in. One such rating is the ingress protection (IP) rating. • The higher the number, the better the protection. Electrical safety Prevention of electrocution Thank you Dr. Ahmed Mostafa