Anaes · Measurement & monitoring physics

Electricity fundamentals and electrical safety

Also known as Electricity · Electrical safety · Ohm's law · Macroshock · Microshock · Diathermy

Electricity powers every device in the operating theatre and carries the two specific hazards of macroshock (whole-body shock from mains contact) and microshock (a tiny current delivered directly to the heart through a catheter causing ventricular fibrillation at currents as low as 100 microamperes). The framework rests on six exam-critical ideas. First, the four fundamental quantities are CHARGE (the coulomb, C), CURRENT (the ampere, A, equal to one coulomb per second), VOLTAGE (the volt, V, the electrical potential difference or electromotive force) and RESISTANCE (the ohm); they are related by OHM'S LAW (V equals I times R) and by the power equation (wattage equals volts times amps). Second, mains electricity is ALTERNATING CURRENT (AC), cycling at 50 hertz in Australasia and the UK (60 hertz in North America); AC is more dangerous than direct current (DC) at the same voltage because the alternating cycle can induce tetanic muscle contraction (preventing release from a live wire) and because it crosses zero twice per cycle, making the heart vulnerable to ventricular fibrillation at the crossover. Third, the body's electrical safety depends on EARTHING (a low-resistance path to ground that diverts fault current away from the patient and trips the fuse), FUSES and CIRCUIT BREAKERS (which break the circuit when current exceeds a set limit), RESIDUAL CURRENT DEVICES (an RCD detects any difference between the current flowing out and returning, tripping in milliseconds at about 5 to 30 milliamps of leakage), and ISOLATED (FLOATING) CIRCUITS (the mains supply is separated from earth by a transformer so that a single fault cannot complete a circuit through the patient to ground). Fourth, MACROSHOCK is the whole-body shock from contact with a live conductor through the skin; the severity depends on the current (1 milliampere is the threshold of perception, 10 to 20 milliamps is the let-go threshold with muscle tetany, 50 to 100 milliamps causes respiratory failure, and over about 100 milliamps causes ventricular fibrillation). Fifth, MICROSHOCK is a very small current (as low as 100 microamperes) delivered DIRECTLY to the myocardium through a low-resistance pathway such as a saline-filled central venous catheter, a pulmonary artery catheter or a pacemaker wire, causing ventricular fibrillation at a current far below the macroshock threshold; this is why all patient-connected equipment must be earthed, isolated and leakage-tested, and why a saline-filled central line is never connected to non-isolated equipment. Sixth, DIATHERMY (electrosurgery) uses high-frequency AC (above about 500 kilohertz) to cut and coagulate tissue without neuromuscular stimulation; MONOPOLAR diathermy passes current from an active electrode through the patient to a large dispersive plate (return electrode), while BIPOLAR current passes between the two jaws of a forceps and does not require a return plate; diathermy burns, surgical fires (the electrosurgical spark igniting an oxygen-enriched atmosphere with a fuel source) and electromagnetic interference with pacemakers and ICDs are the anaesthetic hazards. Built on the electrical-safety review (Burgess 2019), the electrosurgical-hazards review (Pearce 1985), the electrosurgery-principles review (Vilos 2013), the surgical-smoke review (Larivee 2026), the electrical-injury-reconstruction study (Zhang 2026), the electrical-injury-cataract study (Wang 2026), the electrical-burn-amputation study (Sankhla 2026), the device-endocarditis report (Ohev Shalom 2026), the operating-room-fire review (Mortada 2024), and the skin-prep-fire study (Jones 2017).