2015B15 Discuss the prevention of electrical shock in the operating theatre.

List:

(A)Basics of shock

· How

· Thresholds

· Severity

· OT risk factors

(B)Shock prevention

· Area classification

· Supply design: RCD, LIS, EPE

· Equipment design: I, II, III

· Applied parts: B, BF, CF

Basics of shock:

Requirement	· Person forms part of a low resistance, live electrical circuit o Resistive coupling: direct connection to the body o Capacitive coupling: body acts as one plate of the capacitor.
Danger thresholds	· Macroshock: current through two points of body. o 1mA: pain o 10mA: tetany (i.e. unsafe) o 100mA: VF · Microshock: enters body through internal device close to heart. o 10μA: unsafe o 100μA: VF
Determinants of severity	· Type. (AC 60Hz most dangerous; >100Hz safe; DC safe (since high skin resistance) · Magnitude (Amperes) · Density (A/cm2) · Duration · Path (e.g. through heart)
Operating theatre problems	· Electrical equipment++ · ↓Skin resistance (saline, ECG dot, cannula) · Patient unconscious, immobile

Area classification:

Cardiac protected	· e.g. theatre complex, A&E, ICU · main difference: incorporates equipotential earthing)
Body protected	· e.g. general ward

Electrical supply safety features:

Residual current device	· Live and neutral wires loop an equal number of times around a transformer · A third wire from a comparator circuit also loops around the transformer · Earth leakage -> hot ≠ neutral flow -> current induced in comparator circuit -> power cut · Cuts power in 30msec (i.e. still risk of VF), threshold 30mA
Line isolation system	= isolation transformer + line isolation monitor (LIM) · Isolating transformer joins mains and floating circuits, no physical contact · Isolation monitor measures impedance between lines 1&2 and the ground · Connection between floating circuit and earth does not cause shock, but will cause LIM to alarm · Threshold 5mA
Equipotential earthing	· Machines in a stack may be at different electrical potential · If a person touches two of them, current may flow through him/her · Hence each device earthed via heavy duty connections inside the walls · Expensive to install

Equipment:

Class 1	· Earthed outer casing, +/- fuses · e.g. fridge, microwave, toaster
Class 2	· Double layer insulation. Earthing unnecessary. · e.g. DVD player, TV
Class 3	· Safety extra-low voltage (SELV) <24V AC or <50V DC, especially internal battery power · Protects against macro but not microshock · e.g mobile phone, laptop

Applied parts: (i.e. electrical connections)

Type B	Unprotected · Only limitation is the patients’ impedance · Max 500μA if Class 1 or 100μA if Class 2. Prevents macroshock. · e.g. hospital bed
Type BF	Body protected, floating · Current flow limited in the event of a fault · Max 500μA if Class 1 or 100μA if Class 2. Prevents macroshock. · e.g. NIBP cuff
Type CF	Cardiac protected, floating · Max 50μA if Class I equipment, 10μA if class II equipment · *Only Class II equipment with Type CF connection prevents microshock* · Safe for direct cardiac contact · e.g. pulmonary artery catheter

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