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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