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

Resistive coupling: direct connection to the body

Capacitive coupling: body acts as one plate of the capacitor.

Danger thresholds

·  Macroshock: current through two points of body. Safe <10mA, VF 100mA

·  Microshock: enters body through internal device close to heart. Safe <10μA, VF 100μA

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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