Prevention of Electrocution

OVERVIEW

Prevention of electrocution in the hospital setting involves consideration of:

• General measures
• Class of Equipment
• Type of Equipment allowed to be connected to patient
• Equipotentiality
• Isolating (floating) Circuits
• Circuit breakers (RCD’s)
• Surgical Diathermy
• Area Classification

APPROACH

1. General measures

• maintenance and testing
• making sure patient isn’t in contact with earthed objects
• anti-static shoes (high impedance -> current can’t flow through)
• no extension cords that can reach from an unprotected area
• no double adaptors (may not sit well and can leak current)
• all equipment should meet Australasian Standards for Safety
• personnel education

2. Class of Equipment

• Class 1: third pin of plug (direct earthing) + metal casing -> this circuit should be low resistance and if live wire comes in contact with an accessible part should be linked to a fuse that breaks the circuit
• Class 2: double or reinforced insulation (non-conductive plastic), earthing wire not needed
• Class 3: provides protection by stepping down main power to a safety extra low voltage (SELV, see below

3. Type of Equipment allowed to be connected to patient

• 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)
• Type BF: as for type B but uses a isolated (or floating circuit) -> see below
• 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

4. Equipotentiality

• connections of equipment to make them all the potential differences the same
• connections of equipment by low impedance green cables
• includes anaesthetic machines and IV poles with pumps on them
• prevents devices leaking charge to a user or patient

5.  Isolating (floating) Circuits

• patient circuit not earthed by using an isolated transformer
• can be used to isolate an entire theatre -> if leaves entire power susceptible to failure
• used frequently to isolate individual instruments
• line isolation monitor attached that continuously monitors the potential for current to flow from isolated current supply to ground (alarm activated if 2mA of current detected)

6. Circuit breakers (RCD’s)

• live and neutral wire wrapped around the core of a transformer and then around a circuit breakers (normally the magnetic fluxes cancel each other out)
• if excess current is leaked -> magnetic field is produced between the transformer that triggers a current causing break in circuit
• will protect against macroshock

7. Surgical Diathermy

• the use of the heating effects of high frequency (kHz -> MHz) electrical to coagulate or cut tissue
• can cause burning, explosions or muck up pacemakers

Monopolar diathermy

• 200 kHz -> 6 MHz
• neutral and active
• neutral = large conductive area -> low current density and minimal heat
• active = small contact area + high current density

Bipolar diathermy

• lower power output
• output between 2 points on forceps -> high local current density
• no current passes through rest of body

8. Area Classification

Body protected areas

• patients connected to equipment that lowers the natural resistance of the skin
• examples: electrode gels, conductive fluids, metal needles and catheters
• protection from macroshock is goal
• RCD’s, line isolation transformers and monitors used

Cardiac protected areas

• this is where procedures are performed within or near the heart -> protection from microshock is required
• examples: intracardiac pacing electrodes, intracardiac ECG electrodes, intracardiac catheters
• equipotential earthing, RCD’s and line isolation monitors used

References and Links

• Boumphrey, S et al (2003) “Electrical Safety in the Operating Theatre” CEPD Volume 3, Number 1, pages 10-14
• Magee et al (2005) “The physics, clinical measurement and equipment of anaesthetic practice” Electricity Chapter, page 167