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Protection from macroshock (Protective features (All eqpt should be up to…
Protection from macroshock
Definitions
Macroshock - subject becomes part of electrical circuit via skin/surface contact. Allows large current flow leading to harm/death. Current flow required for Macroshock @ 50Hz, 240V:
0.5mA sensory threshold
1mA pain threshold
5mA maximal harmless current
Greater than 10mA sustained muscle contraction, inability to let go
50mA pain, mechanical injury, fainting, muscular contraction, respiratory insufficiency
100mA VF, probable death
2A severe shock with burns, asystole
Microshock - Induction of VF by small electric currents (e.g. 0.1mA) directly passed to myocardium e.g. via wire
Electrical power supply to theatre
240V, 50Hz
3 pin system: active, neutral and earth
Small leaks (inherent capacitance btw live parts and externally accessible parts)
Current seeks return path to source via earthed object
Usually harmless, conducted away or insulated
Failure of protective measures can lead to electric shock
Patients vulnerable because:
Wet skin lowers impedence
Usually well grounded
Cant move/complain
Protective features (All eqpt should be up to Australian Standards and maintained as such)
General measures
Maintenance/regular testing, documented
Not using faulty eqpt
Avoid earthing patients and staff (non-conductive flooring/operating tables/staff footwear)
Avoid unneccesary conducting eqpt e.g. bed frames
Clean spills, keep patient dry
Grounding gel pad used in electrosurgery, inspect during lengthy operations and replace if necessary (caution if increased amount of current used)
Circuit breaker/fuse
Trips/burns if excessive current load
Slow and requires high currents
Supply is interrupted until fuse is replaced/circuit breaker reset
Will not prevent electrocution but will protect wiring from excessive current
Superseded by RCD
Residual Current Device (RCD)
Aka 'earth leakage core balance devices' or 'safety switches'
Senses an imbalance between active and neutral (If active>neutral - presumed current flowing to earth via another path, the alternative current path may result in electrocution)
Australian standards require that the switch trips at current > 10mA, within 60msecs
Power supply will be interrupted until the switch is reset
It will re-trip if the fault current is still present
Devices are cheap and installed in all theatre devices that do not require continuous power
Line Isolation
Isolation transformer used to isolate the active and neutral lines (floating system). Current will not flow from either wire to earth
Line isolation monitors continually check to see if reconnection to Earth has occurred (if >5mA alarm sounds but supply will not be interrupted
Required in areas where life-support e.g. ventilators, Cardiopulmonary bypass
Equipotential Earth
Specifically designed earthing arrangements to bond all conductors within the treatment area to eliminate potential differences btw objects
Large copper conductors connecting all earthing points
Keeps potential difference <1-2mV
Double insulation
Internal wiring completely insulated from accessible conductive parts
Doesnt need earth (2 pins)
Isolation of patient contact areas
Circuits of some devices coupled with master circuits via no-electric link
Isolates circuits that are in contact with patient, thus if fault with appliance unlikely involve patient circuit