Electrical safety
Electricity
The flow of electrons, driven by a potential difference, through a conductor
Current measured in amps (1 amp = 1 coulomb/s)
Resistance
The resistance to flow of current (collision of electrons)
Not related to frequency
Measured in ohms
V = IR
1 ohm allows 1A to flow under the influence of 1V
Impedence
Sum of forces resisting flow in AC circuit
Dependent on frequency and includes resistors, capacitors and inductors
Also measured in ohms
Injury
50 Hz particularly dangerous
Above 100 Hz no fibrillatory potential
Thermal injury from energy dissipation through tissues
Severity of burn ∝ current density and duration
General
Maintenance
Testing
Antistatic flooring
Equipment
Class
1 earthed
2 double insulated (not earthed)
3 low voltage (<24V)
Type
B max leakage current
BF isolated (floating)
CF cardiac floating (max leakage current <10 microamps (too small for microshock – ECG leads etc)
Equipotentiality
Equipment in a stack connected so at same potential to avoid current flow, if connection between them, through the user.
Isolated/floating circuits
Isolating transformer used so patient not connected to earth.
Circuit breakers
Trip very quickly (milliseconds) reducing likelihood of significant shock.