Electrical Safety Flashcards
What is the UK mains supply?
An AC supply with a frequency of 50Hz that oscillates between +340V and -340V.
Why is the UK mains described as 240V?
240V is the root mean square voltage
To generate a more meaningful ‘average’ voltage:
- The waveform is squared, making negative voltages positive
- The arithmetic mean of the squared waveform is +57 600 V2
- And the square root of this mean is 240 V
How can electricity cause harm to patients?
- electrocution
- burns
- interference with monitoring
What law determines how much current flows?
Ohm’s law (V = IR)
ie the current that flows is inversely proportional to the resistance of the path
What are the effects of current flow dependent on?
- how much current flows
- type of current (AC vs DC)
- frequency of current
- current pathway
- current density
- duration of current flow
What is Class I equipment?
- the case is connected to earth by an earth wire providing a low resistance path for current
- if a fault occurs, the casing becomes live
- if you touched the casing, current would use you as an earth connection
- to prevent this, the earth has a very low resistance so that a larger current flows this route, total current flow increases, which causes the fuse to blow - breaking the circuit
What is Class II equipment?
- has double insulation so there is minimal chance that someone could contact the faulty live component
- no earth wire
What is Class III equipment?
- powered internally by a battery or Safety Extra Low Voltage (SELV)
- specifications for SELV are:
- voltage not greater than 25V AC
- voltage not greater than 60V DC
- no earth connection (usually floating circuit)
- low risk of accidental contact with higher voltage
What is the difference between a microshock and macroshock?
Macroshock occurs when current flows from intact skin to skin.
Microshock occurs when the skin is breached or aberrant currents are delivered directly to the myocardium. Currents as low as 100 μA can cause dangerous dysrhythmias but <44 μA is unlikley to cause harm
What increases the risk of microshock induced ventricular arrhythmias?
- site of stimulation (ventricles more sensitive)
- proportional to the area stimulated
- increases the longer the current passes
What equipment can deliver microshock?
Aberrant currents can be delivered via
- saline filled CVCs
- pacing wires
- oesophageal doppler probes
How does a floating circuit work?
Equipment is separated from the mains supply by an isolating transformer. Transformers transfer power via a magnetic field so there is no direct electrical connection -so the secondary circuit is isolated from the circuit at the substation.
To be electrocuted, you must form part of the circuit that allows current to flow back to the substation. However, this secondary circuit has no earth, so current cannot pass from you back to the primary circuit and thus you cannot receive an electric shock.
If this secondary circuit was accidentally earthed, then electrocution would be possible. A line isolation monitor should be fitted to warn you if this occurs.
What is an earth leakage circuit breaker (ELCBs)?
- provide further protection against electrocution
- disconnect the power supply from a piece of faulty electrical equipment when current flows down the earth wire
- can be voltage/current operated
How does a voltage operated ELCB work?
- detect a rise in potential difference between the protected metalwork (earth connected case) and a distant earth electrode
- operate at a detected potential of 50V and activate a solenoid-driven circuit breaker to isolate the downstream equipment from the mains supply
How do current operated ELCBs work?
They compare the current passing down the live wire with that returning via the neutral wire (should be equal magnitude but opposite).
If there is current leakage via earth (or worse, a person) these currents are no longer equal.
Will detect the difference, activate a solenoid and disrupt the power supply.
How does monopolar diathermy work?
- 2 electrodes (active and ground) and the patient’s tissue form a circuit
- the current may form unwanted effects in medical equipment (eg pacemakers) or tissue (burns, arrhythmias) if the electrode positions are not considered carefully
- uses power of 100-200 watts
- the same current flows through the active electrode and ground plates but current density is much less at the ground plate because of the large surface area
How is cutting achieved by diathermy?
- continuous high freq (approx 400 kHz)
- lower voltage (400 - 1000 kV)
- higher power (>100 Watts)
How is coagulation achieved by diathermy?
- interrupted or modulated current
- this means tissue can cool and doesn’t vaporize
- current flows for approx 10% of the time
- lower freq (250 - 400 kHz)
- higher voltages (up to 9000 V)
- lower power (usually <100 Watts)
What is blended diathermy waveform?
A blend of cutting and coagulating current patterns gives a mixture of cutting and coagulation effects.
