Electrical
safety
By
Dr. Ahmed Mostafa
Assist. Prof. of anesthesia and I.C.U.
Electrical safety
The hazards associated with the use of electrical
equipment are:
• Electric shock – macroshock.
• Electric shock – microshock.
• Diathermy hazards.
• Burns.
• Fire and explosion.
Electrical safety
Electric shock (macro-shock)
• Occurs with the external application of a
voltage to the skin, causing an electric current
to pass through the body tissues.
• Commonly electric shock occurs from the AC
mains supply.
• Occurs from the mains supply, when the body
forms a circuit between the live mains line and
a local earth connection or the neutral mains
line.
Electrical safety
Electric shock (macro-shock)
Earthed circuit:
Electrical safety
Electric shock (macro-shock)
Isolated circuit:
Electrical safety
Electric shock (macro-shock)
• It produces VF when a current of 100 – 200
mA is applied to the skin and passes to the
heart (VF threshold).
• Its risk is increased by:
1- The size of the current:
- Small (e.g. 1 mA) ---» tingling.
- If current increases ---» Pain, muscle spasm.
- Large (e.g. 100 -200 mA) ---» VF.
Electrical safety
Electric shock (macro-shock)
2- The impedance:
If large ---» the current will be small.
Source on impedance:
- Skin: 10 kΩ.
- Antistatic footwear: About 100 kΩ.
- Antistatic floor: About 25 – 50 kΩ
Electrical safety
Electric shock (macro-shock)
3- The current density:
A current passing through a small area is more
dangerous.
4- the form of electric current:
50 Hz AC is more dangerous than 1000 Hz DC.
5- Duration of exposure.
6- Myocardial disease.
Electrical safety
Electric shock (micro-shock)
• Current is delivered internally to the myocardium,
causing arrhythmias. The conducting pathway may
be through an intravenous catheter or pulmonary
artery catheter and its contained fluid.
• The magnitude of currents required to produce
ventricular fibrillation in micro-shock is in the order
of 100–150 μA. This is much smaller than in the
case of macro-shock.
• Its risk is increased by the same factors like macroshock
Electrical safety
Electric shock (micro-shock)
Electrical safety
Prevention of electrocution
General measures:
- Adequate maintenance and regular testing
of electrical equipment.
- Ensuring the patient is not in contact with
earthed objects.
- the wearing of antistatic shoes, whose high
impedance will reduce any current flowing
through the body.
Electrical safety
Prevention of electrocution
Equipment design:
Different classes of equipment are specified for the
medical working environment.
The safety specifications may refer to the risk of
electric shock, as follows:
Class I – Earthed
Class II – Double-insulated but not earthed
Class III – Low voltage (<24 V), battery-powered
Electrical safety
Prevention of electrocution
Earth circuits:
These are designed into equipment in order
to reduce risk of electric shock and reduce
interference. Earth connections reduce the
risk of electric shock by maintaining
exposed metalwork at zero potential. Such
metalwork cannot then deliver an electric
shock current if inadvertently contacted.
Electrical safety
Prevention of electrocution
Isolated patient circuit (Floating circuit):
Floating circuit is a circuit, isolated from the
mains because it has no physical point of contact
between it and a circuit of higher voltage. i.e. it
gets its power via a transformer.
Electrical safety
Prevention of electrocution
Isolated patient circuit (Floating circuit):
Electrical safety
Prevention of electrocution
Leakage currents:
• Small electric currents (<500 μA) which arise
unintentionally.
• Such currents can either pass or ‘leak’ down to
earth through a designed safety circuit (e.g.
earth connection, antistatic shoes) or may flow
through an unintentional pathway, creating a
risk of microshock.
Electrical safety
Prevention of electrocution
Leakage currents:
• 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).
(B indicates that the equipment is safe to be contact with the body surface)
• Type BF: as for type B but uses a isolated (or
floating circuit).
Electrical safety
Prevention of electrocution
Leakage currents:
• 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
(C indicates that the equipment is safe to be contact with the cardiac muscle)
Electrical safety
Prevention of electrocution
Isolating transformer:
This system supplies all equipment attached to
the patient via a transformer so that no
equipment associated with the patient is directly
connected to the mains.
Circuit breaker:
This is a sensitive mains switch which operates
to disconnect the mains whenever abnormal
currents are detected
Electrical safety
Prevention of electrocution
Suitable footwear:
• Footwear impedance should be designed to
isolate the wearer from earth.
• The impedance should be high enough to
prevent large current passing to earth (avoiding
electric shock), but low enough to allow a
leakage current to earth to prevent the wearer
and clothing from accumulating a static
charge. Such shoes normally have an
impedance of between 100 kΩ and 1 MΩ.
Electrical safety
Prevention of electrocution
Ingress protection (IP) rating:
• Electrical equipment has safety information
affixed to it to provide the user with
information about which environments it is
safe to use in. One such rating is the ingress
protection (IP) rating.
• The higher the number, the better the
protection.
Electrical safety
Prevention of electrocution
Thank you
Dr. Ahmed Mostafa