11. Electrical safety Flashcards
How is mains electricity supplied? >
Mains electricity is an alternating current (AC) supplied at 50 Hz and at 240 V.
- > In AC, the flow of electric charge reverses direction periodically, producing a sine wave pattern.
- > Electricity is delivered as AC to allow it to be transmitted over large distances (hundreds of miles) with very minimal loss of power.
- This is in sharp contrast to direct current (DC), which has a dramatic loss of power over just one mile.
- > AC is generated at a voltage specific to the power plant generator.
The voltage is then stepped up (using a transformer) to allow more efficient transfer of power over long distances before being stepped down (using
a transformer) to 240 V for use.
What is the significance of the earth wire?
> UK mains electricity supply has three wires:
live wire (brown), neutral wire (blue) and earth wire (green and yellow). Remember: ‘Brown is Hot, Blue is Not, Green and Yellow earth the Lot’.
> The neutral wire is so called because it is connected to the earth at the mains transformer, so its electrical potential is ‘neutral’ with the earth.
> The live and neutral wires are relatively simple to understand but the
earth wire often causes some confusion as on the one hand it is described
as a safety measure but on the other hand it is imperative to protect the patient from exposure to earth.
This is further complicated by the fact that
the earth wire has different names like ‘ground’, which can mean different things in different circuits.
> The earth wire is a safety feature of electrical circuits
designed to protect people from exposure to the full current of mains electricity in faulty appliances.
It is a wire that literally returns to earth and completes
a circuit with the generating power station and is connected to any exposed conducting parts of an electrical appliance.
> This means that if the live wire came into contact with a conducting part of the appliance, electricity would flow through the earth wire to earth.
If the earth was not connected, someone touching the appliance would inadvertently act as a conduit for the electricity to flow and receive a severe shock.
By having the earth there, the electricity flows preferentially down the earth wire (because earth is so large and therefore always offers the path of least resistance) rather than through the victim.
> Due to the deliberate application of tissue-damaging currents (diathermy) patients must be protected from contact with earth.
If a patient was
inadvertently connected to earth, they would provide an alternative route for the diathermy current to flow through, which could potentially cause severe burns at the point of contact between the patient and earth.
Another safety reason to avoid patient contact with earth is to prevent the flow of leakage currents from faulty equipment through the patient.
What do the adverse effects of a current depend on?
1
> Type of current:
AC is significantly more dangerous than DC.
AC can cause tetanic muscular contractions
(‘can’t let go’ phenomenon)
that peak at 50 Hz (frequency of mains electricity).
This frequency is also particularly dangerous
to the heart, making it prone to VF.
With DC, there is a single muscle contraction,
which typically throws the victim clear.
2
> Magnitude of current
(V = I × R)
3
> Current density
(total current/area)
4
> Current duration
(increased time means increased heat
and hence increased tissue damage)
5
> Tissues through which current flows
(cardiac muscle is prone to VF).
At what current amplitude would you feel tingling?
> 0–5 mA – tingling
> 5–10 mA – pain
> 10–50 mA – muscle spasm
(15 mA ‘can’t let go’ threshold)
> 50–100 mA –
respiratory muscle spasm and VF
> 5 A –
tonic contraction of the myocardium
(this level of current is rarely
seen except in defibrillation)
How can a pair of shoes keep you safe from current?
How can a pair of shoes keep you safe from current?
> This is all based on Ohm’s law of V = I × R.
Resistance of skin is approximately 2000 Ω,
body tissue is 500 Ω and
shoes are 200 000 Ω.
> If a current was to enter skin, pass through body tissues and then exit through skin again, the total resistance it would encounter would be 4500 Ω (2000 + 500 + 2000).
> The magnitude of this current would therefore be: I = V/R → 240/4500 → 53 mA (person at risk of arrhythmia)
> If current were to enter skin, pass through body tissues and then exit through shoes, the total resistance it would encounter would be 202 500 Ω (2000 + 500 + 200 000).
> The magnitude of this current would now be: I = V/R → 240/202 500 → 1.2 mA (person would feel tingling sensation)
What is a macroshock?
Macroshocks are due to the
passage of current from
one part of the body to another
(e.g. lightning or direct contact
with a ‘live’ instrument whereby
the body completes the circuit
between the mains and earth).
Current intensities required to cause
harm are in the region of mA.
What is a microshock?
Microshocks are due to the passage of
current directly to the myocardium
(e.g. small leakage currents can pass
through the heart via central lines,
PA catheters
and pacing wires).
Current intensities required to
cause harm - very small
in the region of μA.
How is electrical equipment
classified?
Electrical equipment is classified
by its protection from mains electricity
and
by its allowable leakage current.
For mains protection it is classified as:
> Class I –
earthed casing
(all conducting surfaces are earthed)
> Class II –
double insulated casing
(no exposed conducting surface and
so does not need to be earthed)
> Class III –
battery-operated.
> Class I –
> Class I –
earthed casing
(all conducting surfaces are earthed)
> Class II –
double insulated casing
(no exposed conducting surface and
so does not need to be earthed)
> Class III –
> Class III –
battery-operated.
What other measures are taken in
theatre to prevent electrical injury?
- > Anti-static flooring –
this has high impedance to mains electricity
but enough conductance to earth
to prevent the build-up of static electricity.
2
> Relative humidity of 50% –
inhibits the build-up of static electricity
3 > Circuit breakers – these consist of a transformer attached to a solenoid that will break a circuit or sound an alarm
if a stray current above a set
limit is detected flowing to earth.
4
> Non-sparking switches and plugs
5
> Regular checks and maintenance of equipment
