5. Electrical Safety

Question 1

Electricity:

Answer 1

this is the flow of electrons,
which is driven by potential difference (the voltage)
through a conductor past a given point per unit time.

This current is measured in amperes

Question 2

Resistance

Answer 2

Resistance:

this is the resistance along a conductor to the flow of current.

It is not frequency-dependent.

Resistance is measured in ohms.

Question 3

Ohm’s law

Answer 3

:Ohm’s law this states that the electrical potential (V) = current (I) x resistance (R).

Question 4

Impedance:

Answer 4

The impedance is the sum of all the forces impeding electron flow in an AC circuit.

Unlike resistance,
it is dependent on frequency and
includes resistors,
capacitors and inductors.
(Insulators are high-impedance devices;
conductors are low-impedance devices.)

Impedance through capacitors and inductors is related to the frequency at which AC reverses direction.

Impedance is also measured in ohms (volt/ampere).

Question 5

Lethal current:

Answer 5

the relationships described previously explain how dangerous currents can be generated.

Ohm’s law determines the magnitude of the current that flows,
I = V/R. An individual standing on an antistatic floor may
have an impedance of 20 kΩ or more, and so,

should he or she touch a live enclosure, the current flow will be 240/20,000, or 12 mA.

Wet hands or fluid on the floor may reduce the impedance to 2 kΩ, and so the current, 240/2,000, becomes potentially lethal at 120 mA.

This is not enough to blow the fuse and the circuit remains live.

Question 6

Risks to Patients within the Operating Theatre

Answer 6

Patients can become part of an electrical circuit in two ways

Direct connection (resistive coupling):

Indirect connection (capacitive coupling):

Question 7

Direct connection (resistive coupling):

Answer 7

if any part of the body is directly in contact with an electricity source
or with an earthed object, then current may pass through the patient to earth.

This can be caused either by faulty equipment or by leakage currents.

As all electrical equipment is at a higher potential than earth,
current seeks to flow to earth through a circuit of which a patient may form part.

Medical equipment is well insulated and these leakage currents
are usually small, but they do still carry the risk of microshock

Question 8

Indirect connection (capacitive coupling):

Answer 8

in some circumstances,

the body can act as one plate of a capacitor.

If DC is applied to a capacitor such as a defibrillator,
current continues to flow only until the positive plate
reaches the same potential as the electrical source.

If, however, AC is applied,
then the plates alternate polarity at the same frequency as the current.

The repetitive pattern of charge and discharge sets up a
current flow across the gap with the effective completion of the circuit.

A patient on an operating table can therefore act as one plate of a capacitor while the theatre
light with its 50 Hz AC supply forms the other.

Question 9

Minimizing Risks

Answer 9

Use of appropriate (and well-maintained) equipment.

1. Identification:

Question 10

1. Identification

Answer 10

equipment designed for medical use is generally of high specification
with an identifier to show the grade of protection that it offers.

Class I, II, III

B BF CF

Question 11

Class I

Answer 11

offers basic protection only.

Any conducting part that is accessible to the user,

such as the casing, must be connected to earth,
and must be insulated from the main supply. (Such equipment has fuses on the live and neutral supply in the
equipment, as well as on the live wire in the mains plug.)

Question 12

Class II:

Answer 12

this equipment has reinforced,
or double, insulation that protects all the
parts that are accessible.

It does not require an earth.

Question 13

Class III:

Answer 13

This equipment uses safety extra low voltage (SELV)

which does not exceed 24 V AC.

There is no risk of gross electrocution, but microshock is still possible.

Question 14

Type B

Answer 14

Such equipment has low leakage currents;

0.5 mA for class 1B and

0.1 mA for class IIB.

Question 15

Type BF

Answer 15

Type BF is the same as type B,

except that the piece of equipment that is applied to the
patient is isolated from all its other parts.

Question 16

Type CF

Answer 16

This is class I or II equipment which is considered safe for direct
connection to the heart.

Leakage currents are extremely low,

being 0.05 mA per electrode for class
ICF equipment and 0.01 mA per electrode for class IICF.

Question 17

Common earth

Answer 17

Voltage differences between multiple pieces of
medical equipment increase the risk of leakage currents

which may flow from the higher to the lower
potential via the patient.

This risk is minimized if the equipment is connected to a
common equipotential earth point via a single cable.

Question 18

Isolated (floating) circuits:

Answer 18

in these circuits the equipment that is

mains-powered is separated from the patient circuit

by an isolating transformer

(comprising primary and secondary coils that are insulated from each other).

AC from the primary (earthed) mains supply induces current
in the secondary coil,

which means that although the patient circuit is live,
it remains earth-free.

Question 19

Earth leakage circuit breakers (ELCB):

Answer 19

these devices do not protect against short circuits or overloading

(appropriate fuses are still required),

but they do cut off the electrical supply to faulty equipment
in the presence of current leakage.

There are different types,
but in simplistic terms each consists of a tripping coil which,

when activated by excessive current,
trips a relay which interrupts the supply

Question 20

Clinical Applications

Effects of electricity

Answer 20

1 mA a subject will feel tingling

at 5 mA definite pain.

At 15 mA there is tonic contraction of muscles,

which at 50 mA involves all the muscles of respiration.

At 100 mA ventricular fibrillation supervenes.

Question 21

Electrocution:

Answer 21

this can happen should a patient become part of an electrical circuit.+

The main problem is the fibrillatory potential of the current,

which, if applied externally, need reach only 50–100 mA.

Such current disrupts the normal function of cells,
causing muscle contraction,
respiratory paralysis
ventricular fibrillation.

Question 22

Which type of current is more damaging

Answer 22

The current frequency is also important,
with 50 Hz (the frequency of alternating current [AC] in the UK)
being optimally lethal.

AC at 50 Hz can generate high voltages economically and can readily be transformed,

but it will interfere with ion flux across all cell membranes and force ions in both directions.

(The ion pump can cope better with direct current [DC] voltages.)

Higher frequencies are much less dangerous and above 100 Hz have no fibrillatory potential.

In electrocution, there is additional thermal injury,
caused as the electrical energy dissipates through tissues.

The severity of the electrical burn is directly proportional to the current density and
its duration of application.

Question 23

Microshock:

Answer 23

Gross electrocution by externally applied energy requires currents of around 100 mA,

but very much lower currents in the region of 50–100 μA can
induce ventricular fibrillation if they are applied directly to the ventricle.

This rare phenomenon is known as microshock.

It can occur only with a combination of factors that arise in specialized situations

in which the patient accidentally becomes part of an electrical circuit.

Microshock requires an electrical contact applied directly over a small area of the myocardium and which can be earthed through the patient.

Faulty equipment, even with very low leakage currents,
but which is connected to intracardiac devices such as pacing wires or catheters,
is capable of delivering this microcurrent directly to the ventricle and inducing fibrillation.

Someone holding a pacing wire in one hand while touching the leakage source
with the other may inadvertently complete the circuit and electrocute the patient.

The risk is lessened in this instance by wearing gloves, and in general by the use of
earth-free mains supply.