Electrical safety

Question 1

If you had hand-to-hand or hand-to-foot contact with a pair of conductors carrying alternating current (AC) e.g. a live wire and the ground, what would be the effect of these currents (in mA)
1
5
15
30
50
70
100
1000

Answer 1

1mA -> tingling sensation
5mA -> pain
Over 15 mA ->generalised muscle contraction, can cause inability to let go and asphyxia
Over 75mA -> VF
Over 100mA (at frequency of 50-60Hz) -> burns

Note these thresholds are DIFFERENT to diathermy current

Question 2

Which tissues provide the greatest protection to effects of electricity

Answer 2

Different biological tissues have different electrical resistances.
* Thick, dry skin provides greatest protection
* Once skin breached, tissues much more vulnerable to electricity
* Mucous membranes, vascular areas (e.g. volar aspect of arm, inner thigh) are more vulnerable

Question 3

Effects of mains AC passing through the body: magnitude of current vs duration of contact.
What hazards occur in the areas a-e?

Answer 3

a - usually no effect
b - usually no harmful effect
c - usually no danger of VF
d - VF possible
e - VF probable

Shaded area represents protection offered by a circuit breaker.
Note logarithmic scales

Question 4

What factors determine the harmful effect of electrical (AC) current?

Answer 4

• Type of tissue in contact
• Magnitude of current
• Duration of contact
• Frequency of current (sensitivity of biological tissue to unwanted excitation e.g. VF depends on current frequency)

Question 5

What are the limits of duration and amplitude of current that will usually avoid VF?

Answer 5

50mA for 5 seconds or 500mA for 50s or combination between these limits

Question 6

At which electrical current frequency is VF most likely to occur?

Answer 6

50-60Hz, the frequency used in mains electricity transmission systems

Graph shows sensitivity to unwanted excitation of biological tissues at different current frequency (x-axis is in Hz on log scale)