Electricity

Question 1

Effect of 1 mA current

Answer 1

Tingling

Question 2

Effect of 5 mA current

Answer 2

Pain (anything above 5 mA is painful)

Question 3

Effect of 15 mA current

Answer 3

Severe pain and muscle contraction

A current of 15 mA or more causes generalized muscle contraction and can therefore cause death by asphyxiation

Question 4

Effect of 30 mA current

Answer 4

‘Let go’ threshold

Question 5

Effect of 50 mA current

Answer 5

Respiratory muscle contraction, asphyxia

Question 6

Effect of 70 mA current

Answer 6

Multifocal beats, cardiac failure

Question 7

Effect of 100 mA current

Answer 7

Local burns, ventricular fibrillation

Question 8

Effect of 1000 mA current

Answer 8

Extensive burns, charring

Question 9

Frequency at which VF is most likely to occur

Answer 9

50-60 Hz

Question 10

Overhead transmission of electricity

Answer 10

16000V at 50 Hz
At the substation, there is a transformer to step down to 3 x 240V (RMS) windings
One end is bonded to the “star point” - this is also where the paired neutral returns to

Question 11

Electrical safety: Class I

Answer 11

Live and neutral wires supply the load in a metal case. The safety features are:

1. Metal case is earthed to ground any leakage from the circuitry to the case
2. Return earth pathway is linked to the star point
3. A fuse on the live wire trips if the current leakage is too high

NB if the earth lead from the case to earth is broken, the only way for a current leak to discharge is through someone touching the case! (therefore it’s dangerous)

Faulty earth on one device connected to the patient may discharge through the working earth connected to a different device - causing current flow!

Question 12

Microshock

Answer 12

The skin is the biggest defence against electric shock - once it is breached, even a tiny (100 microAmp) shock can cause VF if connected directly to the heart. A microshock implies the shock is inside the body.

Question 13

Improving Class I devices

Answer 13

1. Non-earthed isolating transformers can be used to isolate the patient connected part of the circuit
2. Using current-operated earth leakage circuit breakers that sense differences in the return (neutral) current and break the circuit - sensitivity <30mA within 30ms

Question 14

Type B devices

Answer 14

For external connection to patient

• May be Class I, II or III
• May not be directly connected to patient heart
• Maximum allowable DC leak is 50 micro Amps (or 10 micro Amps with multiple circuit faults)
• Maximum allowable AC leak is 500 micro Amps (or 100 micro Amps with multiple circuit faults)

Question 15

Type BF devices

Answer 15

For external connection to patient - isolated patient part

• Patient connected part is isolated from all other circuits - it is ‘floating’ (cf. BF, CF)
• Maximum allowable DC leak is 50 micro Amps (or 10 micro Amps with multiple circuit faults)
• Maximum allowable AC leak is 500 micro Amps (or 100 micro Amps with multiple circuit faults)
• If the patient connected part is connected to the mains, the circuit must have a leakage limit of 5 mA when 110% of mains voltage is applied

Question 16

Type CF devices

Answer 16

For internal connection to the heart, isolated patient part

• Patient connected part is isolated from all other circuits - it is ‘floating’ (cf. BF, CF)
• It may be connected to the patient’s heart
• It may be Class I or Class II (not Class III)
• Maximum allowable DC leak is 50 micro Amps (or 10 micro Amps with multiple circuit faults)
• Maximum allowable AC leak is 50 micro Amps (or 10 micro Amps with multiple circuit faults)

Question 17

Diathermy

Answer 17

Uses current in the MHz range (compare: currents causing VF in the 50 Hz range)
Not earthed
Poorly applied plates can cause burns

Question 18

Static electricity and sparks

Answer 18

• Avoid flammable anaesthetic agents
• Antistatic floor
• Antistatic wheels conduct to the floor

Question 19

Class II devices

Answer 19

Double insulated - does not require a connection to earth. Symbol: ⧈

Question 20

Class III devices

Answer 20

Safety extra low voltage, <24V AC RMS from mains (or battery powered <50V)

Question 21

Lasers: Class 1

Answer 21

Lasers cannot emit radiation at any known hazard levels (for the eye); this means very low power output

Question 22

Lasers: Class 2

Answer 22

These are low-power visible lasers, at a radiant power not above 1 mW

Question 23

Lasers: Class 3

Answer 23

These are intermediate and moderate power lasers, and are hazardous only if the beam itself is directly viewed; class 3a devices can have a power output up to 5 mW provided the beam is sufficiently divergent that the eye may be protected by the blink reflex; class 3b devices have power output up to 500 mW, when direct viewing may be hazardous to the eye

Question 24

Lasers: Class 4

Answer 24

These are high power lasers (>500 mW, continuous beam), which are very hazardous to view and are a hazard to skin as well. Most medical lasers are class 4

Question 25

CO2 Laser

Answer 25

10,600 nm - far infra red - minimal penetration - for cutting

Question 26

Ar Laser

Answer 26

488-575 nm - blue-green - for haemostasis of blood vessels e.g. in retina

Question 27

Nd:YAG

Answer 27

1064nm - maximum penetration