Physics - electricity

Question 1

Is a shock from an AC or DC supply more dangerous? Why?

Answer 1

AC shock is about three times as dangerous as DC for the same magnitude of current.
DC produces a single muscle spasm which often throws the person clear. It can produce arrhythmia. Prolonged exposure to low DC currents can produce chemical burns.
AC causes muscle spasm due to tetanic effects (maximal at mains frequency - 50Hz), muscle spasm may cause individual to grip the contact uncontrollably. AC currents cause localised sweat release which lowers skin resistance and increases tissue current.

Question 2

How are mechanisms of patient injury from electricity classified?

Answer 2

Electric shock
- macroshock
- microshock
Burns
- Direct heating
- fire
- explosions
Interference
- Monitoring
- therapeutic equipment

Question 3

What is the definition of an ampere?

Answer 3

That constant current which if maintained in two parallel conductors of infinite length and negligible cross section placed 1m apart in a vacuum will produce between the conductors a force of 2x10-7N per metre of length.

Question 4

What is the usual pattern of variation of AC current?

Answer 4

Sinusoidal

Question 5

How is the magnitude of a DC and AC current determined?

Answer 5

DC current - just measured.
AC current - Measurement of the average voltage of a sinusoidally varying ALTERNATING current will yield ZERO, irrespective of the actual
magnitude since the forward and reverse currents (or voltage) are the same and therefore cancel out. We must remove the sign (+/-).
So we square, calculate mean and then square root result (ROOT of the MEAN of the SQUARED value) RMS.

Question 6

What is a transformer?

Answer 6

Two coils linked only by a magnetic field. There is no electrical connection between them.
The passage of an AC current through one coil will produce a sinusoidally varying magnetic field. The second coil is exposed to this varying field and as a consequence a voltage and current is induced in this coil. The induced voltage in the second coil depends upon the voltage applied to the first coil and the ratio of the number of turns forming the first and second coils. Energy transfer is very efficient.

Question 7

How is AC converted to DC?

Answer 7

Using a diode (effectively a one-way valve for electricity)

Question 8

What are the requirements of a local earth connection?

Answer 8

1. Leakage current of <0.5mA
2. Impedance <0.1 ohm
3. Earth surge test - 25 A for 5 seconds

Question 9

What effects are likely to be caused by the following currents?
1. 0-5 mA
2. 5-10 mA
3. 10-50 mA
4. 50-100mA

Answer 9

1. 0-5mA: Tingling sensation
2. 5-10 mA: Pain
3. 10-50 mA: Severe pain, muscle spasm
4. 50-100mA: Respiratory muscle spasm, ventricular fibrillation, myocardial failure

Question 10

How is electrical equipment classified?

Answer 10

Class I: earthed
Class II: double-insulated, not earthed
Class III: low voltage (<24V) battery-powered

Class I, II and III equipment may also contain floating circuits that are not earthed but are isolated from the rest of the circuit by transformers.

Can also be classified according to the risk of leakage currents:
Type B - may be class I, II or III. Has a low leakage current even in the presence of a fault. Can be safely connected to the patient either internally or externally but is not considered safe for direct connection to the heart.
Type BF: similar to type B but is safer because the patient is isolated via a floating circuit. Maximum permitted leakage current is 100microAmps but still not considered safe enough to be connected directly to the heart.
Type CF: considered safe for direct connection to the heart because the leakage current is very small (<10 microA). Used for thermodilution catheters, ECG leads and pressure transducers.

Question 11

What is a macroshock?

Answer 11

Occurs with external application of a voltage to the skin, causing a current to pass through the body tissues. Commonly electric shock occurs from the AC mains supply.