Sun21November0452PM 7

Monitoring equipment in theatre and intensive care is subject to rigorous manufacturing and design standards to prevent electrocution of staff and patients.

Which one of the following classifications of electrical equipment is least likely to result in the microshock of a patient?

(Please select 1 option)

I (BF) Incorrect answer selected

III

II (BF)

II (CF) This is the correct answer

II (B)

Explanation

Microshock is the passage of very small currents or voltages to the myocardium via a saline filled central venous catheter or pacemaker wires. Currents sufficient to cause ventricular fibrillation can be as low as 100-150 microamperes (µA).

The risk of micro- or macroshock will depend on the construction of a piece of electrical equipment.

The main classes of electrical equipment are:

I: The outer casing of the appliance is connected to an electrical earth. A fault in the appliance will cause a live conductor to contact the casing will allow a current to flow in the earth conductor which in turn will cause the melting of a fuse which will cut off the electric supply to the device.

II: The appliance has double insulation and does not necessarily need an earth connection.The symbol for double insulation is a black line square inside another one.

III: The electrical supply for this type of device is derived from a separated extra safety voltage source (SELV). The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock, that is, 35V AC or 60V DC in dry conditions. These should be lower if used in wet conditions. There is no risk of macroshock but there is still a risk of microshock.

The subtypes of classes I and II are designed so that leakage currents are low should a single fault in the apparatus develop. These sub-types are:

B (body): Maximal permissible leakage current is 500 µA. This current is large enough to cause tingling of the skin but not cause significant injury. It is however large enough to cause microshock.

BF (body floating): These devices have an isolating capacitor or transformer. These separate the secondary circuit from the earth. Permissible leakage current is the same as that of type B equipment.

CF (cardiac floating): The maximum permissible current leakage in the event of a single fault is 50 microamps and is the type of equipment least likely to cause microshock.

Answer Statistics

1

8%

2

22%

3

9%

4

57%

5

7%

Times answered: 247