Electrical Safety

Question 1

What currents can cause:

1. Tingling
2. Unable to let go
3. Asphyxia, slow death
4. Ventricular fibrillation, quick death

Answer 1

1. 1 mA - tingling
2. > 15 mA
3. > 50 mA
4. > 75 mA

Question 2

What type of current causes injury?

Answer 2

AC

Question 3

What current required to cause burns?

Answer 3

at least 100 mA of mains current
(at a frequency of 50 or 60 Hz)
is required for burns to occur.

Question 4

Why is wet skin more likely to be electrocuted?

Answer 4

It has less electrical resistance than dry skin.

Question 5

What voltage and frequency is carried in overhead transmission lines?

Answer 5

16,000V at 50 Hz.

Question 6

What types of devices aim to reduce an electrical hazard?

Answer 6

1. Type H electrical equipment, including
   Class I
   Class II
   Class III
2. Type B electrical equipment
3. Type BF electrical equipment
4. Type CF electrical equipment

Question 7

What is a micro shock?

Answer 7

The definition includes the concept of an electrical connection within the body rather than to its surface.

Because of the direct electrical connection to the heart, VF is a very likely outcome, although it does not usually occur at currents below 10 mA.

Question 8

By what magnitude is the electrical resistance of dry skin greater than that of mucous membrane?

Answer 8

Dry skin has an electrical resistance of 10–40 kΩ/cm2, while mucous membrane has a resistance of 0.1 kΩ/cm2.

Dry skin therefore has a resistance of between 100 and 400 times that of mucous membrane.

Question 9

Describe conditions under which microshock can occur?

Answer 9

• can occur when a device is connected anywhere in circulation.
• death caused by VF
• 100 mA required to be considered problematic

Question 10

What faults need to occur to cause patient electrocution in diathermy?

Answer 10

1. There is a faulty monitor with a leakage current to the case.
2. There is also a fault in the monitor earth lead.
3. The patient monitor earth lead is now live.

Electrocution of the patient occurs by completion of the circuit to earth through the patient and the correctly-earthed diathermy device.

Question 11

What are the safety measures included in class 1 devices?

Answer 11

1. The metal case is earthed in order to ground any leakage current from the circuitry to the case
2. The return earth pathway is theoretically linked to the (distant) neutral star point
3. A fuse on the live wire melts if the earth leakage current becomes too large

These properties all enhance safety in the presence of a leakage current between the circuit and the case, but not if there is also a breakage in the earth lead, when the case will be live.

Question 12

How can induced currents cause electric hazards in the ICU?

Answer 12

Stray currents can flow through patient to earth from near by unattached electrical devices.

1. Capacitative coupling - neighboring unattached device and patient act together as two plates of a capacitor.
2. Inductive coupling - unconnected neighbouring electrical circuits, with associated magnetic field, induces current flow in patient.

Question 13

How can the safety of Class 1 equipment be improved?

Answer 13

By using a non-earthed isolating transformer to supply power to patient applied parts.

Question 14

How is a toroid transformer used in electrical safety?

Answer 14

It acts as a circuit breaker if there is a current leakage. Usually live current and neutral current returning should be equal, inducing an equal and opposite magnetic field in the toroid which cancels out.

But when a current has leaked, this current will not be equal and a magnetic field is induced in the toroid transformer. This produces a current which opens a switch that disables the circuit.

Question 15

What devices can be used in current operated earth leakage circuit breakers?

Answer 15

Toroid transformers or operational amplifiers.

Question 16

Characteristics of Type B equipment?

Answer 16

1. Maybe types I, II, III
2. Mains powered/ internally powered
3. Max DC - 50microAmps single fault, 10microamps multiple circuit faults.
4. Max AC - 500 microAmps single fault, 100 microAmps multiple circuit faults.

External connection to patient body, not to heart.

Question 17

Type BF equipment characteristics?

Answer 17

1. The patient-connected part is isolated from all other circuits – it is ‘floating’
2. Max allowable DC patient leakage current is 50 μA with a single fault, 10 μA with multiple circuit faults
3. The maximum allowable AC patient leakage current is 500 μA with a single fault, 100 μA with multiple circuit faults.
4. The maximum allowable patient leakage current when mains voltage is directly applied to patient-applied part is 5000 μA, assuming a single fault.
5. Furthermore, a patient leakage current of 5 mA would not be exceeded even if 110% of the mains voltage were applied between the patient-applied parts and earth.

Question 18

Type CF equipment characteristics?

Answer 18

1. It may be Class I or II mains-powered with an internal power source.
2. It may be connected to the patient’s heart.
3. As it is ‘F’ type, it has an isolated ‘floating’ patient-applied part.
4. The maximum allowable DC patient leakage current is 50 μA with a single fault, or 10 μA with multiple circuit faults.
5. The maximum allowable AC patient leakage current is 50 μA with a single fault, or 10 μA with multiple circuit faults.

The maximum allowable patient leakage current (when mains voltage is directly applied to the patient-applied part) is 50 μA, assuming a single fault condition.

Question 19

What does the effective function of diathermy depends on?

Answer 19

high current density at the active electrode and low current density at the indifferent electrode (uniporal)

Question 20

How can the build up of static electricity be minimised in the anaesthetic machine / breathing circuits?

Answer 20

1. effective earthing of the anaesthetic material through antistatic wheels.
2. effective antistatic floor material with uniform resistance 20 kOhms, to 5 MOhms.
3. Antistatic material for breathing systems in the presence of flammable agents.

Question 21

When is an earth plate used in diathermy and why?

Answer 21

A diathermy circuit which is intended to be earthed should have a properly-applied earth plate to avoid unintended burns at points of high current density.

Question 22

What is the purpose of Class 1 earthed wire?

Answer 22

To provide a path of low resistance for current to return to the local power substation in the event of a fault.

Question 23

Unique features of Class II?

Answer 23

1. protected by double or reinforced insulation.

2. NO earthed wire, minimal chance of someone coming into contact.

Question 24

Class III features

Answer 24

Class III equipment is powered internally by a battery or by Safety Extra Low Voltage (SELV).

The specifications for SELV include:

1. Voltage not greater than 25 V AC
   or 60 V DC
2. No earth connection (usually floating circuit)
3. Low risk of accidental contact with higher voltage

Question 25

When might equipment predispose patients to microshock?

Answer 25

aberrant currents delivered to myocardium via saline filled CVC (very electrically conductive), pacing wires or oesphageal doppler probes.