Chapter 4 Part 1 CENG Electrical Safety and Standards

Question 1

When is there a concern about electrical safety?

Answer 1

Whenever a person is connected to an electrical device by a grounded conductive pathway that has low resistance (e.g. ECG electrodes)

Question 2

An electrical situation is worsened when? (3)

Answer 2

1. When a person is well grounded (no shoes, exposed toes/heels, contact with equipment
   metallic parts);
2. Has wet skin (perspiration, after bath, sweaty palm);
3. Stands in pools of water (urine, conductive fluids, flood).

Question 3

What are some defective equipment that patients may be in contact with?

Answer 3

In hospitals, patients may be in contact with defective equipment such as diagnostic & therapeutic equipment, table lamp, reclining bed controller, hair dryers, TVs, etc.

Question 4

What can damaged electrical cords lead to?

Answer 4

Damaged electrical cords can lead to possible shocks or electrocutions.

Question 5

What can a flexible electrical cord be damaged by?

Answer 5

A flexible electrical cord may be damaged by door or window edges, by staples and fastenings, by equipment rolling over it, or simply by aging.

Question 6

What are the 3 possible reasons for electrocution or electric shock or contact with electrical hazards?

Answer 6

1. Faulty electrical equipment/machinery or wiring.
2. Damaged receptacles and connectors.
3. Unsafe work practices.

Question 7

What is done to minimize the impact of power failure to critical operations? (Eg. operating theatres and ICU wards)

Answer 7

Medical equipment in these areas are usually plugged into power socket outlet with red rockers which has backup generator.

Question 8

What is an anti-surge plug?

Answer 8

An anti-surge plug is a protection device that has a varistor between LIVE and NEUTRAL

Question 9

What does an anti-surge plug protect?

Answer 9

Anti-surge plug is used to protect equipment against excessive transient voltages

Question 10

How does an anti-surge plug work?

Answer 10

By shunting the current generated by the high voltage away from sensitive components.

Under normal conditions, the resistance of the varistor is very high. When the connected voltage gets higher than the specification of the varistor, the resistance becomes low. This circumstance is used to protect electronic applications from over-voltage.

Question 11

What is a consumer product safety?

Answer 11

Safety Mark (Singapore) is a unique 8-digit registration number traceable to the registrant and the registered models.

It is wise to buy only consumer electrical goods with the Safety Mark (meter testing).

Question 12

What is the Consumer Protection (Safety Requirements) Regulations (CPSR)? - (JUST FOR YOUR INFORMATION)

Answer 12

It requires 33 categories of household products, also known as Controlled Goods, to be registered with Enterprise Singapore.

These products need to be tested to specified safety standards and affixed with the SAFETY Mark before they can be sold in Singapore.

Question 13

Why is current able to flow through the human body and cause various effects?

Answer 13

Amount of electricity will flow through any medium, depending on the resistance that it encounters.

Question 14

What happens when direct current(polarized, non-changing) or high frequency alternating current passes through the human body?

Answer 14

Heating effects and ultimately burns will occur. It is this effect that is intentionally created when electrosurgical generators are used to cut tissue and coagulate fluids.

Question 15

What happens when low frequency alternating current is applied to the body?

Answer 15

If low frequency alternating current is applied to the body, muscular polarisation and depolarisation take place that can ultimately create a “circus movement” in the heart muscle, resulting in fibrillation and death. It is this effect that normally accounts for death due to electric shock.

Question 16

What are the effect of electric shocks?

Answer 16

The effect of electric shock on the human body can be anything from barely perceptible tinges, to muscle spasms, to death. Each can occur from small or large currents, depending on how the currents are introduced into the body.

(Basically, sensations, spasms & death)

Question 17

Difference between microshock and macroshock?

Answer 17

Macroshock, externally applied currents spread throughout the body

Microshock, all the current applied through an intracardiac catheter flows through the heart.

Question 18

What results in macroshock?

Answer 18

Large currents (milliamperes or larger) that are introduced into the body from one external point to another (arm to leg, for example) can result in macroshock.

Question 19

What causes microshock?

Answer 19

If small currents (10 to 100 µA) are introduced into the body from an external source such as a catheter or cardiac pacing wires (during invasive diagnostic or therapeutic procedures),
the resistance to the heart muscle can be very low, and electrocution can occur from microshock (small electric currents applied to a conductor near the heart).

Question 20

What is the physiological effect of 50Hz AC currents on adults at <1 ma?

Answer 20

Imperceptible when externally applied. 1mA = threshold of perception
“Tingling“ sensation. (Internal application near heart can induce ventricular fibrillation at 10 - 100 µA – microshock )

Question 21

What is the physiological effect of 50Hz AC currents on adults at >1 ma?

Answer 21

Mild to painful sensation.

Question 22

What is the physiological effect of 50Hz AC currents on adults at >10 ma?

Answer 22

If contacted by hand or arm may paralyse this region and cause inability to release grip; muscle spasms. (10mA = let-go current)

Question 23

What is the physiological effect of 50Hz AC currents on adults at >20 ma?

Answer 23

Breathing frequently stops (respiratory paralysis), possible fainting, pain.

Question 24

What is the physiological effect of 50Hz AC currents on adults at >75 ma?

Answer 24

Causes ventricular fibrillation

Question 25

What is the physiological effect of 50Hz AC currents on adults at >1 A?

Answer 25

Paralyses the heart; Causes burning of tissues.

Question 26

How strong is the current produced for a microshock than a macroshock at 50Hz AC current?

Answer 26

For microshock, the same effect can be produced with current levels that are only 1/10,000 as great as those listed (Physiological effects of 50Hz AC current on adults)

Question 27

What is the safety standard for leakage current?

Answer 27

Since 10mA or less is the let-go shock level, a safe 5mA leakage current has become the standard.

Question 28

How does a manufacturer receive a listing approval?

Answer 28

The equipment must have a 50Hz leakage current from power line to equipment metal case of less than 5mA.

Question 29

What are the 7 susceptibility parameters?
(TLDBPFS)

Answer 29

1. Threshold of perception
2. Let-go currents
3. Duration
4. Body Weight
5. Points of Entry
6. Frequency
7. Skin & Body Resistance

Question 30

What is threshold of perception between men and women?

Answer 30

Lower for women (mean 0.7mA) than men (mean 1.1mA).

Question 31

What are the let-go current values between men and women?

Answer 31

Values for women (10mA) is about 2/3 that for men (16mA).

Question 32

Duration in susceptibility parameters
How does shock cause ventricular fibrillation?

Answer 32

Shocks must last long enough to take place during the vulnerable period that occurs during the T-wave in each cardiac cycle to cause ventricular fibrillation

Question 33

How does body weight affect fibrillation?

Answer 33

Fibrillation threshold increases with body weight. Fibrillating currents increases from 50mA for 6kg dogs to 130mA for 24kg dogs

Question 34

What is points of entry?

Answer 34

When current is applied at two points on the surface of the body, only a small fraction of the total current flows through the heart. If the two points are on the same extremity, the risk of fibrillation is small, even for high currents.

Question 35

How does frequency affect let-go current?

Answer 35

For frequencies below 10Hz, let-go currents rise, probably because the muscles can relax during part of each cycle. At frequencies above 1kHz, the let-go currents rise again.

Question 36

How does high frequency of the current affect the level of electrical shock?

Answer 36

If the frequency is raised above 1kHz, these current levels no longer produce such sensations (can’t let go, respiratory difficulty) or life-threatening phenomenon (VF or death).

High frequencies in the MHz region will not cause shock at all, but do cause serious burns.

Question 37

Why does high frequency current cause serious burns? (primary effect)

Answer 37

The primary effect of high frequency current (500 kHz to 2 MHz) is to heat tissue AS IT IS CONCENTRATED IN A CERTAIN AREA.

Question 38

What does the amount of heat generated in the burning of tissues depend on?

Answer 38

The amount of heat that is generated depends on the amount of current applied and the area that the current passes through.

This relationship is: Heat = I x I/A

where “I” is the applied current and “A” is the area that the current flows through. I/A is the current intensity in A/cm^2.

Question 39

How does traditional electrosurgery work?

Answer 39

Traditional electrosurgery uses radiofrequency (RF) electrosurgical current to cut and coagulate tissue.

Question 40

State the 5 components and its functions of the circuit for electrosurgical current to conduct/flow through.

Answer 40

1. An electrosurgical unit (ESU) or an RF generator (produces surgical current)
2. Insulated cables (connect the active electrode to the generator)
3. An active electrode (delivers the electrosurgical current to the target tissue)
4. The patient
5. A grounding pad or patient plate with its own cord or cable (To safely return the electrical current from the patient back to the generator through a cord or cable.)

Question 41

Explain the use of the grounding pad?

Answer 41

The grounding pad adhered to the patient’s skin away from the surgical site is intended to safely return the electrical current from the patient back to the generator through a cord or cable.

Because the conductive surface area of the grounding pad is much larger than the active electrode (where cutting, coagulation, or ablation occur), the current is dispersed over a wide area, minimizing the heating of the tissue under the grounding pad.

The risk of a patient burn is increased when there is poor contact quality between the grounding pad and the patient because the current is concentrated at the contact points rather than dispersed over the entire grounding pad.

Question 42

State the 2 types of electrosurgery

Answer 42

Monopolar and bipolar electrosurgery

Question 43

Why are dispersive electrodes usually placed under the patient’s shoulder or buttocks in monopolar electrosurgery?

Answer 43

To ensure that high current densities occur only at the point of contact for the “active” electrode that is controlled by the surgeon.

Question 44

Why does bipolar electrosurgery require less energy?

Answer 44

Bipolar electrosurgery uses lower voltages, so less energy is required.

Question 45

What procedure are bipolar instruments used in electrosurgery more ideally for and why?

Answer 45

It is more ideally used for those procedures where tissues can be easily grabbed on both sides by the forceps electrode because it has limited ability to cut and coagulate large bleeding areas.

Question 46

What is electrosurgical current in the patient restricted to in bipolar electrosurgery?

Answer 46

Electrosurgical current in the patient is restricted to just the tissue between the arms of the forceps electrode.

Question 47

What is the resistance between any 2 limbs?

Answer 47

Any 2 limbs is about 500Ω, with obese people having higher values.

Question 48

How does skin resistance vary?

Answer 48

For 1cm^2 of electric contact with dry skin, resistance is about 15kΩ to 1MΩ. If wet, it is less than 1kΩ.

Question 49

What is the resistance of dry skin?

Answer 49

When dry, skin has a resistance of upwards 100kΩ.

Question 50

What happens when there is an accidental application of 230 V between 2 hands?

Answer 50

Only 2.3mA will flow. However, In a wet environment or on a hot and humid day, that same current path may come to have a resistance as low as 1kΩ, resulting in current flow of 230mA. This may cause ventricular fibrillation.

Question 51

What happens when there is a direct electrical path to the heart via a needle or catheter in an artery or vein?

Answer 51

This directly reduces the resistance and current threshold. Small amounts of current (100mA) can be potentially lethal. Electrical shock in these circumstances is termed “microshock”.

Question 52

How is skin resistance reduced or eliminated? (3)

Answer 52

a) Application of electrodes (needle electrodes more vulnerable than surface ones)
b) Intravenous catheters containing conductive fluids
c) Electronic thermometers placed in the mouth or rectum (bypasses the skin resistance).

Question 53

How does electrical current cause burnt tissues?

Answer 53

Whenever current passes through any resistive element, some of the electrical energy is dissipated in the form of heat.

As a result, damage to the biological tissue in the form of a burn will result when the temperature produced by this phenomenon is high enough to affect the tissue.

Question 54

How does point of contact affect the current concentration in the tissue?

Answer 54

When point of contact is small, the current concentration is high. Thus, leading to burnt tissues

When point of contact is large, the current concentration is low.

Question 55

What happens if the dispersive electrode is too small during monopolar electrosurgery?

Answer 55

If the dispersive electrode is too small, there is a significant risk of skin burn at the contact point.

Question 56

What is the formula for current density?

Answer 56

The absence or presence of burns is directly related to the current density (J) = Current / Area. For any given amount of current, the current density will vary depending on the cross-sectional area of the dispersive electrode.