Electrical Safety in Healthcare

Electrical Safety Overview

• Electrical safety is crucial, especially in medical settings, due to the potential hazards electricity poses to patients and staff

Performance Standards

• Approximately 10,000 device-related patient injuries occur annually in the US
• Most injuries result from improper device use, stemming from inadequate training and lack of experience
• Medical procedures present more hazards compared to typical homes or workplaces
• Minimum performance standards were introduced in the 1980s

Electrical Safety Standards

• NFPA-99 is the most commonly used electrical safety standard in the U.S.
• IEC 60601-1 is the most widely used electrical safety standard

Patient Care Vicinity (PCV)

• PCV extends 1.5 m (~6 feet) beyond a patient's bed, table, or treatment area and 2.5 m (~7.5 feet) vertically from the floor
• Clinical Engineering ensures patient-impact equipment within the PCV meets NFPA-99 or IEC 60601-1 standards
• Clinical Engineering may check equipment in non-patient care areas depending on hospital policy

Effects of Current on the Body

• Electric current can stimulate excitable tissue, like nerves and muscles
• Resistive heating of tissue can occur when current passes through it
• Electrochemical burns and tissue damage can result from very high voltages

Physiological Effects of Electricity

• Exposure of 1 to 3 seconds to 60 Hz current via copper wires held in the hands can have various physiological effects
• These effects depend on the current level

Threshold of Perception

• A tingling sensation occurs when local current density excites nerve endings in the skin
• Threshold of perception describes the minimal current detectable by an individual
• This threshold varies significantly among individuals
• With moistened hands grasping small copper wires, the lowest thresholds are about 0.5 mA at 60 Hz
• Thresholds for DC current range from 2 to 10 mA

Let-Go Current

• Higher currents cause vigorous stimulation of nerves and muscles, leading to pain and fatigue
• Involuntary muscle contractions can prevent voluntary withdrawal at higher currents
• Let-go current is the maximal current at which a person can voluntarily withdraw
• The minimal threshold for the let-go current is 6 mA
• Minimal let-go current occurs at commercial power-line frequencies of 50-60 Hz

Respiratory Paralysis, Pain and Fatigue

• Involuntary contraction of respiratory muscles, caused by still higher currents, can lead to asphyxiation if the current is not interrupted
• Respiratory arrest has been observed at 18 to 22 mA during let-go experiments
• Strong involuntary muscle contractions and nerve stimulation can cause pain and fatigue with long exposure

Ventricular Fibrillation

• Some of the current passing through the chest flows through the heart
• Sufficient current can disrupt normal electrical activity in the heart muscle
• If cardiac activity is significantly disrupted, the heart rate may increase to 300 bpm
• The heart's pumping action ceases, leading to death within minutes
• The threshold for ventricular fibrillation varies from 75 to 400 mA for an average-sized human

Sustained Myocardial Contraction

• The entire heart muscle contracts when the current is sufficiently high
• Although the heart stops beating while the current is applied, a normal rhythm resumes when the current flow is interrupted, similar to defibrillation
• Animal studies on ac-defibrillation showed minimal currents for complete myocardial contraction ranging from 1 to 6 A
• There is no known irreversible damage to heart tissue from brief applications of these currents

Burns and Physical Injury

• Limited data is available on currents exceeding 10 A, especially for short durations
• Resistive heating causes burns, mainly at the skin's entry points due to high resistance
• Voltages exceeding 240 V can puncture the skin
• High currents can cause the brain and nervous tissue to lose all functional excitability
• Excessive currents may cause strong muscular contractions, potentially detaching muscle attachments from bone

Grounding and Electrical Safety

• Electrical safety focuses on the current path to the ground.
• Birds on a power line are not electrocuted because they're not grounded, preventing a current path

Point of Entry: Macroshock

• When the current enters and exits at two points on the surface of the body, only a small fraction goes through the heart
• Large, externally applied currents are termed macroshocks

Point of Entry: Microshock

• Patients are more vulnerable to electric shock when invasive devices directly contact cardiac muscle
• If a device provides a conductive path to the heart, insulated everywhere except at the heart, very small currents, known as microshocks, can induce ventricular fibrillation

Classes of Medical Electrical Equipment

• Electrical equipment is classified based on its electric shock protection method

Class I Equipment

• This equipment has a protective earth
• The insulation between live parts and exposed conductive parts (like the metal enclosure) provides the basic protection
• Protective earth activates if a fault would make an exposed conductive part live

Class II Equipment

• Protection against electric shock is achieved through double insulation
• The first layer of insulation provides basic protection
• Supplementary protection is provided by a second layer of insulation to prevent contact with live parts if the basic protection fails
• Basic insulation includes physical separation of live conductors from the equipment enclosure, using air as an insulator

Supplementary Insulation

• A non-conducting plastic enclosure acts as the supplementary insulation
• There is little risk if a faulty wire touches the inside because the case is an insulator

Class II Equipment Symbol

• The symbol for Class II equipment consists of two concentric squares, indicating double insulation

Class III Equipment

• Protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present
• SELV is defined as a voltage not exceeding 25V AC or 60V DC
• This equipment is either battery operated or supplied by a SELV transformer

IEC Standards Note

• Current IEC standards for medical electrical equipment safety do not recognize Class III equipment
• This is because the limitation of voltage is not considered sufficient to ensure patient safety
• All mains-connectable medical electrical equipment must be classified as either Class I or Class II

Types of Medical Electrical Equipment

• Protection degree of medical devices is based on type designation
• Since different applications have different safety needs, electrical safety requirements vary

Type B Equipment

• Applied parts are generally non-conductive
• Applied parts can be immediately released from the patient
• Example: Non-invasive BP monitors

Type BF Equipment

• Devices that have direct contact with the patient, or parts that have long-term contact, are considered Type BF
• Example: ECG Monitors

Type CF Equipment

• Applied parts of devices have direct contact with the heart
• Example: Invasive pressure monitors and defibrillator paddles

Isolated Power System

• Even a good separate grounding system for each patient cannot prevent hazardous voltages from ground faults
• A ground fault is a short circuit between a live conductor and ground that injects large currents into the grounding system
• Shortcuts through conductive items like metal appliance casings can cause electric shock

Isolation Transformer

• Isolation of conductors from ground is commonly achieved using an isolation transformer

Transformer Output Voltage

• A transformer is not referenced to the ground
• The patient can safely touch the "live" conductor and the ground without receiving a shock, because there is no return path
• If there is any leakage or a hard connection from the transformer to the ground there may still be a return path.

Line Isolation Monitoring (LIM)

• A line isolation monitor (LIM) device continuously monitors the integrity of an isolated power system
• The LIM directly measures the impedance to the ground on each side of the isolated power system

Perfect Isolation

• Impedance would be infinitely high and there would be no current flow with perfect isolation

LIM Readings and Alarms

• The LIM meter shows the total leakage in the system, in milliamperes, that is a result of capacitance or electrical wiring.
• Depending on the system's age and brand, the LIM is set to alarm at 2 or 5 mA
• Visual and audible alarms sound when this limit is exceeded to indicate degraded isolation from the ground

Optical Isolator

• Like transformers, opto-isolators effectively break ground loops caused by high or noisy return currents in ground wires
• An opto-isolator contains a light source (emitter), an optical channel, and a photosensor to detect incoming light
• Transformers and opto-isolators provide reinforced protection
• They contain a single physical isolation barrier, but give protection equal to double insulation.