BioMedical Electrical Safety

Objectives

• Describe the concept of leakage current as it relates to electrical equipment in direct contact with patients or operating rooms.
• Discuss and identify electrical hazards associated with operating room hardware along with electrical safety equipment and grounding systems designed to protect patients and personnel from electrocution.
• Describe preventative maintenance scheduling as it relates to extracorporeal equipment.
• References: Internet, Equipment Manuals, IFU’s.

Introduction

• This lecture introduces electrical safety as it pertains to patients and operating room personnel.

Outline

• Leakage current
• Line isolation system
• Equipotential grounding systems
• Ground fault interrupters
• Proper power wiring, distribution, and ground system in reducing electrical shock hazards
• Preventative maintenance

Leakage Current

• Definition: Leakage current is the current that flows through the protective ground conductor to ground.
• In the absence of a grounding connection, leakage current could flow from any conductive part or the surface of non-conductive parts to ground if a conductive path was available (such as the human body).
• There are always extraneous currents flowing in the safety ground conductor when electrical equipment is in use.

Why Is Leakage Current Important

• Electrical equipment commonly includes a grounding system to provide protection against shock in the event of insulation failure.
• Grounding System: Usually consists of a grounding conductor that bonds equipment to the service ground (earth).
• If there is a catastrophic failure of insulation between the hot (power) line and touchable conductive parts, the voltage is shunted to ground, preventing electrocution by causing a fuse to blow or opening a circuit breaker.
• Shock Hazard: Exists if the grounding connection is interrupted either intentionally or accidentally; this may be greater than expected due to leakage currents.
• A fatal shock could occur if the patient is in a weakened condition or unconscious or if the leakage current applies to internal organs through patient contacts.
• Double Insulation: A protective measure providing two separate layers of insulation to help prevent shock hazards.

What Causes Leakage Current

• There are two types of leakage current: AC leakage and DC leakage.
• DC Leakage: Usually applies only to end-product equipment, not to power supplies.
• AC Leakage: Caused by a parallel combination of capacitance and DC resistance between a voltage source (AC line) and the grounded conductive parts of the equipment.
  • The leakage caused by DC resistance is usually insignificant compared to the AC impedance of various parallel capacitances.
  • Capacitance may be intentional (e.g., EMI filter capacitors) or unintentional (e.g., spacings on printed wiring boards, insulations between semiconductors and grounded heat sinks, the primary-to-secondary capacitance of isolating transformers within the power supply).

How Is Leakage Current Measured

• A meter designed to measure leakage currents is used for this purpose.
• The current in the ground conductor is measured by connecting the meter in series with the grounding connection.
• For Information Processing Equipment: Open the ground connection and measure the current flowing to the neutral side of the power line.
• For Medical Equipment: Measure the current flowing to ground.
• The meter may also be connected between the outputs of the power supply and ground.
• Test Conditions: Include swapping the AC line and neutral connections, and turning power switches off and on while monitoring the current.
• The test should be performed after equipment has warmed to a normal operating temperature, and in some cases, following tests causing abnormally high temperatures within the equipment.
• This practice aims to identify and measure the worst-case leakage current.
• For very low leakage currents, the meter is replaced with a network consisting of either a resistor or a resistor-capacitor combination; the voltage drop across the network is measured using a sensitive AC voltmeter.
• Ungrounded or double-insulated equipment is checked by connecting the meter between any touchable conductive part and ground.
• For nonconductive housings, a copper foil of a specific size is placed on the housing, and the current flowing from it to ground is measured.

Leakage Current Meters

• Example: TONGHUI TH2685 Leakage Current Tester features settings for voltage and current limits as well as testing capabilities specified by the manufacturer.

What is a Safe Leakage Current

• Medical equipment leakage current limits are significantly lower than for non-medical equipment.
  • Type B: Least stringent classification; applicable to applied parts generally not conductive and can be immediately released from the patient.
  • Type BF: Generally for devices with conductive contact with the patient or medium/long-term contact.
  • Type CF: Most stringent classification; required for applications where the applied part is in direct conductive contact with the heart or similar critical applications.

Line Isolation System

• A shock occurs when two separate conductive materials at different voltage potentials come into contact, completing a circuit (e.g., a grounded “zero” voltage part touches a “hot” wire ~120 V, where I = rac{V}{R}).
• Primary wiring in an operating room is normally grounded. Equipment within the operating room is not normally grounded, but the equipment casing is grounded, protecting the patient.
• The secondary wiring system that is not grounded requires two faults to result in a shock (1st fault: converts to ground, 2nd fault: patient touching the secondary wiring system).
• An electrical wire not grounded is considered an isolated line; if an ungrounded patient touches an isolated (ungrounded line), nothing would happen.
• To utilize this safety mechanism, equipment must be connected to the primary wiring system (which is grounded) via an isolation transformer.

Line Isolation Systems

• Line Isolation Systems (which include isolation transformers and line isolation monitors) protect individuals from electrocution by transforming a normal “grounded system” (existing outside an operating room) from one needing a single fault to a safer “protected” system requiring two faults to deliver a shock.
• The line isolation monitor assesses the isolation degree between the two power wires and the ground and predicts how much current might flow if a second short circuit were to develop.
• An alarm activates if an unacceptable current amount to ground is possible, indicating the isolated system is no longer isolated.
• If the alarm is sounded, a single fault exists, but another must occur to deliver a shock; typically, the most recently connected equipment is the suspect and should be unplugged.
• As of 1984, isolated power systems are no longer mandated in operating rooms, hence modern equipment carries electrocution risks that are mitigated by using ungrounded batteries for power, double insulation, and isolating patients from equipment.

Equipotential Grounding Systems

• During surgical or invasive procedures in critical care areas, the patient's natural resistance decreases, thus increasing sensitivity to leakage current, fault current, and potential injury from microelectronic shock.
• Equipotential grounding: Reduces these hazards by bonding all conductive surfaces in the room to each other and to earth.

Ground Fault Interrupters (GFI)

• Designed to reduce the risk of electrical shock by interrupting a circuit when there is a discrepancy in the currents between the “hot” and “neutral” wires.
• Such a difference implies a diversion of current from the “hot” wire, possibly indicating a leakage current from motors or capacitors.
• More critically, this current diversion could occur if a person has contacted the “hot” wire and is being shocked.
• Under normal circuit conditions, all return current flows through the neutral wire; a difference between “hot” and neutral current suggests a malfunction that could lead to a dangerous or fatal electrocution.
• Therefore, GFIs are mandated by electrical code for receptacles in bathrooms, kitchens, outdoors, and near swimming pools.
• The Underwriters Laboratories (UL) requirement for a GFI is that it must trip when there is $5 ext{ mA}$ of leakage current.

Modern Power Distribution

• In the U.S., the power distribution system operates at $110 ext{ volts alternating current (AC)}$ at $60 ext{ cycles (hertz)}$.
• The system consists of three wires:
  • Black wire (hot)
  • White wire (neutral)
  • Green wire (ground, functions as a current sink to minimize shock hazards).

Reducing Electrical Shock Hazards

• Guidelines to adhere to include:
  • Never use two-wire extension cords.
  • Never use extension cords with non-polarized plugs or those with broken ground pins.
  • Never remove the ground pin from electrical equipment.
  • Always use an appropriate ground adapter and ensure the ground wire connection.
  • Regularly check the integrity of any unfamiliar power plug using a ground monitor device; if hazardous, do not plug it in.
  • Always install GFI outlets in wet areas.
  • Refrain from overloading electrical outlets.
  • Include leakage current assessments as part of your preventative maintenance schedule.

Preventative Maintenance

• Definition: A schedule of planned maintenance actions aimed at preventing breakdowns and failures.
• The primary goal is to avert equipment failure before it occurs.
• Designed to preserve and enhance equipment reliability by replacing worn components prematurely.
• Preventative maintenance activities include equipment checks, current leakage assessments, partial or complete overhauls, oil changes, lubrication, etc.
• Documentation is crucial for tracking when to replace or repair worn components before they cause equipment failure.

Long-term Benefits of Preventative Maintenance (PM)

• Improved system reliability.
• Decreased costs of replacements.
• Reduced system downtime.
• Better management of spare inventory.

When is Preventative Maintenance a Logical Choice?

• Condition 1: The component in question experiences an increasing failure rate over time, indicating wear-out patterns. Preventive maintenance on a component with assumed constant failure rates does not yield benefits.
• Condition 2: The overall cost of the preventative maintenance action must be lower than that of corrective actions, which should encompass ancillary costs such as downtime, production loss, lawsuits from safety failures, and loss of goodwill.

Cost per Unit Time

• Graph Description: The graph illustrates the comparison between preventative and corrective replacement costs over time, where a minimum cost of replacement is achieved under the preventative maintenance model compared to higher costs associated with corrective actions.

Preventative Maintenance (PM) Program

• Sometimes mandated even when cost benefits are not realized?
• Costs associated with:
  • Equipment replacements
  • Catastrophic breakdowns
  • Service calls
• A customized PM program ultimately pays for itself, supporting the principle that the best way to handle problems is to prevent them.
• Question: Who mandates preventative maintenance?

Questions

• Opportunity for questions and comments.
• Testimonials regarding the PM program and electrical safety implementations.