Electrical Safety Cedric Dupont-Eisner M.D.
Principles • Electricity is the flow of electrons • Direct Current-electrons flow in 1 direction • Alternating Current-electrons switch directions at regular intervals • Capacitance • The ability of a capacitor to store a charge.
Principles • Stray Capacitance/Capacitive Coupling • inherent in all electrical equipment. • Not designed into the system, but incidental to the equipment construction
Application: Electrical Shock • If electrical systems are not properly wired, persons can be subjected to electric shock.
Application: Electrical Shock • People become injured and death occurs when voltage pushes electrons through the human body, particularly through the heart.
Application: Electrical Shock • Damage from electrical current is due to disruption of normal electrical function of cells or dissipation of electrical energy into human bodies (increased temp->burn).
Application: Electrical Shock • Macroshock-large amount of current flow that can cause harm or death
Application: Electrical Shock • Microshock-small amounts of current flow. Dangerous only to electrically susceptible patients
Application: Electrical Shock • DC-is less dangerous • AC-is more dangerous • 3x DC is needed for VFib • High frequency current have low tissue penetration and does not excite contractile cells • Low frequency current penetrates more
Application: Electrical Shock • What levels of current (mA) is dangerous? • Remember 1,10,100 macro/100 micro • <1 mA = imperceptable to touch • 10 mA skeletal muscle shock (you can let go) • V Fib can be induced by: • 100 mA of macroshock • 100 μA (microamperes)of microshock current
Application: Electrical Shock • THE MAX LEAKAGE ALLOWED IN OR EQUIPMENT IS 10 μA (microamperes)
Grounding • Electrical Power Grounding can exist in two forms • GROUNDED • UNGROUNDED • Easy?
Grounding • Think about your house: • 2 prong outlets = no ground • 3 prong outlets = grounded • Modern homes have a ground to reduce amount of shock
Why is equipment grounded? • Stray Capacitance/Capacitive Coupling: • Remember all equipment leaks a small amount of current • All OR equipment has 3 prong plug
Ungrounding • The OR has many perils that make grounding impracticle. • Saline puddles • Power cords w/ tears in their insulation (colored part of cord) • Numerous electronic devices that risk
Ungrounding • This is where the questions are derived: • OR uses ungrounded power that is derived from Grounded utilities • ISOLATION TRANSFORMER is the answer…
Ungrounding: Isolated Power • Power source does not have a ground,the equipment is grounded • Isolated Power System provides protection from Macroshock. • Faulty equipment plugged into an isolated power system does not present a shock hazard.
LIM (Line Isolation Monitor) • Continuously monitors the potential for current flow from the isolated power supply to ground. • Determines the degree of isolation b\w 2 power wires and the ground. • Predicts the current flow.
LIM (Line Isolation Monitor) • Alarm is activated if 2mA-5mA of current is detected. • Remember Macroshock vs. Microshock?
Application: Electrical Shock • What levels of current (mA) is dangerous? • <1 mA = imperceptable to touch • Ventricular Fibrillation can be induced by: • 100 mA of macroshock • 100 μA (microamperes)of microshock current
Application: Electrical Shock • SO LIM DOES NOT PROTECT AGAINST MICROSHOCK SINCE IT DETECTS 2mA-5mA
LIM (Line Isolation Monitor) • Indicates the existence of a single problem (SINGLE FAULT) • Namely there is a problem with the ungrounded system becoming grounded • So we are back to regular home power • No chance for shock
LIM (Line Isolation Monitor) • A second problem (TWO FAULTS) are required for SHOCK to occur. • A faulty piece of equipment-very likely at HH • Unsafe environment like electric device + pool of normal saline.
Boards • LIM DOES NOT protect from MICROSHOCK, it warns of a POTENTIAL problem • If an LIM goes off, remove the last piece of equipment that was plugged in • In OR power source is ungrounded and equipment is grounded
QUESTIONS • The isolation monitor (LIM) on the wall of the operating room is to alert one to • Faulty drainage of static electricity from equipment to the conductive floor • The presence of equipment with a broken ground wire connection • An excessive load on the isolation transformer supplying the room • An excessive electrical leakage from either side of the power line to ground • Too low conductance in the floor.
Answer • The isolation monitor (LIM) on the wall of the operating room is to alert one to • D - An excessive electrical leakage from either side of the power line to ground
QUESTION • Electrocautery machines do not cause VF because the current they deliver differs from the electric current supplied by wall electrical outlets primarily by being: • DC instead of AC • Lower in voltage • Lower in frequency • Higher in frequency
Answer • Electrocautery machines do not cause VF because the current they deliver differs from the electric current supplied by wall electrical outlets primarily by being: • D - Higher in frequency
QUESTION • The LIM (line isolation monitor) • Measures leakage current flowing from patient to ground • Measures leakage current flowing from the electrical equipment to the patient • Sounds an alarm if the leakage current exceeds 50 mA • Measures the impedance between AC wiring and ground • Cuts off power to the circuit if a faulty piece of equipment is connected
Answer • D • The LIM measures the impedance from the AC wiring of an isolated power system to ground and only sounds an alarm if current in excess of 2-5 mA could flow to ground.
QUESTION • The isolated power supply system used in OR’s requires that: • The metal portions of the OR table be connected to earth ground • The patient be insulated from the metal portions of the OR table • Conductive flooring be used in the OR • A transformer be connected between electrical equipment in the OR and the electric power supplied by the utility company
Answer • The isolated power supply system used in OR’s requires that: • D - A transformer be connected between electrical equipment in the OR and the electric power supplied by the utility company
QUESTION • Microshock hazard is increased in patients: • With a temporary pacing wire in place • In electrically operated beds • Receiving TPN via CVC • Who are in rooms with isolation transformers
Answer • B • Both 1 and 3 can conduct electricity close to the heart and bypass the high resistance of the skin • 100 mA for VF in MACROSHOCK • 100 μA (microamperes) for VF in MICROSHOCK
QUESTION • Operating rooms use an isolated power supply because: • Grounding cannot occur • Contact with both wires of the isolation transformer would cause no shock • Leakage current is zero • It affords protection against high amperage electrocution
Answer • Operating rooms use an isolated power supply because: • 4 - It affords protection against high amperage electrocution • Remember LIM’s detect 2-5 mA (milli Ampres), a small amount of current
QUESTION • A ground fault circuit interrupter: • Is almost never used in OR • Is foolproof method for preventing serious electrical shock • May be used in a grounded electrical system • Has both audio and visual alarms to indicate the presence of a ground fault
Answer • B • A GFCI prevents current from flowing in a circuit when there is an imbalance in the current flowing in the two sides of the circuit. • Not used in OR b\c it would turn off life support equipment
QUESTION • Leakage current: • Is harmless • Occurs b\c of the magnitude of the inductance b\w electrical conductors supplying equipment • Is of the DC type • Is unintentional flow of current from the internal wiring of a device