Electrical Safety: Shock, Leakage, Grounding, and GFCI

The patient is the weakest circuit in the building

Electrical safety in a hospital is different from a factory for one reason: patients have had their natural defenses removed. They are wet with conductive fluids, their skin has been prepped and punctured, they cannot let go, and some have a wire or a fluid column running straight to the heart. This lesson is the physics of what current does and how the facility keeps it away from them.

What current does, and why

Nerves and muscles fire on electrical pulses. Fifty or sixty hertz is slow enough that each cycle acts as a stimulus, so the effects scale with current:

about 1 mA       perception: tingling; sensory nerves fire
5 mA             maximum harmless: startling but no injury
10 to 20 mA      let-go threshold: forearm muscles clamp the hand around the conductor;
                 the victim cannot release, and exposure continues
about 50 mA      pain, fainting; chest muscles may freeze (respiratory arrest)
100 to 300 mA    ventricular fibrillation: the heart's cells fire chaotically and it stops pumping
6 A and above    the heart clamps in sustained contraction; severe burns; may restart when released

Time matters too: a longer exposure at a lower current does the same damage. And the path matters: hand to hand or hand to foot crosses the chest and the heart; that is the dangerous path.

Macroshock versus microshock

MACROSHOCK   current enters through the SKIN            skin resistance limits it; milliamps to hurt
             ordinary electrocution; a person touching a faulty chassis while grounded
MICROSHOCK   current bypasses the skin and reaches the   no skin resistance; MICROamps can fibrillate
             HEART directly through a catheter, pacing   the threshold usually cited is 10 microamps
             wire, or fluid-filled pressure line

The microshock patient is the reason leakage limits for cardiac-connected equipment are measured in microamps, a thousand times tighter than what protects an ordinary person. A nurse touching a faulty device with one hand and the patient's pacing wire with the other could deliver a lethal microshock and feel nothing herself.

Where leakage comes from

No device is perfectly insulated. Current leaks to the chassis through:

• capacitance between the power cord conductors and the ground wire,
• capacitance in transformer windings,
• the filter capacitors that suppress radio interference,
• any degraded or wet insulation.

This is normal and small, tens to hundreds of microamps. The ground wire returns it to the panel harmlessly. The danger begins when the ground path is broken.

GOOD:  chassis --leakage--> ground wire --> panel        patient touches chassis: current takes the ground path
BAD:   chassis --leakage--> [open ground pin]            chassis floats at some voltage;
       patient touches chassis and a grounded bed rail:  ALL the leakage goes through the patient

A broken ground does not stop the device working, so nobody notices until the safety test, or the shock.

Grounding and the ground test

The third prong and the green wire give fault current a path of almost no resistance back to the source, so a real fault trips the breaker in milliseconds and leakage is diverted around people. NFPA 99 requires the resistance from the chassis to the plug's ground pin to be no more than 0.5 ohm on patient-care equipment; a worn plug, a corroded internal strap, or a damaged cord raises it.

Cords, plugs, and outlets

Inspect every cord at every PM: cracked insulation, exposed conductors, a strain relief pulled loose, a bent or missing ground pin, scorched blades. Replace, never tape. Hospital-grade plugs and receptacles (green dot) hold tighter and are built for the abuse. Two-prong adapters and extension cords are forbidden for patient-care equipment. Relocatable power taps are allowed only if rated, secured, used within their capacity, and never plugged into one another. Power cord color coding is part of the inspection whenever a plug has been replaced: US cords black (hot), white (neutral), green (ground); IEC cords brown, blue, green-yellow. A cord rewired with hot and neutral swapped passes a continuity check and fails the reversed-polarity leakage test, which is one reason the analyzer runs that condition.

Two protective systems, two different jobs

• GFCI: watches for current leaving on the hot conductor that does not come back on the neutral. At about 5 mA of difference it opens the circuit. It saves a person touching a faulty device in a wet area. It does nothing for a microshock patient; 5 mA is far above the cardiac threshold.
• Isolated power with a line isolation monitor: neither conductor is grounded, so the first fault to ground causes no current at all. The LIM measures the insulation and alarms at 5 mA of total hazard current, which means "a first fault exists; find it before a second one turns this into a circuit."

Habits

Report tingles; a tingle is a fault waiting for a wetter victim. Keep patient leads off metal. Never disable a ground to "stop the hum." Test after every repair, because a repair can loosen a ground strap.

How the exam asks it

• "The current level at which a person cannot release a conductor?" About 10 to 20 mA.
• "Which condition allows microshock?" A direct conductive path to the heart such as a catheter.
• "Why is an open ground dangerous even though the device still works?" Leakage current has no safe path and may flow through the patient.
• "Maximum ground resistance for patient-care equipment?" 0.5 ohm.
• "Does a GFCI protect a patient with a central line from microshock?" No.

What to memorize

• 1 mA feel, 10 to 20 mA let-go, 100 mA fibrillate, 6 A clamp. Microshock at 10 microamps to the heart.
• Leakage is normal; an open ground makes it a hazard. Ground resistance 0.5 ohm.
• GFCI at 5 mA protects people from macroshock; the LIM alarms at 5 mA hazard current on isolated power.