A checklist for shock
Electrical safety is all about assuring that current flows only where it is designed to flow, and about minimizing the flow of unintended current. Unintended current is parasitic otherwise known as “leakage” current that travels across impedances that are inherent in any device. It cannot be totally eliminated. If there is sufficient leakage current, it poses the risk of shock, especially for patients whose clinical defenses are compromised.
A shock hazard exists if:
• A line-powered device is within the reach of a patient or user, and
• There are conductive parts available for the patient or user to contact, and
• The patient or user comes in contact with those parts, and
• Those parts become energized or ungrounded due to physical damage or component failure, and
Enough current then passes through the patient or user to cause injury or death.
What can go wrong?
A few examples:
• The facility line voltage spikes above its nominal value by well over 10 percent.
• The connection to the protective earth wire in a poorly attached power plug opens up, causing the wire to be functionally disconnected.
• Years of constant flexing and stressing of the power cord, at either end, breaks most of the strands of the protective earth wire, severely increasing its resistance.
• A multi-parameter monitor falls to the floor and seems to be functional to the clinical users, who hurriedly restore it to use on a patient. But one of its circuit boards is partly dislodged or bent and comes in contact with the grounded enclosure.
• Blood and saline solution, rich in electrolytes, spills from the bowl chamber of an autotransfusion system, dripping onto the line-energized power supply board.
• An ECG input connector is inadvertently shorted to the chassis after a repair, compromising its isolation from chassis ground.
Any of these and countless other scenarios can create excessive leakage current.
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