Long electrical cords tend to share antennalike qualities by attracting the wavelengths of certain signals; they also become their own enemies if made too long—the longer the conductor wires, the more resistance the conductors have, affecting the power delivered to the equipment (voltage drop). The Electromagnetic or Radio Frequency Interference (EMI/RFI) can create havoc in applications that are ultra-sensitive in nature. Long cords also create more leakage current, measured in microamps vs. milliamps for safety purposes, which is often the case in sensitive medical applications use. It can also be an issue when relying on accurate measurements when using testing equipment.
“In regard to medical equipment,” said technical support specialist Dan Ford, “if you go beyond the 12–15 feet length for medical cords, it becomes much more difficult to meet the equipment standard requirements for immunity, resistance, and leakage current for the entire system.” Ford is Interpower’s electrical guru. “What you don’t want are high levels of leakage current and EMI/RFI in equipment attached to or very near the patient. You also don’t want excess resistance dropping voltage destined for the equipment.”
While medical applications standards allow minute leakage current in proximity of the patient, many cords in hospital applications are not “patient connected,” and can therefore be measured in milliamps if patient or staff safety is not an issue. Cord safety in medical office applications is almost never an issue, unless there are faulty or bare wires where the plug has been yanked out from an angle too many times and bare wires are exposed. Or one of the contact pins has broken and arcing is occurring at the point where the broken contact is far enough in the socket to draw current, which could become a fire hazard.
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“Currently there is no UL standard for Hospital Grade cord length,” Ford said, “which is likely due to the hundreds of standards already published on hospital-grade equipment.”
However, cutting back on the length of the cord connected to the power main may not solve your EMI issues. By adding an EMI/RFI filter, you add a layer of redundancy to minimize interference. You should be good to go, right? Maybe. Ford on the calculation of every cord in a given design:
“If you plug your main power cord into an Accessory Power System (APS), then plug additional accessory cords into that APS, you must calculate the total cord length—not just the main power cord—to get total resistance and leakage current,” Ford said. An example: A 50-foot cord, plus four 10-foot cords drawing off the same system would calculate to 90 feet of resistance and current leakage. This may affect equipment performance, or take you beyond the limits of the specific standards you are complying with.
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