Isolated Power Systems: How Hospitals Protect Wet Procedure Locations

Most of us know GFCIs from kitchens and bathrooms. In an operating room, though, a GFCI tripping in the middle of a procedure could cut power to equipment a patient depends on. Hospitals solve this with a special kind of system that warns about a fault instead of shutting off: the isolated power system, sometimes called an isolation panel or IPS.

What is a wet procedure location?

In health care terms, a wet procedure location is a patient care area where procedures are normally done with standing fluids on the floor or drenching of the work area, conditions where patient or staff contact with fluids is part of the work. Paraphrasing NFPA 99, operating rooms are considered wet procedure locations unless a risk assessment by the health care facility’s governing body determines otherwise. Patient beds, toilets, bidets and wash basins are not, by themselves, wet procedure locations.

That risk assessment is meant to be a team effort, typically involving clinical staff, biomedical engineering and facility safety people, because the people doing the procedures know where fluids actually end up.

What the NEC requires (517.20)

NEC 517.20 says receptacles and fixed equipment in wet procedure locations need special protection against electric shock. In plain terms, the designer has two choices:

• An isolated power system, which limits ground-fault current to a low value on the first fault without interrupting power, or
• Ground-fault circuit interrupter protection, but only where interrupting power under fault conditions can be tolerated.

Because losing power during surgery usually can’t be tolerated, many operating rooms use isolated power. Article 517 Part VII (517.160) covers how those systems are built.

How isolated power works

A normal building system has one conductor (the neutral) intentionally bonded to ground. If a hot wire touches grounded metal, or a person, a large current flows and a breaker or GFCI opens.

An isolated power system starts with an isolating transformer. Its secondary isn’t connected to ground, so neither output conductor is grounded. If one conductor faults to a grounded surface, there is no complete path for a large current, so nothing trips and the equipment keeps running. The system has, in effect, become a grounded system on that first fault, so it needs to be found and fixed promptly.

The line isolation monitor (LIM)

Because a first fault doesn’t trip anything, the system needs a watchdog. The line isolation monitor continuously checks how much current could flow from either isolated conductor to ground, called the total hazard current. It shows green while the system is well isolated and switches to a red light and an audible alarm when the hazard current reaches the threshold, which for the monitor is set at 5 mA under 517.160. Staff can then unplug the most recent piece of equipment to find the problem.

What you see in the room

• An isolation panel in or near the room, containing the transformer, breakers, and the LIM display.
• Isolated circuit conductors identified by color: orange and brown for single-phase systems, each with a distinctive stripe, and a third, yellow, conductor for three-phase.
• Receptacles fed from the isolated system, often mixed with some normal-power receptacles, all clearly marked.

Why GFCIs are still common

Isolated power costs more and takes more space and maintenance. Where a brief loss of power is acceptable, GFCI protection is simpler and fully meets the rule. For critical care spaces outside of wet procedure locations, isolated power has been optional in the NEC for many years.

Testing and upkeep

The LIM is tested on a schedule, and the whole system is inspected as part of the hospital’s electrical maintenance program under NFPA 99 and the maker’s instructions. Health care work also involves state agency review in California, so these projects are always coordinated with the facility and the AHJ.