Isolated Ground Receptacles

Article 517 in the National Electrical Code (NEC) applies to electrical construction and installation criteria in healthcare facilities. Part II of Article 517 provides the requirements for wiring and protection in healthcare facilities and is applicable to all patient care areas. However, there is at least one section within Part II that should not be used in a patient care area. Section 517.16 in Part II covers receptacles with insulated-grounding terminals and seems to permit isolated ground (IG) receptacles in a general or critical patient care area.

The IG receptacles covered in 517.16 consist of a receptacle with the metal yoke of the receptacle isolated from the grounding screw of the receptacle. This effectively isolates the grounding terminals of the receptacle from any contact with the metal plaster ring mounted to the metal box and the metal raceway or separate equipment-grounding conductor attached to the box. In addition, Section 250.146(D) permits this isolation as a means to reduce electrical noise that may cause electromagnetic interference on sensitive electronic equipment.

Electrical noise is often introduced into an electrical circuit and, thus, into the equipment through capacitive and inductive coupling into the metal raceway enclosing the circuit. This noise is then connected through the box, the metal plaster ring and the metal yoke of the receptacle into the grounding connection. By installing an isolated-ground receptacle, the electrical noise path is interrupted and cannot be coupled into sensitive electronic equipment.

Section 250.146(D) still requires the receptacle-grounding terminal to be grounded to a separate insulated equipment-grounding run with the circuit conductors. This separate insulated equipment-grounding conductor is permitted to pass through without any connection to the box. It can also pass through any panelboards without connection to the panelboard with the final termination at the service or at a separately derived system. This insulated equipment-grounding conductor will carry any ground fault, but the isolated-ground system will interrupt the noise circuit.

The verdict

Eliminating the noise in a circuit supplying power to electronic equipment may be desirable for the operation of the equipment. However, there is only one grounding means for the receptacle. Therefore, using an IG receptacle in a patient care area would be a violation of Section 517.13(A) and (B).

Section 517.13(A) requires all branch circuits serving patient care areas to be provided with a ground path for fault current by installation in a metal raceway system or by a cable with a metallic armor or sheath assembly. The metal raceway system or metallic cable armor or sheath assembly must qualify as an equipment-grounding conductor in accordance with 250.118.

Section 517.13(B) further requires an insulated equipment-grounding conductor sized based on the size of the overcurrent protective device to be installed in the metal raceway or cable assembly. This requirement creates two separate equipment-grounding systems for receptacles or fixed equipment in a patient care area. If one system is lost, there is always a backup system to maintain the equipotential plane surrounding the patient.

The bottom line is that if sensitive electronic equipment is installed in the patient care area and noise becomes a factor, any IG receptacles installed in the patient care area will result in only one grounding system for the receptacle, not the two required by 517.13(A) and (B). Since there are no exceptions in 517.13(A) and (B) permitting this application, installing an IG receptacle in a patient care area is a violation of the NEC.

Installing an isolation transformer with a ground shield between the primary and the secondary of the transformer will solve the problem of the NEC violation and provide a solution to the noise for the circuit. The ground shield for the transformer must be connected to the reference-grounding point (panelboard bonding) as detailed in 517.14 to maintain the patient equipotential plane. The ground shield is located midway between the primary and the secondary of the transformer. Between the primary windings and the ground shield, distributed capacitance occurs and acts as a path to ground for any noise that may occur on the primary side of the transformer.

On the secondary side of the transformer, any noise generated within the equipment is then coupled through distributed capacitance from the secondary winding into the ground shield. Since this transformer is a separately derived system, compliance with Section 250.30 is a necessity.

Section 517.16 should be clarified for the 2011 NEC, since the present text does not make it clear if the panel’s intent is to permit IG receptacles in a patient care area or not.   EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at

About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and .

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