Clean Ground or Dirty Ground? Isolated grounding receptacles in IT rooms and more

Server room | Image by Schäferle from Pixabay
Published On
May 13, 2022

In last month’s column, “Dedicated IT Rooms,” I explored the code rules for IT equipment facilities, where isolated grounding circuits and receptacles are often installed to reduce electromagnetic interference in grounding circuits. The objective of an isolated grounding circuit is to reduce circulating currents and interference through it by insulating it from other grounded conductive paths between the source and the outlet connection point. Isolated ground receptacles have a grounding terminal that is deliberately isolated from the mounting strap of the device.

Isolated ground receptacles and branch circuits have specific requirements. Section 250.146(D) includes the permissive text that allows an insulated equipment grounding conductor (EGC) to pass through panelboards and other enclosures without connecting to them, where they are terminated at the point of grounding of the circuit. The point of grounding is either at the service equipment or at a source of a separately derived system, which could be a transformer or power distribution unit (PDU) in an IT room. This is usually where the main bonding jumper or system bonding jumper is installed.

Installing EGCs

The isolated/insulated EGC “clean ground” is installed, in addition to the normally required EGC “dirty ground” for the circuit. The required EGC can be achieved by installing a conduit or other raceway or cable system that qualifies as an EGC in accordance with those identified in 250.118.

In the completed wiring installation, two separate EGC paths are established from the outlet to the source grounding point.

There are listed armored cable (Type AC) and metal-clad cable (Type MC) assemblies that provide an outer armor that qualifies as an EGC when used with listed fittings. These cable assemblies may also include a separate insulated wire-type EGC. These cables with two EGC paths are suitable for use in isolated grounding circuits. The metallic path inherent to these wiring methods establishes the required EGC for the circuit. The isolated/insulated EGC connected to the isolated grounding-type receptacle establishes a clean ground, which is the second EGC path. Both paths terminate together at the grounding point of either the service equipment or the grounding point at the source of a separately derived system. At the receptacle, however, the two EGCs are isolated/insulated from each other.

In some instances, multiple isolated grounding circuits are supplied from a single panelboard or PDU. Some designs exceed minimum NEC requirements and specify that an isolated grounding terminal bar be installed in the panelboard for connecting all isolated/insulated EGCs from such branch circuits. In such designs, typically two EGCs are run with the feeder to the panelboard. One EGC serves as the normal one and connects directly to the enclosure, while the other isolated/insulated EGC connects to an equipment grounding terminal bar that is mounted in, but isolated from, the panelboard enclosure. Isolated grounding terminal bars built specifically for this purpose are available from various panelboard manufacturers. The size of both EGCs installed with the feeder is based on the overcurrent protective device ahead of the feeder supplying the panelboard; refer to Table 250.122.

Sometimes a separately derived system supplies a panelboard. The same installation techniques and methods apply, except the isolated/insulated EGC terminates at the grounding point for the separately derived system, usually within the transformer enclosure. The NEC does not specifically address this type of design, but it is logical to conclude it can be installed and sized the same as the normal feeder EGC for the circuit.

Clean ground

The term “clean ground” often refers to the isolated EGC sometimes specified for IT equipment or receptacle outlets by the equipment manufacturer or owner. Section 250.96(B) indicates that when a reduction of electrical noise on grounding circuits is desired, an equipment enclosure (typically for IT equipment) is permitted to be supplied by a branch circuit containing an insulated EGC that is isolated from metal raceways by use of listed nonmetallic raceway (polyvinyl chloride [PVC]) fittings. Isolation is accomplished by using a nonmetallic fitting between the conductive frame of the equipment and the grounded metal raceway, thus reducing the vulnerability to any electromagnetic interference or circulating currents that may be present on the metal raceway.

The NEC does not specifically address who determines the circumstances that justify the use of isolated equipment grounding. This is a design issue and not an NEC requirement. Usually, the owner, design engineer or equipment manufacturer specifies the circuit to use for isolated equipment grounds. This NEC provision does not mandate the use of a metal raceway to supply the IT equipment. If PVC conduit is used, the insulating connector between the metal raceway and the IT equipment is not required. However, an EGC must be installed inside the PVC conduit to serve as the required equipment grounding means for the IT equipment.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at mj@necanet.org.

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