What does the National Electrical Code (NEC) require when installing isolated/insulated grounding-type receptacles and auxiliary grounding electrodes? Some manufacturers threaten to void warranties unless equipment is connected only to a driven ground rod and not the branch-circuit equipment grounding conductor. This article provides clarification regarding this misinformation. Let’s look at why isolated/insulated grounding conductors are often specified and installed for electronic equipment.
Isolated/insulated grounding circuits and receptacles are installed to reduce electromagnetic interference (EMI) that can interfere with data systems and equipment. For example, where sensitive electronic equipment is installed and used in IT rooms, electrical-grounding-circuit designs often exceed minimum NEC requirements. These types of insulated grounding circuits can reduce or eliminate the EMI on the equipment grounding circuits by insulating the conductive paths and limiting the grounding circuit to a single insulated path that extends back to the source grounding point, usually at a service or separately derived system. Electrical noise can negatively affect some electronic equipment and lead to data errors or even data loss. In the IT world, three characteristics are sought regarding electrical power supplying these systems: reliability, power quality and electrical noise minimization in the equipment grounding circuits or other grounding paths.
All equipment grounding conductors (EGCs) must provide an effective ground-fault current path, even isolated/insulated EGCs. When isolated/insulated EGCs are installed, a minimum of two separate EGCs—a dirty ground and a clean or quiet ground—are necessary for the branch circuit. One will serve as the required EGC of the wiring method and enclosures; the other will be the additional isolated/ insulated EGC, which terminates directly on the isolated grounding receptacle or the chassis of sensitive electronic equipment, without a connection to the grounded metal outlet box. Section 250.118 provides a list of wiring methods that qualify as acceptable EGCs.
When solutions to electrical noise interference problems are sought, the minimum NEC requirements must never be compromised. Safety always comes first, which means the EGC must perform grounding functions and provide an effective path for ground-fault current. From a design perspective, the optimal isolated EGC is kept insulated from all other grounded conductive surfaces, which will minimize the possibility of creating a ground loop and circulating current disturbance for connected electronic equipment. Obviously, it is a better design to keep this EGC isolated (insulated) and routed all the way back to the source grounding point or applicable service grounding point. For example, in a remodel project where isolated grounding circuits and receptacles are specified, the EGC’s connection point might be at an existing panelboard or switchboard located downstream of the service’s grounding point, or it might be at a separately derived grounding receptacle installed in a branch circuit include an isolated, insulated EGC.
For a truly clean isolated/insulated equipment ground, the EGC should be installed from the receptacle to the grounding point at the service or source. No connection is made to any grounded conductive parts of any intervening panelboard(s) or any other grounded raceways or enclosures such as boxes, wireways and conduit bodies. Section 250.146(D) contains permissive text that addresses isolated/insulated EGCs and allows the insulated EGC to pass through one or more panelboards or other enclosures without a grounding connection so as to terminate at the grounding point at the service or separately derived system. These provisions of the NEC do not require the isolated grounding conductor to return all the way to the service or source grounding point, although it may be the desired method. The important factor is that isolated grounding circuits provide an effective path for ground-fault current in each completed installation.
An auxiliary grounding electrode is not a requirement, but it must comply with NEC rules if installed. Section 250.54 indicates it is permissible to connect one or more auxiliary grounding electrodes to the EGCs specified in Section 250.118, e.g., at light-pole bases in parking lots. Installing the EGC with the branch circuit supplying the pole luminaires is a mandatory requirement, so the equipment is grounded and an effective path for ground-fault current is provided with the branch circuit. The auxiliary grounding electrode is connected to the metallic pole in addition to the required EGC. This provides additional protection so a lightning event is dissipated into the earth locally at the light pole base. The local earth connection of this equipment establishes a reference to ground and an equipotential between the conductive pole and the earth. Auxiliary grounding electrodes do not have to be bonded to the service grounding electrode system for the building or structure, as indicated in Section 250.50 or 250.53(C). It is important that installations of auxiliary grounding electrodes meet the requirements in Section 250.54 and satisfy sections 250.4(A)(5) or 250.4(B)(4). This means the earth is never permitted as an effective path for ground-fault current, which is the essential function of the required EGC for the branch circuit.