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The How's and Why's of IG Wiring

Aug 1, 2008 12:00 PM, By Thomas M. Gruzs, Emerson Network Power

Whether IG wiring is beneficial or not depends on many situational aspects

It's hard to buy a new electrical device these days that doesn't contain electronics. Adjustable-speed and variable-frequency drives are now almost always connected to AC motors. Electronic ballasts are being coupled to gas discharge lighting products. Even circuit breakers and relays are mostly electronics-based in design. But with the expanded use of electronic equipment comes greater sensitivity to power disturbances and electromagnetic interference (EMI), along with its resulting operational problems.

Because equipment ground potentials (or changes in them) can affect the operation of certain electronic devices, designers, installers, and service personnel often have very specific and sometimes special grounding requirements. Most of these techniques have evolved based on empirical (trial-and-error) testing rather than on detailed analysis. Some of the more creative grounding arrangements are devised in the name of electrical noise reduction (see What's Electrical Noise?), but they often ignore the basic principles of electricity, such as electricity follows the paths of least impedance, flows in complete paths, and flows because there is a potential difference. Furthermore, when trying to reduce the effects of “noise,” the fundamentals of noise coupling are sometimes ignored.

One such special grounding technique used in low-voltage AC power systems to reduce interference is known as an isolated ground (IG). IG is allowed by the National Electrical Code and is an exception to the standard grounding requirements. Section 250.96(B) allows IG wiring only “where installed for the reduction of electrical noise (electromagnetic interference) on the grounding circuit.”

Figure 1 is an example of a typical low-voltage power system using IG receptacles, as allowed by the NEC. Note that the ground terminal of the receptacle is not connected to the conduit grounding system at the receptacle. Instead, an IG wire is connected to the receptacle ground terminal and is routed with the power conductors, passing through one or more panelboards, remaining insulated from the metal conduit and enclosure grounding system until its termination at the power system grounding point (at the service entrance or separately derived source, as shown in this example).

IG wiring and ground-fault current

It's just as important that the IG wiring provide an effective ground-fault path from the connected equipment back to the power source. If a ground fault were to occur at the load equipment, the IG grounding system would provide an effective ground path, as shown in Fig. 2. The IG conductor is permanent and continuous, has ample current capacity — because it's sized according to NEC requirements — and has a low enough impedance path to allow the overcurrent protective device (OPD) to clear the ground fault.

One other form of IG wiring for hardwired load equipment is allowed by the NEC in Sec. 250-96(B), where “an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway containing circuits supplying only that equipment by one or more listed nonmetallic raceway fittings located at the point of attachment of the raceway to the equipment enclosure” (Fig. 3). In this way, the equipment ground is isolated from the metal conduit and raceway system at the load equipment but is still effectively grounded for safety — the same as previously discussed for IG receptacles.

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