A potential problem with isolated grounding is that unaware users may connect a signal cable between a piece of equipment powered by an isolated ground outlet to another piece of equipment powered by a non-isolated ground outlet. Noise now enters the isolated ground system via the signal interconnect.
This problem is very common in large computer networks and, like most computer noise problems, will likely be blamed on something else.
The resultant ground loop and circulating noise currents defeat the purpose of the isolated ground system.
THE NEUTRAL-GROUND SWAP
National Electrical Code recommends that premises wiring be sized such that regulation of the most distant outlet on a branch circuit is 5 percent (6 volts of drop for 120-volt circuits) or better under full load. This means that 3 volts are dropped across both line and neutral conductors.
On the other hand, safety ground wiring normally carries well under 100 rnA of cumulative equipment leakage currents. This could occur for a worst-case scenario of 20 pieces of equipment, each having a three-prong AC plug and 5 rnA leakage current (maximum allowed by UL). In this scenario, total voltage drop over the length of the safety ground conductor would be only about 20 mv.
But, as shown in the schematic directly below, a simple outlet wiring error that swaps the neutral and safety ground conductors allows load current to flow in the safety ground wiring.
The middle outlet has N-G swap wiring error. (click to enlarge)
When equipment load currents of 15 or 20 amps flow in the safety ground wiring, voltage drops as high as 3 volts can occur over its length (assuming the safety ground wiring is the same gauge as line and neutral).
Although the outlet is still functional and safe, this error can cause system ground noise to increase by a factor of 100 or about 40 dB!
Simple outlet testers cannot find the problem because they test only for voltages at the outlet.
Clamp-on AC ammeter. (click to enlarge)
Since neutral and safety ground are bonded together at the main breaker panel, they are indistinguishable to these testers. Even more sophisticated testers cannot reveal the error.
However, if nominal loads (say 100-watt light bulbs) are plugged into each of the outlets on the branch circuit, a clamp-on ammeter like the one shown here at left can quickly reveal and locate the mis-wiring by measuring current at points A, B, and C.
Referring to the schematic, abnormally high currents would be measured at locations A and B but not C.
Bill Whitlock has served as president and chief engineer of Jensen Transformers for 20 years. Read more articles by Bill about best AC and electrical practices here.