Ground fault circuit breaker problems look simple on the surface: Too much current and bang, breakers trip. And when they trip, everyone pays attention. But what about the times when breakers don't trip yet other problems continue to crop up in a facility?
The National Electrical Code (NEC) mandates that a ground cannot serve as a current-carrying conductor. However, it does not consider momentary nuisance currents that affect data equipment as a Code problem. Any efforts to track down and correct ground current problems are usually driven by nuisance trips of ground fault detect circuit breakers. In truth, however, there is correlation between the NEC, life safety, good equipment performance, and reliable data networks.
No one rationally wants the grounding of a facility carrying large amounts of current. Keeping this unwanted current flow out of a facility is not only good safety practice, but also an important power quality rule. However, tracking down the source or sources of unwanted ground current in a facility can be a challenging endeavor. Many people still resort to the old process — turning off the breaker and watching. Obviously, this approach becomes especially difficult in work environments where interruptions are very costly. Just imagine turning off all of the circuits in a hospital, semiconductor fabrication facility, or data processing center. Even suggesting such an approach could be a career-altering experience.
Here's what to do
You can trace the source of unwanted ground current through the use of current transformers (CTs) and flexible current probes. A CT picks up the common-mode current because this current generates a magnetic field around the conductors that is proportional to the current magnitude. Differential-mode currents, on the other hand, do not generate much of a magnetic field, because the signal pair is physically configured so that the magnetic fields cancel each other. This is called a zero-sum magnetic field condition, a phenomenon familiar to electricians using clamp-on ammeters when they gather all of a circuit's conductors (excluding ground) at the same time through the clamp-on's aperture. Any displayed current in this scenario is typically referred to as “unbalanced current” or “ground leakage current,” depending on the situation.
Fig. 1 shows the most basic measurement setup. If you place a single CT around the hot and neutral conductor of a circuit, the output of the CT will be zero if the supply and return currents are balanced. If they're not, then a non-zero output will be an indication that somewhere downstream of that measurement point, an unwanted condition exists, such as a neutral-to-ground bond. For convenience and throughout this article, we'll refer to this measurement process as a zero-sum measurement.
Photo 1 illustrates the process of zero-sum measurement at bus bars of a service entrance. The blue flexible current probes are used to measure phase currents, and the red flexible current probe provides the zero-sum measurement of the bus bars. Even though there are three phases, the principle of balanced supply and return still exists. In addition, you would place the zero-sum CT (red-colored CT) around all phase and neutral conductors.
Downstream at smaller panels, you can use individual CTs to measure phase and zero-sum currents (Photo 2). In both the Photo 1 and Photo 2 scenarios, you would track the zero-sum output with a power monitor to record momentary conditions during which a load with problems might be turned on and off.
The use of a single CT or flexible current probe to attain a zero-sum measurement is better illustrated in Photo 3. The zero-sum measurement process allows you to rapidly move from service entrance to panels and individual circuits. Photo 4 and Photo 5 illustrate how to measure an individual ground wire (not very precise) versus ground wire and zero-sum measurements (much more precise). The problem with individual ground wire measurements is two-fold: First, there may not be any ground wire to measure. Second, the current flowing in a ground conductor may be induced and therefore not involved in neutral-to-ground bond issues. With two measurements, you can more quickly determine whether current flow in a ground wire is an issue or simply a case of unfortunate wire placement.
Return current node points
Transformers are node points for currents returning from loads. Current supplied from a transformer must return to that transformer whether or not it returns via the ground or intended neutral return paths. Current flowing in the neutral-to-ground (N-G) bond reflects problems downstream from the transformer. The N-G bond of a transformer is an excellent point to detect unwanted current flow. In some instances, where downstream bonds are solidly connected to electrical conduits and also to building steel, you can measure the effects of a non-code-compliant N-G bond in both the N-G bond of the transformer and the bond to steel and/or water pipe (Fig. 2).
Another important effect of ground loops inside a facility is the formation of unwanted electromagnetic fields (EMF). The magnetic fields caused by ground loops decay linearly with distance rather than exponentially, as would be the case with normal balanced circuits. As a result, cathode-ray tubes (CRTs) and TV screens that rely upon precise placement of electron beams can be upset, causing screens to become wavy and jittery.
Shaughnessy is vice president, PowerCET, Santa Clara, Calif. He can be reached at [email protected].
Sidebar: Tips for Tracking Down Unwanted Ground Currents
In addition to the tips and techniques discussed in the article, here are a few more helpful hints for identifying unwanted ground currents:
Look for unique patterns in the current. Repeating events or current flow with unique time intervals can help identify the source.
Compare the phase angle of the current versus that of the various phases. In 3-phase distribution systems, the 120° phase displacement of the respective phases allows you to focus on circuits that share the same phase relationships with the unwanted ground current.
Every load has some leakage current (normally micro-amperes or milli-amperes). The cumulative effect of leakage from load throughout a facility will usually be small because the leakage currents in polyphase systems will tend to offset.
Neutral-to-ground bonds or large loads with wiring problems will cause large amounts of current flow (amperes and larger). Large amounts of current flow can adversely affect equipment and trigger ground fault circuitry.