At the heart of the issue was the distinction between fields caused by high-current or conductor separation and those caused by a “net current.”
In a ground-floor space of a Class-A building in Westchester County, N.Y., the hum of high-speed electronic devices was noticeably absent. Elevated electric and magnetic field (EMF) levels had rendered the space unusable for most computers. The building's engineering staff thought electricity feeds were the culprit. But when they prepared to take corrective action, they realized the situation was far more complex than they expected. As it turned out, trying to remedy the initial problem revealed a series of seemingly unrelated and far more potentially dangerous ones.
Exasperated, the engineers contacted the Power Quality Group from ConEd — the local utility for Westchester — to verify field levels and propose corrective strategies. The group conducted the first general survey of the affected space and established a field strength averaging 29.1 milliGauss (Fig. 1).
After reviewing the data, the engineering staff hired Field Management Services (FMS) to supplement the Power Quality Group's work. It soon became clear that a variety of sources, both inside and outside, produced the fields inside the space.
The investigation led to the detection of wiring errors in the building's upper floors. These Code violations were causing return current to travel on grounding conductors throughout the building, increasing the fields coming from the switchgear room. High-current conductors in the 460V network protector vault at the building's edge and high-current conductors in the transformer vault outside the building were adding to elevated magnetic fields in the space. Multiple ground paths to the 460V network protector and switchgear room were creating unusual field levels above the vault. In addition, the investigation revealed that the neutral bus connection in the main distribution panel had been improperly installed.
Reducing Field Levels
At the heart of the issue was the distinction between fields caused by high-current or conductor separation and those caused by a “net current.” Although the fields from a high-current conductor are fundamentally indistinguishable from those from a net-current wiring error, the available mitigation strategies are quite different. Therefore, as the investigation revealed the various sources, the corresponding mitigation strategies were adjusted accordingly.
As is often the case, this project required a phased approach, starting with the more obvious, physically accessible sources and proceeding toward the more intractable, complicated ones. The FMS personnel started with the wiring errors on the top floor and worked down toward the vault. In the beginning, the main distribution circuit had 140A of net current. After completing the corrections — a process that required several months — the levels had been reduced to 11A.
After resolving the wiring problems, the building engineer discovered a dangerous flaw. On a routine inspection, he found the bolts on the neutral bus inside the building's switchgear were glowing cherry-red and smoking. An emergency call immediately brought the Power Quality Group back to the site.
Prior to the wiring corrections on the upper floors, a portion of the return current flowed on the grounding conductors, causing a potentially dangerous installation error to go undetected. Now that the return current was flowing properly on the neutral bus, it was overheating — an indication that someone had improperly installed the bus connection. The building engineer, with the assistance of power quality personnel, installed an emergency shunt until a new bus bar section could be installed safely at a later time.
The average field strength after reducing the building's net currents measured 35.2 milliGauss. Surprisingly, the fields actually increased from the previous average of 29.1 milliGauss.
A Problem on the Move
The wiring corrections reduced the net currents on the major circuits from the basement distribution and the fields on the upper floors. But the computer model, which FMS personnel initially used to develop a mitigation plan, had predicted much lower levels in the first-floor space. These remaining fields appeared to come from the vault and switchgear rooms below the space.
FMS investigators created a map of the current flows inside the switchgear rooms. The computer model showed current flows producing high levels of magnetic fields from an alternate, parallel neutral current path, which was located between the utility-owned transformer and network protection vault and the neutral-to-ground bond in the building's main distribution equipment (Fig. 2).
Approximately 40A of neutral current flowed on the building's steel components between the transformer/network protection vault ground point and the main distribution equipment neutral-to-ground point. This produced the fields of a net-current circuit condition in the street-level tenant space. These fields, like those produced from wiring errors on the upper floors, could not be mitigated by shielding measures.
The computer model further suggested the following as the most efficient solution to this new problem: Modify the existing grounding connections so they encourage the currents to flow on a single path and maximize their capacity to self-cancel the currents normally in the vault and switchgear room.
These changes could not impact any safety requirements nor could they violate any ConEd transformer vault construction policies. Accordingly, all parties assessed the technical options before agreeing on a scheme that satisfied each participant's goals.
As a result of this agreement, the building engineer installed two additional grounds from the building switchgear to the building steel, and an additional ground to the building's cold water pipe (Fig. 3).
The placement of these additional grounds met ConEd's grounding policies requirements, allowed for the removal of the original ground connections, and further reduced the net currents.
Measurements taken by the Power Quality Group verified that the 40A originally running on the ground system had been reduced to 4A. More important, engineers achieved this reduction without compromising the integrity of the building's electrical system design or the principles of safety and reliability that were of concern to all participants.
Wrapping It Up
At the end of this project, the net-current circuit conditions were minimized, and magnetic fields in the affected street-level tenant area were proportionately reduced. After diminishing the vault net currents, the average field strength decreased to 26.8 milliGauss, and the “spread” of the fields had been substantially reduced.
The reduction of the building's net currents permitted the productive use of magnetic field shielding. After performing a detailed survey of the mechanical and electromagnetic characteristics of the space, FMS personnel developed a two-phase mitigation plan. This plan guaranteed to reduce magnetic field-strength levels in the affected area to a target range of 2 milliGauss to 6 milliGauss.
The first phase involved installing a shield in the tenant space and, if necessary, one inside the vault. In the end, only the first phase was needed. The building's management and the tenant considered the residual field-strength values from the first-phase shielding acceptable. After shielding and net-current reductions, the average field strength measured 6.2 milliGauss (Fig. 4).
Resolution of these complex problems required close coordination and a successful technical negotiation between FMS, the building's management and engineering staffs, and ConEd's Power Quality Group and Engineering department.
It also vividly illustrates the relationship between power quality issues and unusual EMF levels, and demonstrates how a major power utility explored and implemented a new way to serve the customer.