FMS Articles

Case Study:
Net-Current Problems in a Newly Remodeled Office

Alias/Wavefront, a leading graphic software developer, needed additional office space for their growing business. They decided to remodel an existing building in an historic area that had previously been a car dealership. The former garage, office and display areas were converted to accommodate the numerous sophisticated computer workstations and administrative offices. The entire facility was rewired and a new electric service was installed from the underground facilities in the street.

The Problem Appears

As the new tenants moved in, they immediately noticed problems with their computers. In many of the office areas, the large graphic monitors as well as many of the smaller monitors exhibited screen interference, particularly jitter. Their first course of action was to contact the local electric utility, which provided a free magnetic field survey of the facility. Elevated fields were measured over a wide area, centered around the electrical room.

Alias/Wavefront then contacted Field Management Services (FMS) to investigate the problem and recommend a solution. At this point, the problem and the analysis were focused on three interior offices near the electrical room.

FMS surveyed the facility, confirming and adding detail to the utility results - the fields were indeed elevated in the offices nearest the electrical room (typical office field levels range from 0.5 to 3 mG). However, the detail analysis revealed an unusual characteristic: these fields diminished at a very slow rate, much more slowly than anticipated. It was because of this characteristic that the fields extended over such a considerable area. (See Figure 1 for the initial conditions adjacent to the Electrical Room.) Even in areas where no problems had been noted, the magnetic fields were higher than is typical for a facility of this type. Since this condition is often the result of wiring errors (violations of Code), efforts were directed to locate these types of Code violations.

The nature and scale of these magnetic fields removed conventional shielding from the list of remediation options: fields due to wiring problems (net-current fields) cannot be economically shielded. The offending errors must first be located and corrected. Correcting the errors could substantially lower the overall fields, leaving those fields which normally occur next to the high current electrical room. Shielding these remaining fields would be both very effective and inexpensive.

Since neither the building electrical contractor nor electrician was experienced in these matters, FMS was commissioned to resolve the problem – and the process of detective work began.

The Search

Measurements in the electrical room main switch panel revealed a net-current of approximately 60 amps, variously distributed among the feed conduits to the five building sub-panels. Measurements throughout the building disclosed two locations where receptacles were touching the plaster ring, which seemed like a possible answer, but correction had little noticeable impact on the overall net-current. One clue led the investigation in another direction: elevated fields in portions of the parking lot and along the front sidewalk and street suggested that the source might be external to the building.

The focus of the investigation now split between inside and outside the building, with substantial help from the local utility. Measurement of the net-current, phase and neutral currents in the pad mount service transformer located in the parking lot was, at first, more confusing that revealing. The net-current at the transformer was equal to that of the service panel, as expected, but there was little neutral current in either location, in spite of a meaningful phase imbalance. Furthermore, there was a net-current on the transformer primary conductors very nearly equal to that on the secondary side. Traditional design would make this all but impossible.

Inside, the facility had file servers that serviced offices in remote parts of the world, so testing with the service interrupted was brief, but possible. Switching the individual sub-panels off in succession at first had little impact on the overall net-current, but switching off the second, third, fourth and fifth panels yielded large and nearly identical reductions until all of the net-current was gone with the last panel. Clearly, the source of the problem was inside the building.

The breakthrough came while the panels, now exposed, were being re-covered. A check of the voltage between the neutral conductors feeding the meter panel and the distribution section registered several volts, rather than the expected few millivolts.

The utility then unsealed and opened the metering section and the problem was exposed: the bolts on the various bus splices between the metering and distribution sections showed clear evidence of heating as did the bus splices themselves. Further, an adapter plate for the service cables had both of the required two bolts in place, but only one nut. The owner was immediately informed that the source of the problems had been located and that an outage of several hours duration would be needed for a tear down, cleaning, and proper reassembly of the main metering panel in accordance with the panel manufacturer's specifications.

The Source of the Problem

A poorly executed installation of a new panel had caused corrosion of the bus splices, particularly on the neutral splice, until very little of the neutral current was able to pass via this intended path back to the source transformer. Instead, the neutral current was passing from the neutral bus section in the main distribution section of the service panel to the bond between the service panel case and ground, which was properly connected to the neutral at this service point. The current then was passing via the conduits, water pipes and building steel into the ground at every opportunity. Since, for the circuit to be complete, the current must return to the source transformer, it found every opportunity to get to that location. The best and lowest impedance path for that to occur was via the utility primary cable which included bare concentric conductors (bare wires spiraled along the outside surface of the insulated conductors), and which paralleled the front of the building.

At the transformer, the current from the building passed from the grounded primary concentric conductors to the grounded secondary neutral, thus completing the circuit. The net-current measured on the service cables was a shortage of current - the current provided by the phases was not matched by the neutral. The stray neutral current was returned through the ground to the utility concentrics to the transformer ground and from there to the secondary neutral. Therefore, the net-current that had been measured on the utility primary cables and which included this neutral current was in the form of an excess of current.

The Solution: Back to Normal Operation

Following an overhaul of the service panel, the fields in the parking lot and along the front sidewalk returned to normal values. Within the building, the fields in most areas had been reduced to below 2 milliGauss (mG), with the exception of those locations in immediate proximity to sub-panels or other significant electrical equipment. In the office adjacent to the electrical room, the highest fields were now below 50% of their former value and the smaller fields were now small fractions of the former readings. Moreover, the field levels diminished with distance from the panel at the expected rates, and, beyond the area immediately next to the electrical room, they were no longer at issue.

Since the office near the electrical room had been made into a conference room during this period, and since there was no immediate plan to return it to an office usage, it was decided not to proceed with shielding at that time. Later, when the conference room was converted to office space, shielding was installed around the electrical room and the fields were reduced to an average of 1.1 mG in the affected offices.