Ecmweb 6167 Unauthorized Movement Neutral Conductors Pr
Ecmweb 6167 Unauthorized Movement Neutral Conductors Pr
Ecmweb 6167 Unauthorized Movement Neutral Conductors Pr
Ecmweb 6167 Unauthorized Movement Neutral Conductors Pr
Ecmweb 6167 Unauthorized Movement Neutral Conductors Pr

The Illegal Shift

March 19, 2014
Unauthorized movement of neutral conductors creates perplexing troubleshooting scenario for electrical contractor and substantial repair bill for medical facility owner

Although football season is over, the anticipation of watching the next batch of talented athletes coming up through the ranks is thrilling. And with each ensuing season, new rules of the game are introduced, requiring players to learn how to comply. Failure to do so results in the refs tossing the yellow hankie at violators. Mistakes can be costly for individuals and the team alike. For example, an illegal shift happens when a player changes position, having been previously set in another. A flag is thrown for failing to follow the specific and technical rules of how to change your position or stance. The following electrical case study reminded me of an illegal shift, and I found myself reaching for that yellow hankie.

Photo 1. This terminal unit [216] assembly had been modified by someone in the field.

Today’s electrical distribution equipment — what the trade calls switchgear — is often a complex, factory-engineered field configuration. Going forward, especially given the recent expansion of selective coordination requirements, electrical contractors must use caution when altering, modifying, and servicing modern switchgear installations. It is a best practice for owners and facility managers to allow only qualified professionals to access and service sophisticated switchgear installations. This recommendation is offered in order to avoid equipment malfunction issues that can result in service disruptions and entirely avoidable, potentially costly consequences.

This case is precisely about this precaution. For some years now, my firm has provided electrical services to a large medical specialty practice. Housed in a three-story, 52,000+-square-foot, 10-year-old building, the facility contains state-of-the-art medical equipment used for patient care diagnostic services.

During one particular alteration, it is evident that an electrician committed an illegal shift with respect to the switchgear configuration. This unauthorized move was evasive to me as I methodically searched for the possible cause(s) of why a certain branch panel feeder overcurrent protective device (OCPD) would erratically trip. In each instance, the OCPD ground fault detection feature was responsible for the panel’s power loss, resulting in an HVAC service shutdown for the second floor at the medical facility.

Although I am certain this was an innocent mistake — and fortunately one that proved to be nothing more than a nuisance — it ultimately resulted in a substantial repair bill (paid incrementally over a long span of time) to multiple tradesmen who searched for the root cause in vain.

Like any good mystery, clues are helpful. Mine was this: The facility manager explained that the 250A OCPD serving the second floor duct heaters and zone dampers would randomly trip. At that point in time, though several technicians had tried, nobody could figure out why. When this OCPD tripped, HVAC service to the patient care wing on the second floor went down — in some instances without immediate detection. Eventually, staff and patients would complain about the uncomfortable indoor temperature, prompting the facility manager to check the breaker.

The building’s electrical service is a 3,000A, 480/277V, 3-phase, 4-wire wye configuration. The switchgear had been modified on occasion, new breakers had been mounted into spare slots, and some existing breakers were relocated, changing the factory-built layout. A not-so-uncommon sight, a black magic marker was used to scratch out formal identification labels and indicate revised descriptions for where or what a breaker supplied power to.

I first removed panel covers and observed the existing distribution equipment configuration. I also asked the facility manager lots of questions. It was revealed that yet another renovation had just been completed. New MRI imaging equipment, along with additional HVAC equipment, had been added to the switchgear. I sought details surrounding the time of day, frequency, and even the seasonal load present when the breaker would trip. Each time it tripped, the facility manger would reset the breaker and restore power to the sub-panel. I was advised that there was never an instance where the OCPD would not reset.

I also familiarized myself with the building automation system, which controlled the HVAC zones for the second floor. The BAS graphics (floor plan with symbols) provided real-time status and mode of the second floor terminal unit zones (duct heater/damper assembly).

These loads — cycled as zone sensors — would call for set point heat or cooling. The zone damper would actuate, and either cool or warm air was supplied. It was important to note that when the zone called for heated air, the electrical load on Panel 2MP increased as electric strip heaters (3kW to 5kW) were the source of the warm air. Thus, the cumulative demand for Panel 2MP varied, depending on how many of the 20 zones called for terminal/resistance heat.

The nuisance tripping had manifested sporadically, since about a year ago. There was no particular event associated with the recurring power loss incidents this panel was experiencing.

It was a curious problem, with no apparent rhyme, reason, or precursor as to why or when the OCPD would trip. Morning, afternoon, or night — there was no pattern. Weeks would go by without an incident, and then again the problem could occur twice in the same day.

My initial hunch was that there was a section of skinned insulation, a pinched neutral conductor, or a sharp edge penetrating the wire jacket somewhere that created a path for current leakage via a ground path. I diligently searched for a copper-to-ground connection on the neutral runs. Meticulously — often from the vantage point of a ladder — I traced all the EMT circuit runs, opened every junction box, and inspected the terminal unit control panels and each disconnect. Although I was certain I was on the right track, I found nothing obvious.

I soon began to doubt my own theory and reminded myself that others had been down this same road, also failing to resolve the problem. At this point, I decided it was time to learn more about how this type of electronic trip breaker functions.

Analysis of breaker operation

Each OCPD has a settings panel and a microprocessor LCD screen readout. The LCD will sleep if a load is absent; the screen will be blank. The microprocessor has a variety of functions. The readout is advanced by pushing the button, current levels on the phases, and the neutral can be viewed by scrolling the display.

The display will also indicate the reason the electronic trip mechanism was activated. The screen will read “overload,” “short circuit,” or “ground fault,” indicating the reason for the trip.

This OCP, which supplied power to the second floor patient care area electrical panel (Panel 2MP), displayed a “ground fault” readout after tripping. The microprocessor monitors line and neutral (return) currents and processes them to evaluate/detect a ground fault. The presence of unbalanced phase currents produces a net neutral current range that the logic is programmed to interpret. An inappropriate (out of the permissible range) current load detected on the neutral is interpreted as a ground fault, resulting in a trip. This range is influenced by dial settings, field adjustable thresholds that must be carefully calculated and selected.

The features of the OCPDs installed in this switchgear provided frontline protection for the facility electrical distribution equipment and personnel. The fact that the breaker tripped on detected ground fault was consistent with my hunch that somewhere there was neutral current path that had inappropriately found a shortcut to ground.

A refocus on the path

Using my ammeter, I checked to see if any current was flowing through the ground conductor. There was no detectable current flow on the ground. Furthermore, I disconnected the ground conductor and measured for a voltage potential between it and the mechanically grounded metal of the enclosure. There was no electrical potential detected.

I also checked the feeders from the switchgear (first floor electrical room) to Panel 2MP, paying particular attention to the condition of the neutral conductor. There were no indications of insulation damage. Hence, everything with regard to Panel 2MP checked out fine — there was no clear evidence of a deficiency of any kind.

Next, I carefully inspected all of the concealed conduits containing circuit conductors from Panel 2MP installed above the drop ceiling of the second floor. The purpose of this effort was to identify or locate the possibility of a pinched or nicked neutral wire making contact with any grounded metal component. I checked every branch circuit pipe run and every junction box. Nothing obvious was revealed. Although I was disappointed, it at least confirmed that there was no instance where any of the neutrals had been compromised.

A new clue surfaces

The facility manager then revealed that several new terminal units had been added on the second floor and that apparently the breaker tripping problem began after these new terminal units were installed. This new bit of information sent me in a new direction.

Because I am a certified air conditioning contractor, in addition to holding my electrical licensure, I was able to expand my investigation to include the mechanical equipment and components. I embarked upon a careful survey of all electrical supply connections, control connections, and circuit configurations for the new heaters.

Although I spent a significant amount of time poring over the newest terminal units, I discovered nothing obvious. I then expanded my inspection to some of the other terminal units. Another hunch I had was that perhaps a resistance heating coil was making contact with a metal frame or cabinet enclosure. I found a suspicious condition in Terminal Unit 216. The assembly had at some point been modified by someone in the field (Photo 1).

While I wasn’t certain that I had located the problem, I wasn’t impressed with the condition of Terminal Unit 216. The rigged heater coil electrical terminal connection was a field adaptation. I checked for continuity from the coils to the metal cabinet as well as conducted a thorough visual inspection of the ceramic insulators. There was no continuity between any of the three coils and the enclosure. Frustrated and somewhat desperate at this point, I suggested that this unit was the best evidence of an electrical abnormality discovered thus far. I recommended replacing the unit.

A custom-built unit was ordered (three weeks lead time) and then installed; it was an identical replacement of the original unit (UL listed and without compromise).

Unfortunately, the day after I installed the new terminal unit the 250A OCPD tripped. I now had egg on my face and was right back where I had begun. I was not only embarrassed, but also concerned I had a credibility issue. Fortunately, my client was most gracious and instructed me to continue my quest to pinpoint the problem.

Calling in additional support

At this point I realized it would be wise for me to place a call to the breaker manufacturer and tap into their expertise. Seasoned contractors know the value equipment manufacturers can bring to the table. I called technical support and asked the manufacturer to explain exactly how these breakers worked — specifically how the microprocessor sensed a ground fault. It was a key question, and the answer provided me with valuable information about how the detection technology works.

Examining the switchgear, there are labels on both the breaker case and the neutral terminals that are strategically placed in proximity to one another in the cabinets. Tech support staff explained that the neutral terminals were coordinated with a current transformer (CT) whose output is subsequently input to the breaker microprocessor. This crucial measurement is how neutral current, which varied with any unbalanced loads on the phases, is measured. This information was extremely helpful and corrected an erroneous assumption I made in my initial investigation. I presumed that the breaker somehow sensed current flowing through the feeder grounding conductor. I traced this to its attachment to the switchgear grounding terminal and observed no means by which a current flow through the feeder grounding conductor could be detected. Although my observation was right, my assumption was wrong.

The electronic trip unit in 3-phase breakers senses four currents: phase currents on A, B, and C — and, via the CT — the return current on the system neutral. The phase current sensors are internal within the breaker case; naturally, the neutral CT is an external component with harness wire leads connected to terminals on the breaker. The microprocessor interpolates these four current readings and is programmed to expect a given current draw range for the neutral, per any unbalanced phase currents the loads may produce.

Photo 2. Internal switchgear wiring for microprocessor type breaker.

The switchgear frame bears the intricate wiring scheme (neatly laid out and concealed behind the dead front covers) of this sophisticated distribution design, location, and layout having been specifically coordinated (Photo 2). Thus, it’s important where a sub feed neutral is connected. The designated neutral terminal corresponds to its affiliated breaker. Contractors are much more familiar with smaller switchgear assemblies where breaker mounting typically has no specific locale, and neutral connection points are randomly connected to a neutral bus or terminal. When working with electronic trip ground fault detection breakers, the position and order of connection are crucial. Additions to or alterations of the original switchgear configuration must respect this factory layout. If not, there will be performance issues.

The manufacturer’s tech support staff cautioned me to verify that the feeder breaker for Panel 2MP corresponded with the neutral terminal intended for that breaker space. This could be easily verified by tracing the CT wiring harness run.

Now that I understood how the breaker technology detected a ground fault, my next task was to trace neutral terminal connections and wiring harness connections to sub feed breakers. What I saw when I looked more closely at Breaker No. 12 — the 250A OCPD that fed Panel 2MP — took me by surprise.

Photo 3. Notice how someone wrote “2MP” on the label. It was mounted in space No. 12.

Notice the sticker “12” in Photo 3. Also note that someone penned in “2MP” on the label. This breaker supplied Panel 2MP and was mounted in space No. 12. These were the original factory labels. Now observe the wire harness entering the breaker case. This is where the CT output is delivered to the breaker microprocessor; following that wiring harness will lead to a neutral terminal bus that runs through a CT. The system neutral for the sub feed must be connected to that terminal; otherwise, the microprocessor will sense no neutral current whatsoever.

Photo 4. The processor display shows the CT is currently generating no current.

Next, notice the LCD on the microprocessor digital display. The picture indicates the neutral current level the processor is sensing. See the zero reading (Photo 4)? The CT is generating no current, while the phase currents are carrying substantial, unbalanced loads. How can the neutral current be zero, given the significant loads on phases A, B, and C? It was now time to locate neutral Terminal No. 12.

Photo 5. Note the manufacturer installed current transformer locations. There is no neutral feeder connection on CT3. In addition, the neutral feeder to 3MP wasn’t moved from CT4 to CT3 when the phase feeders were moved to breaker No. 3.

My search led me to an orderly layout of neutral terminals positioned in proximity to nearby breaker slots in each cabinet (Photo 5). I found neutral Terminal No. 12; it was without a neutral conductor connection — bare and unused (Photo 6). No wonder the microprocessor read zero neutral current. The CT associated with space No. 12 had nothing to sense! Where then, is the sub feed neutral from Panel 2MP connected?

Photo 6. Terminal No. 12 (where the neutral for 2MP was supposed to be connected) was bare.

They say hindsight is 20/20. I had gained critical understanding that now aided me in this diagnostic endeavor. In removing the dead fronts of the switchgear, the problem was revealed (Photo 7). In fact, now it was clear as day.

Photo 7. Note how Terminal No. 17 has two neutrals connected to it. They did not derive from the same source.

Terminal No. 17 has two neutrals connected to it, and they don’t derive from the same source of conductor runs. For Terminal No. 17 to have two neutrals, breaker space/breaker No. 17 would have to be a parallel sub feeder run. It was not, and that meant we had an illegitimate point of attachment — the neutral was  connected to Terminal No. 17.

In short order I discovered that the additional neutral conductor was in fact a bandit from Panel 2MP’s sub feed conductors. The 2MP neutral had been randomly attached to terminal No. 17, probably out of convenience. Note from the picture that neither neutral conductor properly identified as such. Finally, the problem had been identified, and the explanation made perfect sense.

Because Breaker No. 12 (Panel 2MP) had no sensed neutral current input, it was perpetually zero. Thus, as loads built up on phases A, B, and C, the unbalanced neutral load that the microprocessor expected was absent, resulting in only one possible interpretation — the presence of a ground fault. The microprocessor would then, as designed, electronically trip the breaker, open the contacts, and display the ground fault error message. In other words, the breaker was performing precisely as designed. The random trip instances were contingent upon the varying demand (on/off) of the zoned terminal unit heater loads serving the second floor.

After de-energizing the breaker and properly marking the neutral, I removed the 2MP neutral conductor from Terminal No. 17 and connected it to Terminal No. 12. When I turned on the breaker, the display immediately indicated a load on the neutral, and the zero reading was gone.

Why did Breaker No. 12 start tripping eight years after the original installation? One option is that the 2MP panel feeder neutral was improperly connected to Terminal No. 17 from day one. It is also possible that an electrician moved it at some point in time after the original installation. It is my opinion that it was moved when the three new terminal units were installed.

Tying up loose ends

Photo 8. Someone scratched out the original “3MP” marking on this breaker and penned in the word “spare” below it.

It will probably come as no surprise that upon closer examination of the breaker locations and the connected neutral feeder for each branch circuit, another improper breaker/neutral configuration was revealed. Ironically, the third floor terminal unit’s panel — a 400A load — had its neutral connected to the wrong CT terminal as well. I was given the approval to repair any and all deficiencies present within this equipment.

As for Breaker No. 3 and Breaker No. 4, the evidence is a bit more conclusive. Originally, Breaker No. 4 served Panel 3MP, the terminal units on the third floor. However, the 3MP marking on the breaker label had been scratched out, and the word “spare” was penned on the sticker (Photo 8). The original phase conductors connected to Breaker No. 4 were also moved, at some point, to Breaker No. 3.

Photo 9. This is the neutral conductor terminal point for CT/Breaker space 3. Note that it is empty.

Why Panel 3MP’s feeders were moved from Breaker No. 4 to Breaker No. 3 is not known. But whomever moved them forgot (or didn’t know) that the neutral terminal connection needed to be moved as well (Photos 9 and 10).

The neutral feeder conductor for Breaker No. 4 was relocated to the neutral Terminal No. 4, as should have been done when the breaker was shifted to a new slot in the switchgear. Current on the neutral was confirmed and indicated in the microprocessor display window.

Photo 10. This is the neutral conductor terminal point for CT4. Note that it should be empty since the breaker in slot No.4 is marked “spare.”

Wrapping it all up

Good electricians know when to stop and go back to the drawing board, having realized the evidence doesn’t support their assumptions. The best decision any technician can make when you’re in over your head is to call tech support. The good fortune of excellent knowledge and diagnostic guidance from the manufacturer’s technical support staff saved the day for me.

I began this case study with a football analogy of an illegal shift. In this electrical configuration quandary, we ultimately learned the unauthorized move of changing the position of breakers in sophisticated switchgear can’t be done without regard for the manufacturer’s design and operational protocol. More than a year after the reconfiguration was completed, the equipment is performing without incident — not a single instance of a power supply failure to Panel 2MP.      

McHaffie is the owner of Thomas McHaffie, LLC, an electrical contracting firm based in Tallahassee, Fla. He can be reached at [email protected].

SIDEBAR: Don't Touch That Dial

During the electrical troubleshooting efforts in this case, it is worth mentioning that at one point changing the sensitivity settings on the 250A breaker was proposed to my client. Although tempting, this is typically a bad idea — especially if the equipment has been performing without problems for a long period of time. Tampering with sensitivity settings is seldom advised.

When switchgear is designed, the manufacturer ships the equipment with breaker settings considered appropriate for the intended use and application it is destined for. It is rarely advisable or necessary to tweak these settings, and if warranted — the adjustment(s) should be made only at the recommendation of a person qualified to specify this type of change.

One contractor told my client that the 2MP breaker’s settings were too sensitive. Thus, he concluded this was the cause of the nuisance tripping. However, as was later revealed, it would have been a mistake to change these settings.

About the Author

Thomas McHaffie | President and Owner

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Don't Let Burn-Through Threaten Another Data Center or Utility Project

Get the No Burn-Through Elbow eGuide to learn many reasons why Champion Fiberglass elbows will enhance your data center and utility projects today.

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.