Power%20Quality%20Article
 

EMF/EMI Inspection

Hello Mike,

I am doing an EMF / EMI inspection of an office building. I will only be spending a couple hours per week inspecting the building until I can complete it. I will send descriptions and pictures of what I find each week.

The main power quality complaints for the building are minor but include:

The 225 kVA transformer, (which feeds 120/208-volt power to the building), is
very hot to the touch.

One tenant has several strip surge suppressors that have the "fault" light lit.


The purpose of the inspection is to use the EMF given off by incorrect or defective wiring to locate the wiring problems that are causing the power quality complaints. As the problems are identified, an electrician will make the needed corrections.

This is a five-story rental office building, which opened in 1990 and is approximately 110,000 square feet in size. The power is fed through a 1200 amp 480-volt main and the mechanical system is fed with a separate 600-amp 480 volt main. The 120/208-volt transformer is 225 kVA and there is a 45 kVA transformer for the emergency power system. There are two large tenant owned UPS units, one of which is fed from a separate outdoor emergency generator.

Before the EMF work begins, we need to look at the basics to see if everything is correct. The initial inspection was in the main electrical room. The main service bonding and grounding was correct. Now we move to the 225 kVA transformer.

The transformer secondary has two parallel feeder conduits and the temperature on the top of the transformer is 138 degrees F.


Note: To check the outside surface temperature of the transformer, I use a handheld
infrared thermometer (cost about $180 from Graingerís). I aim the infrared into the transformer up underneath the top. I have found that anything over 100 degrees F is another symptom of a possible electrical problem.

The current on the grounding electrode conductor is whopping 25.5 amps. The goal is to correct all the "net currents" in the building and reduce the 25.5 amps to about 2 amps or less.

The metal raceway containing the grounding electrode conductor is bonded at both ends to the conductor.

The secondary feeders (EMT and Flex) do not have an equipment grounding conductor terminating to the X0 terminal on the secondary. At this point, the only path for ground fault current back to the power supply (transformer) is the feeder conduit, (which includes flex).

I do not know if the transformer is grounded correctly in the transformer or if the transformer case is properly bonded. I instructed the electrician to schedule a shut down of the transformer as soon as possible to:

Properly bond the transformer case to the neutral.

Install equipment grounding conductors in the secondary feeders or to add a bonding jumper for the 6í flex.

Ground the transformer to building steel.

Tighten all electrical connections.

Since this shutdown needs to be done off-hours with advanced notification to the tenants, it may take a week or two to schedule and complete.

Note: A walkthrough with a triaxial Gaussmeter confirms what the transformer already told me, there are "net currents" present in the building. Next week I will start to identify the circuits causing the "net currents" but the electrician will not make any corrections until the transformer work is completed.

Please note that EMF inspection should only be part of a complete electrical maintenance program. The program should include regular thermal imaging inspection, scheduled tightening of connections and regular inspections by qualified electricians and electrical inspectors.

Comment No. 1

Mike, The "hot" transformers that we have investigated are due to 3rd (or zero sequence) harmonic current produced by electronic loads. The 3rd harmonic current add together in the neutral (possibly overloading the conductor) and ends up circulating in the transformer primary delta winding. Based on the total harmonic current a derating factor should be applied to the transformer. It seems as though this survey should be looking at the harmonic current as well as the grounding issues.

Is "net current" a term some use instead 3rd harmonic current?

Chris Fate, Snohomish County PUD
Everett, WA

Comment No. 2

The hot transformer may be overloaded or more than likely suffer from harmonics. With UPS loads that do not have an input filter (most UPS systems not all) the harmonic content can be significant. Does Ed have a harmonic test instrument? Check the neutral current for harmonic content. What loads are running on the transformer?

When transformers are shipped, the X/O bond is not made and I have seen a number that were never bonded by the electrician. I will not get into the problems that can cause.

Has Ed measured the resistance of the building grounding?

Edís Comments

I quit blaming everything on harmonics a long time ago. Bonding and grounding errors and neutral to ground shorts are where I concentrate now.

Just like a 100-degree day is the best way to find the weaknesses in your refrigeration system, harmonics are great at pointing out the weaknesses in your electrical system.

I use the term "net current" two different ways. One-way is: the lack of, or excess of, current on a complete set of conductors Take a branch circuit coming off a breaker box for instance. If you clamp all the conductors together you should get a zero reading. Today as an example, I clamped a circuit where there is a neutral to ground short in a receptacle and the reading was 3.5 amps. So the 3.5 amps was the net current and the 3.5 amps was neutral current running on the steel of the building and not returning through the grounded conductor.

The other was referring to a single conductor that has current on it and normally shouldn't. For instance, the water pipe ground at your home or the transformer to building steel grounding conductor.

Note: I work with a local electrical testing company that does the thermal imaging, ground resistance testing and power quality testing. Those tests are a standard part of any electrical survey. I am detailing only the EMF portion, as not many are familiar with it.

Edís Report No. 2

I am a little short on time this week but I had a second look at the property to continue my inspection. I have found fourteen (14) 120 volt branch circuits on the second, third and fifth floors that have "net currents" greater than .5 amps. I did not do the first and fourth floors yet.

As the building has a small 24/7 operation on one of the floors, we have not been able to pin down the tenant for a time for the transformer shut down. Next week I will be working with an electrician to clear the "net current" circuits and inspect the other floors.

Note: There were 4.25 amps of "net current" on one circuit. Using the formula B=2I/R, (where "B"= flux density read in milligauss, (mG), "I" is the "net current" and "R" is distance in meters), means that along the entire length of the cable run in the ceiling, the EMF reading given off by this cable alone, in all directions, is 8.50 mG one meter away from the cable. 4.25 mG two meters away.

I hope this shows the significance of using the Gaussmeter for a quick walkthrough. Also, if you know the distance from the source of the "net current" you can take the Gaussmeter reading, invert the formula, and estimate the "net current".

Edís Report No. 3

Sorry I have had nothing to report on the EMF inspection. I did not work on it the past few weeks waiting for the transformer shut down that was scheduled for last Thursday night. The 24-hour tenant canceled the shut down at the last minute.

The plan was for the office to shut down. While reviewing the equipment in the computer room it was discovered that other offices use the same computer server, and it cannot be shut down. They have a large UPS unit but not a generator. We also discovered the UPS unit feeds two 10-ton A/C units.

So before we can fix the transformer, we need to fix the wiring of the computer room power. In the mean time we will precede continue with clearing the net currents.

I see this all the time in critical computer rooms. The computer people know their equipment but not everybody understands how their electrical supply does or should work.

Edís Report No. 4

Things have slowed down a little so I was able to get back to the EMF inspection I started last month. As I mentioned earlier (Report No. 2) I had high Gaussmeter readings on the fifth floor and we identified the circuits that were causing the EMF's. Yesterday we started to trace them and all I can say is what a mess!

This floor was remolded two months ago and about 8,000 square feet of cubicles were installed in a large open area. Each section of six cubicles has 3 circuits fed through a single whip terminated to a wall junction box. The whip has three 12 gage hot wires and one 10-gage neutral plus a 12 gage grounding wire. Each cubicle then has three receptacles, one marked for each circuit. Since there is only one neutral, the intent was to wire the whip to a three circuit one neutral house feed, each circuit on a different phase.

While I was there I learned that this tenant has been experiencing frequent monitor failures since moving in. I'm not surprised.

The electrical contractor used a two circuit home run feed with one same size neutral. Then at ceiling junction boxes dropped one individual circuit to feed two of the three cubicle circuits and brought another circuit from a different home run, (sometimes from a different panel), to feed the third circuit in the cubicles. There was no regard for what phase anything was on and all the different circuit neutrals are tied together in the junction boxes.

This tenant is in operation from 7:00 am to 11:00 PM 7 days a week, so the contractor will have to work the midnight shift to straighten this problem out.

Ed Chavern, Chavernet@aol.com

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