DMM: Demystifying Maintenance Maladies

Master troubleshooters often solve electrical mysteries with nothing more than a professional-quality digital multimeter (DMM) and their knowledge of how to use it. Let's look at how the masters do it. At 9 p.m., Plant Maintenance Manager Joe Brewster ends a long day by tucking his kids into bed. At 9:01, his phone rings. He recognizes the plant controller's voice immediately. "Joe, I've got 16 people

Master troubleshooters often solve electrical mysteries with nothing more than a professional-quality digital multimeter (DMM) and their knowledge of how to use it. Let's look at how the masters do it.

At 9 p.m., Plant Maintenance Manager Joe Brewster ends a long day by tucking his kids into bed. At 9:01, his phone rings. He recognizes the plant controller's voice immediately. "Joe, I've got 16 people standing around here because our computers aren't working right. The monitors jump around and..." Have you been in a similar situation? Has an electrical mystery taken a 25 hr chunk out of your day? Let's look at how others used their DMMs to solve similar problems.

Mystery No. 1: Case of the unstable computer screen. A large office building had an unhappy tenant. In one of the rooms, monitors near a particular wall had jiggling screen images. Moving the power cord to a different receptacle showed no improvement. But, moving the computer to a different room caused the problem to disappear.

An experienced troubleshooter investigates the problem. He suspects a magnetic field, since such a field (if strong enough) can deflect the monitor's electron gun. With everyone watching, he sets up his DMM to detect an AC magnetic field. He uses a length of small gage wire to make a 30-turn coil the size of his fist. He then connects the coil to the DMM's voltage input jacks and sets the function switch to "AC volts." Holding the coil horizontally at the height of the monitor, he moves slowly along the wall, watching the reading. At first, the DMM shows a noise level reading with the last digit flickering. Then, at one point on the wall, the reading rises rapidly to about 3 mV. It drops again as the coil moves farther along. The troubleshooter marks the point of maximum reading on the wall, and repeats the process with the coil held vertically. The vertical readings are similar to the horizontal ones.

The troubleshooter explains something inside the wall; or on the other side of it; is generating a stray field. The stray field induces a small voltage in the sensing coil, which shows up as a reading on the DMM. A quick look at the building plans reveals an electrical room behind the wall. This room holds a service panel.

Behind the panel cover, the source of the stray field is obvious. All branch circuit conductors enter from the top of the panel. These conductors, tightly bound together within the panel, extend to the bottom of the panel where they make a "U-turn" to go back up to reach the circuit breakers. This configuration (a "service loop") allows for easy rearrangement of conductors and circuit breakers without any "wire stretching." In this case, the loop is too long; the concentration of conductors generates much more field than a proper loop would. How does the owner resolve this? An electrician shortens the conductors (and dresses them properly). This reduces the stray magnetic field to a point where it no longer causes a problem.

Mystery No. 2: Case of the arcing light switch. A library attempts to do its part for energy conservation by retrofitting all fluorescent light fixtures with electronic ballasts. On the one hand, they are successful. Their power consumption drops, and their quality of light improves. On the other hand, they aren't so successful. Whenever someone turns on any of the wall-mounted light switches, a blue arc greets them from behind the wall plate. Consequently, the library staff leaves the lights on; thus eliminating the energy savings.

The troubleshooter uses a DMM to measure the steady-state current in each branch circuit. He carefully inspects these switches for any signs of overheating. The running currents measure low, well below 10A, and there are no signs of overheating. The next step is to measure the inrush current when the switches are on and compare the results to the switch ratings. Over the phone, the switch manufacturer says the switches, with a continuous rating of 25A, passed tests at 40A.

To measure the inrush current, the troubleshooter sets his DMM's recording function to capture current impulses as short as 1 millisecond. The inrush current measures 70A. Repeated tests give the same results. This high value is hard to believe for a lighting circuit. So, the troubleshooter brings in a storage oscilloscope to verify the DMM's results.

A detailed circuit analysis reveals the ballast circuit has a diode-capacitor input configuration that draws high current for a short time when the capacitors are charging. Contact bounce in the switch interrupts the inrush current and causes an arc. Modifying the ballast circuit is not an option, so the library needs some other solution. What it does is replace the switches with ones designed to minimize contact bounce. The library also uses an enclosure that prevents arcing from being visible to the operator.

Mystery No. 3: Case of the missing voltage peak. You've probably dealt with buildings the owners use for purposes not planned for in the original design. Here's a case of a manufacturing facility that began life as a large commercial office building. The factory's operation involves diagnostic machines with significant amounts of computer memory.

The test department manager complains of repeated problems seemingly related to line power. Small loads turning off and on cause memory resets that result in lost test time. Several electricians try but fail to correct the problem. So, the manager calls in the local master troubleshooter, who listens carefully to the test manager's story and suggests the investigation start where incidents of the problem occur most frequently. When the troubleshooter and an electrician arrive, DMMs in hand, they see multiple power strips and extension cords feeding various nonlinear loads. Potential overloads of branch circuits?

The troubleshooter asks the electrician to measure the AC voltage at a nearby receptacle using his average-responding DMM. This test gives a reading of 118V. The troubleshooter then repeats the measurement using a true-rms DMM that displays 115V. Theory says an average-responding DMM will read 11% high if the waveform is a square wave. Even though difference in readings isn't 11%, it's more than enough to arouse suspicion about "flat-topping" distortion.

Next, the troubleshooter sets his DMM for "Peak Min-Max" to capture the instantaneous peak of the AC voltage. The display reads 135V peak. The correct value for a true sinewave would be about 163V peak (RMS value2=2 or 115V21.414). This shows a clipping of 27V off the peak of each line cycle. A digital storage oscilloscope verifies the DMM's results. What's causing this flat-topping distortion? The answer: A high level of nonlinear loads drawing large peak currents through a series of long feeder runs. The 27V missing from the line voltage peaks results in an undercharge condition in the equipment power supply capacitors. This makes the memories vulnerable to the slightest sag in line voltage. The solution involves installing stepdown transformers near the load and raising the feeder voltage from 208V to 480V.

Mystery No. 4: Case of the unlabeled conductors. Imagine this situation: The job is an electrical upgrade to a historic building (built in 1925) with "knob and tube" wiring. The apprentice removes all the two-wire receptacles, and your job is to replace them with GFCIs. Because of age and discoloration, it's impossible to tell which wire is neutral and which is hot. The usual method to sort this out involves energizing the circuit and using a capacitive voltage sensor (tick-tracer) to identify the hot conductor. But, your tick-tracer has a dead battery, and the only thing you have available is your DMM. To complicate matters, the nearest grounded water pipe is 50 ft away.

Can your DMM substitute for a tick-tracer? Think of it this way. The capacitive voltage sensor works as a series-connected, capacitive-coupled circuit using your body as a capacitively coupled ground conductor. Your DMM can work in the same mode because of its high input impedance.

Try this procedure: Energize the circuit and set your DMM to "AC volts." Verify you have full voltage between the conductors. Next, lay the DMM and black test lead on the floor. Stretch out the black lead to maximize its capacitive coupling to ground (earth). Take the red lead and alternately connect it first to one conductor and then the other. For best results, use an alligator clip and take your hand away. The side giving the highest reading is the hot conductor. Typical readings might be 0.2V on a neutral wire and 16V on ahot wire. You won't get full line value because the voltage is dividing between the DMM's input impedance and the capacitive impedance between the black lead and earth ground. The test method assumes you have a grounded (at the service) neutral.

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