Mobile Friendly  Web Version  Add to Your Safe Sender List   Subscribe to EC&M October 25, 2011
Vol. VII No. 20
Predictive Maintenance
Electrical Troubleshooting Quiz
When Circuit Boards are Toast, Part 13
NEC in the Facility
Safety
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Maintenance

Predictive Maintenance

Predictive maintenance gives you control over when equipment will shut down. That is, instead of an unintentional shutdown occurring in the middle of a production run (a bad thing for, say, a glass plant), it occurs in a planned, orderly fashion. Because you see equipment is moving along a trend to failure (based on your insulation resistance trending, coronal analysis, oil analysis, thermography, or whatever other predictive tools you're using), you can schedule repairs rather than react to a crisis.

You'll have on hand everything you need:

  1. Replacement parts.
  2. Repair techs and support personnel.
  3. Lights, power, drawings, cleaning supplies, safety guards, and PPE.
  4. Test equipment and tools.
Plus, production won't pay surprised operators to stand around and wait while a critical order is going out late to a major customer. It will also allow engineering to look at possible upgrades. You can coordinate with the utility, fire department, EPA, etc., as needed. In addition, you can consult with vendors, manufacturers, and service firms.

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Repair

Electrical Troubleshooting Quiz

Work stations in your plant's administrative area have point-of-use UPS boxes, most of which are rated 500VA. These units provide ride-through during brownouts. They also allow people to save their work and conduct an orderly shutdown in case of a blackout.

During recent sag and blackout events, these boxes didn't stop workstations from shutting down. Everyone agreed that the obvious problem was that the batteries had outlived their usefulness and needed to be replaced. But even with new batteries, the problem persists. What's going on?

Visit EC&M's website to see the answer.

When Circuit Boards are Toast, Part 13

Previously, you found your transformer was overloaded. You moved enough loads to correct that, but you still have problems with circuit board failures. Will this nightmare ever end? Yes, but not before you know what's going on with harmonics in your system. To do that, you need a harmonics analyzer or other instrument capable of providing harmonic content information (for example, a power analyzer).

Perhaps the easiest place to connect to the load side of that transformer is at the panel it serves. What you first see on the display may look far worse than it actually is, especially if you're viewing everything out to the 50th harmonic. If we were troubleshooting a 480V distribution system, we'd mostly be concerned with the 5th and 7th harmonics, but for single-phase distribution, we need to focus on the 3rd harmonic. You can ignore the others.

The 3rd harmonics present on the load side add together in the neutral and then circulate in the primary of the transformer. That can mean enough additional heat to effectively reduce the rating of the transformer. With enough harmonics, an otherwise properly specified transformer runs in saturation — resulting in wave flat-topping and other power quality problems. If harmonics are above the normal tolerance of 5% THD (per IEEE-519), then you'll have to look at the primary side.

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Operation

NEC in the Facility

Commonly misunderstood/misapplied terms, part 9.

  • Service point. This isn't the service entrance but upstream of it. This is the point where utility wiring connects to premises wiring.
  • Service drop. The word "drop" is a good clue this pertains to overhead conductors. These conductors are between the utility supply and the service point. This term doesn't refer to underground conductors serving a similar function.
  • Service lateral. This is the underground equivalent of the service drop. These conductors run underground from the utility supply to the service point.
  • Service entrance conductors. These differ from "service conductors" and, like those, are overhead or underground (with that designation appended as appropriate). The overhead ones run from the service drop (or overhead service conductors) to the supply-side terminals of the service equipment. The underground ones also run to the terminals of the service equipment but connect to the service lateral.
  • Service equipment. Often erroneously referred to as the "service entrance" due to its location, this is the equipment immediately on the load side of the service conductors. It consists of panels, breakers, fuses, switches, and related accessories.
The supply-side sequence can be defined as follows: utility wiring, drop/lateral, service point, service conductors, service entrance conductors, service entrance, and service equipment. The service equipment then supplies power to load-side equipment.

Safety

Today's electrical test equipment is vastly superior to what electricians used when Neil Armstrong walked on the moon. The analog VOMs (volt ohmmeters) have been replaced by a wide range of instruments that measure far more.

Today's industrial-grade meters also have safety features not imagined back then (see the manufacturers' literature for more info). However, some things have not changed with regard to safe use of electrical test equipment. For example:

  • Set the instrument up for the specific measurement you need to do, and then have a partner check the setup. Even the best DMM can't protect you when you try to measure current between a 2,000A bus and ground through its mA connections.
  • Attach and remove leads one at a time to prevent creating an ionization trail between the leads.
  • Don't try to economize by keeping damaged test leads.
  • If you're too tired or distracted to focus, don't attempt electrical testing.
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