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Make PQ Monitoring Meaningful

Oct 1, 1998 12:00 PM, By Ken Michaels, F.D. Hayes Electric Co.

Finding voltage quality problems is a lot like detective work. You rely on the power line monitor to gather evidence of any unseen disturbances. But, how can you ensure a safe monitor hookup and meaningful captured data?

Power line monitors capture and display power disturbance waveforms that can disrupt, degrade, or destroy electronic equipment. Electrical contractors, technicians, and engineers purchase or lease these devices at considerable expense to satisfy their customer's power quality needs. So, how do you make the most of your money? Following the basic rules below will guarantee the power line monitor is a sound investment.

Where to start. You can save money by simply starting at the right place. You should gather as much information as possible about the site and occurring problems, prior to connecting the power line monitor. You can easily collect basic information by asking the customer questions like:

• What kind of equipment is affected?

• What are the symptoms?

• When did the disturbances begin?

• How often do they occur; for example, time of day, week, etc.?

Learning as much as possible about the electrical environment will help determine monitor location, connection (single- or three-phase), threshold settings, hookup, and monitoring duration.

Monitor location. Most companies believe the electric utility supply is the root of their AC voltage quality problems. As a result, they often connect the power line monitor to the main electrical service entrance.

However, most power line disturbances, especially transient events, result from equipment operation on the load side of the service. So instead of wasting your time and money, you may want to place the power line monitor where the afflicted equipment attaches to the electrical system; namely, the receptacle, panelboard, etc. This gives you a firsthand account of power line disturbance as seen at the input of the equipment.

Once you capture the disturbance, you can trace backward to find the root of the disturbance. The "signature" of the disturbance at the equipment location will also assist in the selection of a power-conditioning device, if necessary.

Monitor setup. Consider the environment where you will use the monitor. Like all microprocessor-controlled devices, it is susceptible to RFI/EMI, temperature, humidity, and electrostatic discharge. Also, secure the monitor so it is not subject to mechanical shock or vibration.

Threshold settings. Experience is the best teacher here. If the threshold settings are set too tight, the result is an overwhelming amount of meaningless information. On the other hand, if the thresholds are adjusted too loose, you won't capture the meaningful disturbances. It is helpful to know the input voltage tolerances of the affected equipment. If you don't know these, you may have to monitor the steady-state voltages for a brief period.

Once you know the typical ranges, you can set the monitor just above these values. You can also use the default threshold settings provided by the power line monitor manufacturer. Furthermore, this informational literature could be useful in offering insight on typical setting values.

Monitor hookup. If you aren't certain how to hookup the monitor, be sure to consult the manufacturer's literature for the proper connections. For safety, double-check the sense lead configuration before connecting to the circuit you wish to test. Improper connections of the leads to the monitor may damage the equipment, cause electric shock, or create a short circuit on the power system once the leads are terminated.

You may need to provide strain relief for the sense leads. This will prevent damage to the leads. It will also stop the leads from slipping off their termination point, which could result in acquiring erroneous information.

To prevent the monitor leads from picking up radiated EMI/RFI, it may be helpful to route the sense leads close to the metallic frame of the device you're monitoring. Try twisting the wires of each monitor channel together. And of course, always wear the appropriate safety gloves and glasses while making the physical connections to the circuit.

Monitoring duration. For most applications, you should monitor the power line for at least one business cycle (seven days). This will give you an accurate depiction of the events during peak- and low-demand power usage. However, if the plant's production schedule varies, you may need to monitor the electric power for a month.

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