A difficult area to evaluate is at the branch circuit/equipment location, because so many things contribute to electronic equipment malfunction.
Here's a 10-step approach to evaluating branch circuit wiring without having to unplug equipment or interrupt power.
Step 1: Conduct a visual investigation. Before connecting any meters in your branch circuit investigation, get a good look at the surroundings. This helps you interpret data by correlating the operation of nearby equipment to captured events. It also helps you determine other tests you wouldn't ordinarily perform.
Here are some items worth noting:
Sensitive and nonsensitive equipment sharing the same branch circuit.
Overloaded outlet strips. (This may create unacceptable harmonic distortion or electrical noise conditions.)
UPS units or ferroresonant transformers sharing the same branch circuit as electronic equipment. (This may further aggravate an existing harmonics condition.)
Electric fans or fluorescent lighting near electronic equipment or monitors causing wavy computer screens.
Styrofoam or plastic items near equipment. (This will cause electrostatic problems.)
Step 2: Take voltage measurements. Use a true-rms measurement instrument device when working on a branch circuit receptacle. A true-rms reading will display the actual value of the voltage in the presence of harmonic waveform distortion. You should expect the following readings:
Hot — Neutral = 120VAC
Hot — Ground = 120VAC
Neutral — Ground = 0VAC
Some voltage between the neutral and ground exists due to distributed capacitance between the two conductors as they get further from the load. However, this voltage should not exceed 0.5V. If it's above this value, investigate further to see if overloaded wiring or electrical noise are causes.
Step 3: Test for correct wiring polarity. You can do this at the receptacle by using a ground impedance tester. Under most conditions, this type of tester will accurately determine the polarity of the wiring as well as other wiring conditions.
Avoid using testers such as the three-lamp circuit tester. (See the article "Three-Lamp Tester: Valid Tester or Night-Light?" in the December '98 issue.) Manufacturers of these devices caution the user to follow certain procedures. For example, if the meter does not give a "CORRECT WIRING" indication when you first plug it in, then don't proceed with other meter tests until you correct the problem.
Step 4: Test for improper neutral-ground bonds. With a press of some buttons, you can learn if the neutral and ground are shorted within 20 ft of the outlet under test.
Step 5: Take ground impedance measurements. Measure the impedance of the equipment grounding conductor by using a ground impedance tester. A low impedance ground path allows ample fault current flow to overcurrent devices under ground fault conditions. Also, a low impedance ground dissipates high frequency, electrical noise currents back to the power source. If impedance values are at or near 0.25 ohms, it's likely a loose connection exists in the equipment.
Step 6: Measure neutral impedance. High impedance along the neutral conductor path causes two problems:
Overheated conductors and their connections, which can cause fires, and
Increased common-mode noise levels between neutral and ground.
You can measure the neutral conductor impedance with a ground impedance tester. (The device comes with an adapter that can change the measurement path to measure the neutral impedance.) The neutral impedance for a branch circuit should not exceed 0.25 ohms as per IEEE Standard 1100-1992 (Emerald Book).
Step 7: Take electrical noise measurements. You should measure electrical noise to confirm or eliminate it as the cause of equipment malfunction. You can do this by using an oscilloscope with a line viewer, which can de-couple (separate) normal-mode electrical noise from the hot-neutral voltage waveform. It can also de-couple common-mode noise from the neutral-ground voltage waveform.
Step 8: Perform a harmonic analysis of the voltage waveform. Be careful where you make the analysis. Many mistakenly rely on harmonic analysis at the main service entrance to indicate if there's a problem at the equipment location. However, these results are captured too close to the power source and will not be indicative of any voltage waveform distortion at the loads. If you suspect harmonic distortion, use an instrument that allows you to view the severity of the distortion.
Step 9: Measure electrostatic discharge. Electrostatic discharge destroys AC power conductors. An electrostatic discharge voltmeter can help determine the amount of accumulated electrostatic charge. Measure all surface areas of the equipment location.
Step 10: Capture voltage disturbances. You can do this by connecting a power line monitor at the equipment location. This allows you to view the disturbance as the equipment experiences it. If possible, configure the device to monitor the affected equipment's AC input as well as the DC output of its power supply. This will help you determine if the equipment helps reduce the power problem. If there's a power conditioner installed at the input of the affected equipment, you should monitor its input and output power to see the conditioner's response (if any) to the disturbance.