advanced search

Ten Easy Steps for Testing Branch Circuits

Jan 1, 1999 12:00 PM, By Ken Michaels, F.D. Hayes Electric Co.

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.

Want to use this article? Click here for options!

© 2012 Penton Business Media, Inc.

---

Acceptable Use Policy blog comments powered by Disqus

Browse Back Issues Select an Issue EC&M Magazine | April 2012 March 2012 February 2012 January 2012 December 2011 November 2011 October 2011 September 2011 August 2011 July 2011 June 2011 May 2011 April 2011 March 2011 February 2011 January 2011 December 2010 November 2010 October 2010 September 2010 August 2010 July 2010 June 2010 May 2010 April 2010 March 2010 February 2010 January 2010 December 2009 November 2009 October 2009 September 2009 August 2009 July 2009 June 2009 May 2009 April 2009 March 2009 February 2009 January 1, 2009 December 2008 November 2008 October 1, 2008 September 2008 August 2008 July 2008 June 1, 2008 May 2008 April 2008 March 2008 February 2008 January 2008 December 2007 November 2007 October 2007 September 2007 August 2007 July 2007 June 2007 May 2007 April 2007 March 2007 February 2007 January 2007 December 2006 November 2006 October 2006 September 2006 August 2006 July 2006 June 1, 2006 May 1, 2006 April 2006 March 1, 2006 February 2006 January 2006 December 2005 November 2005 October 2005 September 2005 August 2005 July 2005 June 2005 May 2005 April 2005 March 2005 February 2005 January 2005 December 2004 November 2004 October 2004 September 2004 August 2004 July 2004 June 2004 May 2004 April 2004 March 2004 February 1, 2004 January 1, 2004 December 1, 2003 November 1, 2003 October 1, 2003 September 1, 2003 August 1, 2003 July 1, 2003 June 1, 2003 May 1, 2003 April 1, 2003 March 1, 2003 February 1, 2003 January 1, 2003 December 1, 2002 November 1, 2002 October 1, 2002 September 1, 2002 August 1, 2002 July 1, 2002 June 1, 2002 May 1, 2002 April 1, 2002 March 1, 2002 February 1, 2002 January 1, 2002 December 1, 2001 November 1, 2001 October 1, 2001 September 1, 2001 August 1, 2001 July 1, 2001 June 1, 2001 May 1, 2001 April 1, 2001 March 1, 2001 February 1, 2001 February 1, 1995 January 1, 2001 December 1, 2000 November 1, 2000 October 1, 2000 September 1, 2000 August 1, 2000 July 1, 2000 June 1, 2000 May 1, 2000 April 1, 2000 March 1, 2000 February 1, 2000 January 1, 2000 December 1, 1999 November 1, 1999 October 1, 1999 September 1, 1999 August 1, 1999 July 1, 1999 June 1, 1999 May 1, 1999 April 1, 1999 March 1, 1999 February 1, 1999 January 1, 1999 December 1, 1998 November 1, 1998 October 1, 1998 September 1, 1998 August 1, 1998 July 1, 1998 June 1, 1998 May 1, 1998 April 1, 1998 March 1, 1998 February 1, 1998 January 1, 1998 December 1, 1997 November 1, 1997 December 1, 1996 November 1, 1996 October 1, 1996 September 1, 1996 August 1, 1996 July 1, 1996 June 1, 1996 May 1, 1996 April 1, 1996 March 1, 1996 February 1, 1996 January 1, 1996 December 1, 1995 November 30, 1995 November 1, 1995 October 1, 1995 September 1, 1995 August 1, 1995 July 1, 1995 June 1, 1995 May 1, 1995 April 1, 1995 March 1, 1995

browse e-newsletters