advanced search

Three-Lamp Circuit Tester: Valid Tester or Night-Light?

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

Popular with electricians and electrical inspectors, this device is inexpensive. You can easily use it to test a receptacle for correct polarity. But is it giving you a false sense of security?

Some three-lamp circuit testers prove our industry's old adage: The bitter taste of poor quality remains long after you've forgotten the sweet smell of low price.

Sure, we'll spend thousands of dollars on harmonic analyzers, power line monitors, oscilloscopes, and spectrum analyzers. But this equipment finds only a small percentage of power quality problems, primarily those related to voltage quality events.

On the other hand, investing a few dollars in a three-lamp circuit tester to research wiring and grounding problems can cause you more problems. These represent the vast majority of power quality failures.

Yes, a three-lamp circuit tester is inexpensive. But the truth is: These devices can be very inaccurate and give you a "CORRECT" indication when, in fact, the outlet has one or more problems. Following the indications given by the circuit tester can lead to additional problems later.

Even IEEE 1100-1992 (IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment) provides a warning in its definition of a receptacle circuit tester:

"Receptacle circuit tester. A device that, by a pattern of lights, is intended to indicate wiring errors in receptacles. Receptacle circuit testers have some limitations. They may indicate incorrect wiring, but cannot be relied upon to indicate correct wiring." (Italics added for emphasis.) Some manufacturers even admit the inaccuracy of the product, printing a disclaimer on the package to caution you of its limitations.

So why are some three-lamp testers ineffective? If you took one of these devices apart, you'd find nothing more than three resistors connected to the three blades of the device. These are ballast resistors, and they prevent each light from burning out when connected across 120VAC.

Two factors contribute to the tester's inaccuracy: circuit wiring capacitance and leakage current from the equipment plugged into the circuit under test. This means the three-lamp circuit testers can provide erroneous indications of the wiring conditions on practically every circuit! To show you the deficiencies of the devices, let's consider these two factors.

Circuit wiring capacitance. Assume you install two conductors for a 120V circuit (a hot and a neutral) and use the metallic conduit as an equipment-ground conductor. The circuit conductor has a distributed capacitance between it and the metallic conduit. If the equipment ground conductor pigtail from the metallic box to the receptacle becomes open, a capacitive voltage exists between the hot conductor and equipment ground terminal at the receptacle. This capacitive voltage causes the tester's lamp (which is connected across these conductors) to light, thus giving an erroneous CORRECT WIRING indication. This open ground condition represents a serious safety hazard during faults, while being detrimental to the operation of electronic equipment. Yet, the three-lamp circuit tester indicates this circuit is good!

Leakage current from equipment. Let's take this a step further. Suppose you connect equipment to the circuit (with the open ground condition). Now, there's no path for leakage current to flow. But there will be one: When you plug the outlet tester into the receptacle. The leakage current will now flow through the lamp connected between the neutral and ground, giving you an incorrect indication of REVERSE POLARITY.

So you follow the tester's indication, and reverse the hot and neutral. Now, you've created an actual reversed polarity. But when you test the receptacle again, the tester indication is CORRECT WIRING.

The original problem was an open ground, and it still exists. Only now, a reverse polarity condition compounds this original problem.

Use a ground circuit impedance tester instead. Your ability to troubleshoot power quality problems on AC wiring and grounding systems is only as good as the tools you use. Instead of choosing the three-lamp circuit tester, use a ground impedance tester. Yes, this more sophisticated device's primary function is to measure the impedance of the equipment grounding conductor or neutral (grounded conductor) from the point of the test back to the source neutral-ground bond. But, it can do additional testing as well:

Detection of wiring errors (reversed polarity, open equipment grounding conductor, and open neutral),

• Measurement of voltage, and

• Verification of neutral-to-ground and isolated ground shorts.

• With this diagnostic tool, you'll get accurate information on which you can base your power quality solutions.

Don't be misled. When checking building wiring, use a ground impedance tester to verify correct wiring and low equipment ground impedances. Use the three-lamp circuit tester as a night-light!

Thanks to Ed Cantwell of PowerTek Services, Wheeling, Ill., for his contribution to this article.

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