advanced search

Get to the Heart of Objectionable Current

Aug 1, 2002 12:00 PM, By Mike Holt, Mike Holt Enterprises, Inc.

What's the harm in objectionable current, and how do you eliminate it?

You should know 250.6 says objectionable current is not to flow on grounding and bonding paths, but do you know what it is, why it's dangerous, and how to remove it? First, though, you need to know its causes.

Objectionable current on grounding and bonding paths comes from an improper neutral-to-case bond that creates a parallel path for neutral current to return to the power supply via metal parts of the system. It occurs in three common scenarios:

• When a neutral-to-case bond exists in either a panelboard not part of service equipment or at the transformer and the secondary panelboard.

• When a generator system's transfer switch doesn't open the normal power grounded (neutral) conductor, and the generator's ground and neutral conductors are bonded together.

• When the bonding path serves as a neutral conductor, which happens when a 125V receptacle is installed at a switch location where a grounded (neutral) conductor is unavailable, and the equipment grounding conductor is used as the neutral.

Objectionable current on grounding and bonding paths can raise the temperature at a loose fitting enough to ignite combustible materials. Arcing at loose fittings is very dangerous in areas that contain ignitable and explosive gases, vapors, or dust.

This current can also result in dangerous voltage on metal parts of electrical equipment. The possibility of death from ventricular fibrillation exists any time the voltage is more than 30V.

Nuisance tripping of a ground-fault protection device can occur if neutral current returns on the equipment-grounding conductor, instead of on the neutral conductor. A circuit breaker with ground-fault protection required for a 480Y/277V, 3-phase system, of 1,000A or more, uses the residual current method to detect a ground fault (230.95). On a 3-phase, 4-wire system, the trip unit will sum the vectors of the currents in the 3-phase conductors and the neutral. When no ground fault is present, the sum of the vectors of the currents flowing on the 3-phase conductors and the neutral will equal zero. Any current flow not equal to zero is a ground-fault. Depending on the impedance of the parallel neutral paths, the ground-fault protection relay may see current flow above its pickup point, causing the device to open the circuit when no fault exists.

If multiple improper neutral-to-case bonds exist and a ground fault occurs, the ground-fault protection relay might not operate because some of the ground-fault current returns on the neutral conductor, effectively bypassing the device.

Not only is objectionable current dangerous, it can cause serious power quality problems, so you must address it by grounding and bonding electrical installations in accordance with Art. 250. Pay special attention to your neutral ground bonds. It's helpful to draw the current paths in CAD or on paper so you can see, for example, if neutral current will flow on grounding or bonding paths.

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 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