A little more on the grounding issue for Kelly. Rich mentioned the problem can occur (voltage potential between different grounds in a building) if the grounded service conductor(neutral) is grounded at more than one location. This would certainly cause the problem. That along with additional safety issues is why it is a violation of the National Electric Code.
Keep in mind there are two different terms in use. The grounded conductor (neutral) and the grounding conductor (equipment ground). These two are only allowed to meet at one location. The service equipment. There is another term "bonding" that is defined as the permanent joining of metallic parts to form an electrically conductive path. Note that there has been extensive rewriting of Article 250 of the National Electric Code from the 1996 code to the 1999 code. Most language was put into positive format to try and eliminate the multitude of exceptions. Specifically 250-23 (1996), 250-24 (1999) require the service supplying a building to have a grounding electrode connected to the grounded conductor. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means. There are some exceptions when there are multiple buildings served from the same service, or separately derived systems involved (generators, etc.) >From the service equipment (where outside power enters the building) separate grounded and grounding conductors must be run. Throughout the building additional bounding is allowed and mandated if the metal can become energized in the event of a failure. This is to prevent "objectionable current" from flowing on the grounding conductor. 250-21(b)(1996), 250-6(b) (1999) allows alterations to stop "objectionable currents" Be careful though, 250-21(d) (1996), 250-6(d) (1999), state that noise that results in data errors is not considered to be "objectionable current". Computer and communication gear may be sensitive to these stray currents. Circulating currents on equipment grounding conductors, metal raceways, and building steel develop voltage potentials. One way this has been addressed in the past has been to isolate the equipment that is suffering from the grounding system. However, this can create a shock and fire hazard and is not an acceptable means for handling the problem. Equipment that is at the end of long runs of the grounding conductor may see a rise in voltage on the grounding. It is permissible to supplement the grounding by bonding to building steel, raceways, or adding supplemental ground rods. However, these are supplements and do not allow you to remove the grounding conductor 250-51(1996). By bonding all equipment and metal in a building wherever possible you can create a more equipotential ground plane and hopefully eliminate the noise. Failing that, isolated grounding runs are allowed to equipment by 250-74 exception no. 4 (1996), 250-146(d) (1999). John L. Allen, P.E. Electrical Engineer Engineering and Fire Investigations 4405 International Blvd. Suite B-115 Norcross (Atlanta), Georgia 30093-3013 Tele: 800-245-9601 in GA: 770-925-9600 Fax: 770-925-9649 Cell: 404-931-4481 Visit our web site at http://www.efiinfo.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
