There is a lot that can be said about grounding and bonding and providing an “effective fault current path”. This article describes what this author believes is a best practice when it comes to establishing a “safety ground”. The safety ground is one that ensures enough ground fault current to quickly trip the circuit breaker connected to the faulted wire.
Technically what this article is focusing on is more correctly called “bonding“. In control panel designs this is normally established by a green or green with yellow stripped wire and is part of what is often called the grounding system. However there is nothing we do that actually ties anything to the earth. Instead the control panel bonding/grounding point is tied to a green wire that at some point is tied to the return path of some power source. For this article the word grounding and bonding are used interchangeably to conform with typical usage.
The focus is also on the safety aspects of the grounding system and does not deal with the EMI and shielding aspects associated with bonding. However in all but the most special cases this method will provide good EMI and equipment shielding unless high frequency generating devices (such as variable frequency drives) are located inside the control panel. In this case, this method is still likely a best practice, but additional measures may need to be taken. __________________________________________________________________________________
Over the years I have almost unwittingly developed my own form of grounding and bonding best practices as it relates to grounding/bonding of control panels and components in establishing a clear ground path “back to the source”. Recently I started becoming aware that it does not appear to be a common practice and am trying to get input on what people in the field think of this idea. I’ll call the idea the “Wired Star Grounding Method“. The practice puts a wire on everything that needs to be grounded and does not just rely on the cabinet and/or subpanel connections to establish the ground path. I implement this using ground bus bars (typically the kind used in panelboards to connect the neutrals together) bolted to the subpanel of each panel in a system. The main ground wire that is run with the power source wires will feed the first ground bar. From there, power fed to other external devices and panels will get their own ground wire of suitable size to each panel or device tied to the same ground bar. In each panel each device requiring a ground wire, the subpanel, the enclosure body, the door will have separate ground/bonding wires connected to the panel ground bus bar. The ground bus bars are tied together from panel to panel in a star (rather than series) connection as much as possible. In this way there will be a wired ground/bond path “back to the source” that does not count on the metal parts of the panel to provide the fault current path.
The reason I prefer this is that it is much easier to inspect for proper connection. Metal to metal connections sometimes are not adequate ground fault paths to trip circuit breakers in the event of a ground fault. Since we typically cannot test for this, it becomes difficult to verify that the ground/bond is solid. We typically check the ground with an ohm meter, but that draws almost no current so it is not possible to tell how “solid” the ground/bond connection is. Also over time metal to metal connections can foul. Finally they can just be installed improperly and this is not always “inspectable”. A wired connection is easier to inspect with a screwdriver or a good tug on the wire to tell if the connection is “solid”.
A couple of stories will help illuminate the problem. 3M thermionically welds the building steel together for all of its manufacturing facilities just to establish the building ground grid because they have had problems with the bolted connections in typical steel building frames providing a reliable ground/bond at all points in the building.
Another example is from my most recent project where due to a communication problem the vendor asked us to make sure the grounds are solidly connected. During the first check everything looked good using an ohmeter and a long spool of wire to check resistance from one panel to each of the other panels on the site. But when the problem persisted the ground/bonding at one panel was checked again and this time a 5 and then 10 ohm impedance was measure to the subpanel when it was less then 2 ohms before. It was found that the ground wire going back to the source was connected to a conduit hub ground screw that was loose and it could not be tightened.
My belief is that wiring the grounds back to the source ground/bond via a “wired star grounding method” avoids problems, is easier to inspect and verify, and provides another path to ground, thereby increasing reliability.
General Walk Through on Grounding: http://www.energyedcouncil.org/pdf/Event%20Presentations/Grounding%20and%20Bounding.pdf
Proper Grounding of Instrument and Control Systems in Hazardous Locations (Includes a recommendation to use a single point star grounding method: http://www.idc-online.com/pdf/Papers/Joe_Zullo.pdf
Allen Bradley’s recommendations on grounding, bonding, and electrical interference reduction: http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1770-in041_-en-p.pdf
Grounding and Bonding Hardware Examples From Panduit: http://www.panduit.com/Products/ProductOverviews/GroundingSystem/index.htm
EMC concerns with grounding systems in control panels: http://www.radiocad.com/_downloads/EMCenclosure.pdf http://www.emcuk.co.uk/awareness/Index.htm
ANSI/ISA-RP12.06.01-2003, Recommended Practice for Wiring Methods for Hazardous (Classified) Locations
LinkedIn Comments on the Wired Start Grounding approach: http://www.linkedin.com/groupItem?view=&gid=1967039&type=member&item=90589038&commentID=78576438&report%2Esuccess=8ULbKyXO6NDvmoK7o030UNOYGZKrvdhBhypZ_w8EpQrrQI-BBjkmxwkEOwBjLE28YyDIxcyEO7_TA_giuRN#commentID_78576438
Revised Saturday, April 28, 2012 8:40 pm CST | Published: Saturday, April 28, 2012 8:20 pm CST