If there is one common thread throughout the electrical industry, it seems to be centered on grounding and bonding. Article 250 provides many of the requirements for grounding and bonding but is still difficult to understand. The entire article was rewritten for the 1999 National Electrical Code (NEC) with an eye toward making it easier to use. To this end, Code Making Panel 5 (CMP 5) has provided a more organized and logical order for the information, but in the process, some of the major changes to Article 250 may have been missed by the user of the Code.
One notable change to Article 250 can be found in the section dealing with the grounding of a separately derived system. But before dealing with the change to the Code, you must first understand the basics of a separately derived system.
The definition of a separately derived system is in Article 100. As far as the NEC is concerned, a separately derived system is a premises wiring system whose power is derived from a battery, a solar photovoltaic system, a generator, transformer, or converter windings. This system has no direct electrical connection, including a solidly connected grounded circuit conductor (neutral), to supply conductors originating in another system.
The main purpose of a separately derived system is to convert from one system voltage to another, usually lower voltage. Since there is no direct electrical connection from the primary side to the secondary side, the newly developed voltage on the secondary side must usually be connected to earth ground. By connecting the secondary of the separately derived system to earth ground, the voltage can be stabilized as the ultimate goal for most systems is for the ground reference point to be at a zero reference to ground.
This zero reference to ground for the electrical system can be equated to our numbering system with zero being the reference point between the positive numbers above zero and the negative numbers below zero. In other words, -4, -3, -2, -1, 0, +1, +2, +3, +4, and so on; all numbers relate back to the center point or zero point for the numbering system.
Just as with our numbering system, the newly derived system voltages relate to the zero reference point established by the connection point to a good ground reference point. Without this zero reference to ground, the separately derived voltages can vary depending upon the amount of load resistance and impedance found in the circuits attached to the derived system. By establishing a grounding electrode system tied into either manmade electrodes or natural electrodes, the reference point for the separately derived system can be designed and installed to be as close to the zero reference as is possible for the conditions at the site.
Lightning, line surges caused by large loads being switched into the system, and arcing from high-voltage lines coming into contact with the lower-voltage conductors, require a good reference to zero ground or as close as possible. This zero reference will help to limit the voltage and current peaks on the system, thus limiting potential damage.
With this ground reference in mind, it can be better understood that the purpose of Section 250-30 is to provide information on how to properly bond and ground a separately derived system. Section 250-30(a)(3) requires the grounding electrode for the separately derived system to be either connected to an effectively grounded metal structure or to a metal water pipe that is effectively grounded. This connection to building steel or metal water must be as near as is practical to the separately derived system and preferably in the same area. The closeness of the separately derived system to the grounding electrode will help to reduce the resistance of the system and keep the reference as close to zero as possible.
To determine if the steel of the structure is effectively grounded, you may need to check the building plans to find out if the building steel is connected into the ground plane of the building through a rebar cage and into the foundation connected into earth. If the building plans do not detail a connection to ground, a test may be needed to ensure that there is a good connection to the ground plane of the building.
The alternate method of establishing a grounding electrode for the separately derived system is connecting the system to the effectively grounded metal water piping system. This is where a major change occurred in the methods used in the past to make this connection to the separately derived system. The 1999 NEC now requires the connection to the metal water pipe to be within 5 feet of where the metal water pipe enters the building.
For example, a transformer may be located on the 50th floor of a high-rise building. If building steel is available near the transformer, the transformer is connected to building steel and a good reference point can normally be assured.
If building steel is not available or not effectively grounded, then an available metal water pipe could be used. In this case, a grounding electrode conductor may have to be installed from the transformer down through the building fifty floors to a point within 5 feet of where the water enters the building. Care should be taken to properly size this grounding electrode conductor, since this is the conductor that will be used to provide a connection to the zero ground reference for the separately derived system.
If neither building steel nor a metal water pipe is effectively grounded or in close proximity to the separately derived system, a concrete-encased electrode, a ground ring, a rod or pipe electrode, or a plate electrode can be used. The problem with using these electrodes in a high-rise building is the amount of distance from the separately derived system. The problem with resistance will be as critical, if not more so, with these alternate methods as with the metal water piping system.
There has been one alternative suggested for a high-rise building but not incorporated into the Code. The suggestion is to install a grounding electrode conductor. It could be sized from a 3/0 AWG copper conductor to a 500 kcmil copper conductor or from a 250 kcmil aluminum conductor. It would travel from the grounding electrode at ground level (or in the basement) up to the 50th floor (or whatever the height of the building), in an accessible location, to allow connection of the separately derived system.
Before using this method or any alternate method not covered by the Code, the electrical designer/engineer and the electrical inspector should be consulted to determine if they agree with this method of grounding the separately derived system. The second paragraph in Section 90-4 permits the authority having jurisdiction to waive specific requirements in the Code or permit alternative methods where it can be assured that safety is not compromised.
It would also be a good idea to get any "special permission" in writing, as defined in Article 100, since changing the system after the work is completed could be difficult and expensive.
Don't forget! The metal water piping system must still be bonded to the separately derived system at the nearest available point in the area served by the separately derived system in accordance with Section 250-104(a)(4). This bonding jumper does not have to be taken back to within 5 feet of the entrance of the metal water pipe into the building. It is simply there to provide a bond to the secondary of the transformer or the type of separately derived system being used. The bonding jumper from the separately derived system secondary will be sized in accordance with Section 250-66 and the accompanying table.
Following these simple rules should provide an effective reference for your separately derived system.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.