GFP levels in hospitals
My question concerns required levels of ground-fault protection (GFP) in health-care facilities.
The facility is a medium-size hospital that has grown from a small hospital through several additions and remodels over the past 20 to 30 years. In addition to the 3,000A main mentioned, there is another 3,000A main and two 2,500A main services all grouped and all with GFP. The distribution scheme is similar to Figure 517-30(b). The GFP on the 1,600A breaker is coordinated with the GFP on the main. The 400A feeder in question feeds a panelboard supplying nonessential loads in a separate temporary laboratory building.
The NEC Handbook commentary on Section 517-17 seems to say that GFP on the 400A panel feeder (Third level) would be Code allowable and possibly desirable “at the option of the governing body of the health-care facility,” but is not a Code requirement. The underlying issue with this question is that if the determination is made that GFP is Code required on this feeder, the same would apply to numerous other feeders throughout the facility that do not presently have GFP. This would incur a substantial cost.
The hospital administration certainly wants to provide the level of safety required by the Code, but would like to have the option on whether to go beyond the Code requirements.
Two or more levels of GFP may be required, depending on how the various feeder branches are supplied.
The intent of Section 517-17 is to enhance the reliability of the electrical system. Additional levels are required so that a fault in the normal branch (nonessential loads) will not cause an interruption of power in the essential electrical system. Because the language used in 517-17(a) says “an additional step,” this may be interpreted to mean that only one additional level of GFP is required. However, the words “additional levels” in the second paragraph indicate that two or more levels may be required in order to provide the “100% selectivity” required by Section 517-17(b). In addition, more than one GFP device may be required at the second level, depending on the way the normal branch and the essential electrical systems are arranged. In your case, if the 400A device feeds the normal branch, and another device at the same level (also fed by the 1,600A device) supplies power to emergency equipment, then the 400A device may require GFP in order to meet the selectivity requirements of Section 515-17(b). However, you did not say what type of health care facility you are dealing with or what types of loads are served by the mentioned feeders. Normally, the essential and nonessential loads are separated at the first feeder level. Figures 517-30(a), (b) and (c) illustrate this arrangement. If your distribution scheme is similar to Figure 517-30(b), and the 3,000A device is at the service entrance level, only two levels would be needed. However, more than one device would likely be required at the second level.
Editor's Note: The preceding answer repeats from March — this time with the appropriate question. We apologize for the error.
Wiring railroad cars
I am involved in rewiring railroad passenger cars. Amtrak says that the wiring must meet the applicable NEC requirements. The particular problem is that passenger cars vibrate a lot, and are subject to jolting forces. I wish to weld the new conduit (EMT inside, rigid outside) to the car body to prevent rattling and looseness, before inserting the wiring. The Amtrak inspector says that I am not allowed to weld conduit to the car body, citing a provision that prohibits welding wire trays to a building, and combining with that a definition of tray that includes conduit.
Does the NEC apply? I think it's mentioned in the NEC introduction.
If the NEC does apply, is the Amtrak inspector correct?
Additionally, the inspector also said that liquidtight used in lengths of more than 6-foot conduit must include a ground wire. The metal conduit on either side of the liquidtight is welded or clamped to the steel car frame. Further, the majority of railroad passenger cars are wired with three-phase delta non-grounded circuits. Thus the circuit breakers are all two-pole, which means the ground or neutral floats.
Is there any need for any bonding ground wire in this circumstance?
The NEC does not specifically mention railroad passenger cars. Section 90-2(b)(1) says the NEC does not cover “railway rolling stock,” and I would take that to include passenger cars. However, read the FPN under this section. Like ships, the fact that the NEC did not intend to cover railroad cars does not mean that parts of the code could not be applied to railroad cars. If the AHJ (Amtrak and their inspector) has decided that the NEC or “applicable” parts of the NEC do apply then they apply. Of course the AHJ will also have to decide which parts are applicable.
As for the welding question, Section 300-18(b) forbids welding directly to a raceway.
Section 250-118 says the equipment ground wire must be run with or enclose the circuit conductors. Since more than 6 feet of liquidtight flexible metal conduit can never be used for grounding, a ground wire must be run in the raceway. You could put an equipment-grounding conductor/bonding jumper on the outside of the flex, but that permission, which is located in Section 250-102(e), is also limited to 6 feet. You could argue that Section 250-136 would eliminate the need for the ground wire, and this seems to agree with your thinking. However, although the conduit is equipment, the car frame is not necessarily “provided for its support.” The railroad car chassis is, in effect, the “structural frame of a building,” and using the structural frame for grounding is not permitted.
Remember, the point of these rules is to make the installation comply with 250-2(d), and the lowest impedance for the ground return path will generally be provided by a conductor “run with or enclosing” the circuit conductors.
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