Mike Dolt Enterprises
Mike Dolt Enterprises
User: barone, Location: ORLANDO FLA
Asked on: Friday, April 29, 2011 at 4:45 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:48 am EDT
Question: I have read several threads to try and get an answer to my dilemma and have not had any success. Most of the threads deal with the obvious issues with providing an alternate source of power for a fire pump and how to make those connections. Here is where I get fuzzy and take issue with the way most installations have been done. Since the fire pump ATS/Controller is service entrance equip. we must provide a neutral conductor on the normal side and provide a main bonding jumper in the fire pump ATS controller. We do not however provide a neutral conductor from the generator, as there is nowhere to land it at the FP ATS or is it required because the load is delta. So what we have is an unbonded generator (not a SDS) and 3-pole transfer switches with solid neutrals, everywhere but at the fire pump ATS/controller. So my question is, how are we providing a low impedance path for fault current to actuate the emergency source overcurrent device (short circuit protection only) mounted on or near the generator? And if the path does exist (if we have an additional ATS with solid neutral anywhere else in the system, or if we are providing our normal source from a tap section in a service switchboard, which we are) then is that path not objectionable?The way I see it if a phase to ground short occurs in the fire pump supply conductors or the fire pump itself for that matter, when the system is in emergency mode, current will flow through the equipment ground to the bond in the fire pump ATS, then through the normal neutral or the water pipe ground to the service switchgear, then back through the service neutral to the other ATS normal feeder neutral, across the neutral block in any other ATS and back to the generator alternator thorough that ATS's neutral conductor, or every ATS's neutral conductor should we have many, which we do (not to mention any stray currents that may flow in the equipment grounds). Thus being objectionable.Code does not address this at all. Other than requiring that low impedance paths for fault current must not be objectionable.Keep in mind that there is not a Fire Pump ATS/Controller manufacturer that will provide a connection for the neutral conductor from the emergency source or will they build a 4-pole transfer switch into them. They come one way only, grounded normal input and ungrounded emergency input. I have yet to see them any other way, and in talking to the manufacturers they just do not make them. With this in mind, providing a neutral conductor from the generator to the fire pump ATS is a waste of wire, as there is nowhere to land it, without violating its UL listing. And if I bond at the generator, they do not make a four pole fire pump ATS, so there is no way to switch the neutral. Again a waste of wire.Sorry for the long post and lack of code sections, this was hard to describe.
Regarding the Low Impedance Path for Fault Current
NFPA 70 (NEC) requires that a low impedance path for fault current be provided back to the emergency source overcurrent device (short circuit protection only). This path should not be objectionable, meaning it should not create hazardous conditions or interfere with the operation of other equipment.
In the Case of a Fire Pump ATS/Controller
Since fire pump ATS/controllers have a grounded normal input and ungrounded emergency input and do not provide a connection for a neutral conductor from the emergency source, the low impedance path for fault current is typically through the equipment ground.
Path of Fault Current
In the event of a phase-to-ground short in the fire pump supply conductors or the fire pump itself, the current will flow as follows: Through the equipment ground to the bond in the fire pump ATS Through the normal neutral or water pipe ground to the service switchgear Through the service neutral to the other ATS normal feeder neutral Through the neutral block in any other ATS Back to the generator alternator through that ATS's neutral conductor
Objectionability of the Path
While this path provides a low impedance for fault current, it is not ideal due to the potential for stray currents and the fact that it relies on the continuity of the equipment grounds and neutral conductors. If any of these paths are interrupted, the fault current may not be cleared effectively.
Solutions
Unfortunately, there is no clear solution to this issue within the current requirements of NFPA 70. One possible approach is to provide a dedicated grounding electrode at the generator and connect it to the fire pump ATS/controller. This would provide a low impedance path for fault current without relying on the continuity of the normal neutral conductors.
Other Considerations
If the generator is classified as a Separately Derived System (SDS), it would require a 4-wire connection to the fire pump ATS/controller. However, most fire pump ATS/controllers are not designed to accept a 4-wire input. Some manufacturers offer fire pump controllers with a neutral input termination, but they are not widely available.
Recommendations: Consult with a qualified electrical engineer to design a solution that meets the requirements of NFPA 70 and provides a reliable low impedance path for fault current. Explore the possibility of installing a dedicated grounding electrode at the generator to improve the grounding path. Consider using fire pump controllers with neutral input terminations if they are available. Flag for review