Fire Pump Controller/ATS Normal versus Alternate Source Requirements

[milwaukeebob]

I have a question regarding a fire pump/motor and associated fire pump controller/auto-transfer switch (ATS) installation at small power generation plant (<40MW). The plant is in the final stages of installation and commissioning has already begun. Note the owner of the plant is not a utility.

The design/installation of the fire pump system and associated small building is non-traditional in that there is odd scope-split between the owner and the EPC Contractor. Furthermore, portions of the design do not appear to have been vetted properly by a traditional, single Engineer of Record (EoR). I was brought in very late in the project cycle to represent the owner and help with several high-level tasks. Although I’m a degreed power electrical engineer and have almost 30 years of industrial power plant experience, I’m not a fire pump/protection expert. Nonetheless, I recognized some oddities when it came to the fire pump/system design and installation.

The existing installation design has a traditional listed electric fire pump controller with an ATS that feeds a small fire pump/motor (460V, 125hp). The ATS has two (2) incoming supply feeds – one is a NORMAL feed and the other is an ALTERNATE feed. The NORMAL feed meets all the requirements of NEC Article 695 and NFPA 20 including the requirement for the entire circuit to be designed to carry locked rotor current (LRC) from the fire pump motor indefinitely. According to all practical industry definitions and based on historical data, the NORMAL feed would be considered a reliable power source per NFPA 20 and NEC 695. Although the NORMAL feed is derived from a reliable utility power source, there is also an in-plant permanently installed portable generator connected to the plant electrical system that also backs up the low voltage switchgear where the NORMAL feed to the fire pump ATS comes from.
Despite having an individual reliable power source, the client decided they wanted to add an ATS and an ALTERNATE source of power for emergency, extremely low probability conditions. This ALTERNATE source of power is from a separately derived utility power source via pole-mounted xfmrs (34.5kV/480V) through a low-voltage switchboard. There is a series of fuses and a molded-case circuit breaker between the secondary side of the xfmrs and the input of the ATS.

The primary question I have is does the ALTERNATE source circuit need to be designed to carry LRC? NFPA 20 seems to suggest it does NOT. NEC 695 is a lot more convoluted. My experience with the ALTERNATE source of power is from a traditional on-site standby generator connected to the ATS. In NEC 695.3(D) “On-Site Standby Generator as Alternate Source” it clearly states this ALTERNATE source does NOT need to be designed to indefinitely carry LRC. While it’s understood the existing ALTERNATE source via utility pole-mounted xfmrs is NOT an on-site generator, what would be the difference between the two? I see this section of the Code suggesting intent here. The whole purpose for my question is that there is now another engineer involved in the review effort who is suggesting the ALTERNATE source needs to be designed for LRC. I have taken the opposing position.
Appreciate all feedback on this and would especially like to hear from those with specific experience in this area.

 

[waross]

The actual current Locked Rotor Current drawn from a generator may well be much less than LRC drawn from a utility transformer.
A generator AVR will have Under Frequency Roll Off.
If the generator is unable to sustain LRC there is a good chance that UFRO will be active and the voltage and the current supplied to the motor will be less than it would be if it were supplied from a utility source.
That said, I have no idea if this is taken into consideration with regards to the regulations.
I hope that this helps answer the question:

While it’s understood the existing ALTERNATE source via utility pole-mounted xfmrs is NOT an on-site generator, what would be the difference between the two?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

I would as the owners underwriters engineering department for an opinion (Gaps, FM and Hartford, probably others do this). Then having the underwriters position in hand, I would ask the AHJ if they will accept the situation, as it becomes a bigger problem if the AHJ holds the final permit completion over this issue.

 

[milwaukeebob]

Thanks waross and FacEngrPE.
I know this will fly in the face of everything you know but there is NO traditional AHJ on this project. I understand...makes no sense but that is the reality. This is not a traditional utility or IPP building this small facility. Unfortunately, we currently have a number of PE's including myself debating this issue and there's a difference of opinion.
As for your question on the Underwriter, another great question. Would you believe the actual Underwriter is unknown by the project manager and engineer?

 

[waross]

In the absence of an engineer of record, you, as the owner's engineering representative may find yourself in the legal position of EOR by default.
To protect your position it would be well to have a ruling from an authoritative source.
Are you able to meet the condition of continuous LRC by using a primary impedance start system, with the impedance rated to continuously carry LRC?
Do you have a lawyer?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!