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Requirements of Critical Operations Power Systems (COPS) - Part 2

Requirements of Critical Operations Power Systems (COPS) - Part 2


This article is a continuation of the article from the previous issue of PowerConnect regarding the requirements of Critical Operations and Power Systems (COPS).

Fuel is a major concern when evaluating a facility’s ability to defend in place. Through the grid failure of 2003, the Florida hurricanes and the flooding in New Orleans, it has become apparent that during significant events the generator’s ability to be refueled is significantly impacted. In certain situations, owners can’t just call for another tank of fuel or expect it to be available upon demand. It may be days or even weeks before a diesel fuel truck can reach your destination. NEC 708.22(C) requires COPS to have a minimum operating time of 72 hours with the DCOA at full load. In addition to this requirement, NEC 708.20(F)(1) mandates that fuel for the generator may not be solely supplied by the public gas utility.

On first evaluation, many system designers will approach these fuel requirements by configuring the system with a diesel generator and utilizing main storage tanks to assure a minimum of 72 hours of run time. Is this the best approach? Will this mitigate the failure risks as required by NEC 708.4(C)?  Is this enough fuel or is this too much fuel? There are a number of competing issues that are in conflict. Too little fuel and the generator will run out prior to refueling; too much fuel and the fuel goes bad.

The NEC does not directly reference any other NFPA standard but it does use fine print notes (FPN) to reference key related codes. NFPA 110 is a repeated FPN reference within Article 708 and will probably be adopted by statute for COPS. NFPA 110 has numerous requirements related to fuel: the system design shall provide for a supply of clean fuel (; the fuel must be consumed in its storage life or provisions shall be made to replace stale fuel with clean fuel (; an annual fuel quality test is required (8.3.8); sulfur, naturally occurring gums, waxes, soluble metallic soaps, water, dirt and temperature all begin to degrade fuel as it is handled and stored (A.

To understand why NFPA 110 makes so many references to fuel condition, understand that the typical standby generator doesn’t run much. If the tank is sized for 72 hours of full load operation, it could easily take 22 years to get one fuel turn on the tank (assumes 60% typical load level, weekly no load exercising and an average of 4 hours of outage per year). So what is the storage life of diesel fuel? Exxon’s web site responds to this question: “If you keep it clean, cool and dry, diesel fuel can be stored 6 months to 1 year without significant quality degradation. Storage for longer periods can be accomplished through the use of periodic filtrations and addition of fuel stabilizers and biocides.”

So where does this lead us? At a minimum, the fuel tank will need an aggressive maintenance program that may require the fuel to be replaced.  However, if the generator shares fuel with other diesel devices (boilers, etc.); the fuel will be turned more often.
Another option may be to use the generator to support a significant optional standby load with a modest runtime requirement. The optional standby load could be shed after a minimum run period under normal outage situations or immediately shed during a significant event. This would maximize the fuel available during more significant outages.

For areas with sufficient and reliable natural gas infrastructures, innovations in the standby generator market also make bi-fuel (diesel and natural gas) configurations a very good fit for this application. Bi-fuel units typically run with 75% of the engine’s power coming from natural gas, thus extending the duration of the engine’s on-site diesel fuel by a factor of four.  This technology also has the ability to operate on 100% diesel, meeting Article 708’s requirement for not being solely supplied by the public gas utility. 

Finally, for smaller applications dual fuel (LP and natural gas) may present another highly reliable option.

Generator location is also a consideration when designing high reliability systems.  Article 708 requires that the generator meet physical security and restricted access requirements. The generator location must also provide protection from natural and human-caused events.
These requirements may lead some system designers to initially show a preference for locating the generator indoors. Though indoor locations have some advantages, they also have some often overlooked disadvantages. Indoor locations:

  • have increased supporting system requirements: airflow, exhaust, thermal considerations, and fuel transfer.
  • present greater challenges in controlling fire risks. The building is a fire risk to the generator and the generator is a fire risk to the building. 

When paralleled generators are utilized, outdoor generators provide inherent fire isolation between units. Outdoor units, located in separate locked enclosures, also tend to be impacted less from inadvertent human interactions – breakers left open, fuel lines closed, controls not in automatic, control parameters over adjustment, etc. 

One potential solution that combines some of the benefits of indoor and outdoor configurations is outdoor units located inside a secure, walled-in area.

Article 708 takes a holistic approach to critical power system reliability and, as such, challenges system designers to consider all risks that can impact system operation. This is a significant departure from the traditional NEC regimented rule based approach. Reasonable people will choose to disagree on relative risk levels and thus system design preferences.  Whatever the form of the final design, system reliability will be improved simply by challenging ourselves to work through these tough questions. 

If you have questions about the NEC Article 708 or any code issues, contact your authorized Generac Industrial Power distributor or dealer.