Five more reasons generators fail when you need them the most

Here are five additional reasons why generators fail

By John Yoon September 14, 2022
Courtesy: McGuire Engineers

Engineers tend to focus on prescriptive design solutions for mission critical and life safety generator applications. Often the guiding concept is that adding capacity and complexity to the design helps ensure reliability. However, the devil is in the details. Real-world generator system reliability is often dictated by seemingly simple, mundane items that are often overlooked.

It’s reasonable to expect that a properly installed generator will function perfectly on Day One, but it should also be noted that its useful service life will often extend well beyond 25 years. In addition, that generator is only one component of a larger emergency power supply system. Failure of any individual part of that system could compromise the overall performance and reliability of that system. Given that extended service life, the logical question for any engineer is, what parts of that system will become vulnerable as the system ages and how can the associated risks be mitigated?

Figure 1: Smart battery charger and starting batteries within corrosion resistant battery boxes next to a generator. Battery failure is a leading cause of inoperable generators. Temperature-compensated battery charging is important in maximizing battery service life. Courtesy: McGuire Engineers

NFPA 110: Standard for Emergency and Standby Power Systems is the most applicable standard in this regard. NFPA 110 addresses installation, testing and (most importantly) ongoing maintenance requirements for the EPSS. The issues that are examined in this article echo those identified within NFPA 110 and consist mostly of simple items that have outsized consequences if not properly addressed.

Here are five common reasons why generators fail, with additional information available in the previous issue:

 1. Generator or related systems are in a vulnerable location

Aside from engineers, few people appreciate the appearance of generators. Often, generators are consigned to out of sight locations that are less that optimal — on roofs, in basements, behind shrubbery. As multiple natural disasters have proved, these types of locations can make a generator just as vulnerable as the electrical utility services that they are intended to back up.

NFPA 110 Section 7.2 addresses some generator location considerations such as avoiding locations with the potential for flooding. While this is a valid concern, flooding is by no means the only risk that should be evaluated. There may be other pertinent vulnerabilities associated with specific geographic locations or applications. In addition, other components of the EPSS may be just as vulnerable as the generator. For example:

  • Outdoor generators may be damaged by flying debris/projectiles in coastal areas where hurricanes are common.
  • Generators at the top of tall buildings, where elevators are the primary means of access, can become nearly inaccessible for emergency repairs if the elevators fail.
  • Even if the generator is located above grade, if fuel tanks and associated transfer pumps are in a basement, flooding may still cause an EPSS failure.
  • In extremely cold regions, it may not be possible to maintain diesel fuel temperature above cloud point if stored outdoors. Cloud point is the temperature at which wax crystals precipitate in the fuel. This wax precipitation can cause fuel filter plugging and generator failure.

Some situations may be unavoidable. However, a proper risk assessment performed during the design phase of a project can help identify these types of issues ahead of time and potentially allow specification/design of appropriate measures to help minimize the associated risks.

2. Weak or dead batteries

Just like an automobile, you cannot start a typical standby generator without a functional battery. And like their automotive counterparts, dead batteries are a leading reason for generators failing to start. If you doubt this, ask any generator vendor what the most common cause for generator starting issues and the most likely answer that you’ll get is batteries.

Batteries are electrochemical energy storage devices. Anything that affects the chemical reactions that happen within that battery will impact affect its functionality. Unfortunately, the adverse conditions under which most operate (vibration, improper charging, wildly fluctuating ambient temperatures) can do just that and will also dramatically reduce their useful service life.

The most common starting batteries used for generators are flooded cell lead-acid type. These types of batteries consist of lead cathode and anode plates separated by a sulfuric acid electrolyte. Every time a battery is discharged, the lead on the electrode plates within the battery will react with the sulfate in the electrolyte and form lead sulfate. When the lead on the plates can no longer react with the electrolyte, the battery is considered completely discharged. For recharging, the chemical reaction is reversed by applying an external voltage source, the lead sulfate is converted back to lead that redeposits on the plates and the sulfate returned to the electrolyte liquid.

Figure 2: All NFPA 110 compliant remote generator annunciators have a “not-in-auto” alarm. Remote annunciators should be placed in regularly supervised locations so that these alarms don’t go unnoticed. Courtesy: McGuire Engineers

However, the lead sulfate tends to deposit and accumulate in crystalline form on the electrode plates in the battery. That lead sulfate has a relatively high resistance. If the thickness of the deposits on the electrodes are great enough, the charging voltage can be insufficient to break them down. Eventually, the battery becomes unusable. Proper charging with adequate voltage and current is critical for slowing this deposit formation and ensure the batteries maximum service life.

Temperature compensated battery charging is critical in maximizing battery life. NFPA 110 section requires this capability for Level 1 (life safety) EPSSs. Batteries operate optimally in ambient temperatures (68°F to 77°F). Deviations from this optimum temperature directly impact the speed of chemical reactions that take place during battery charging and discharging.

Accordingly, a temperature compensated battery charger adjusts charging voltage based on ambient temperature. When the ambient temperature is high, charging voltage is reduced to avoid boiling off the electrolyte. When the ambient temperature is low, the charging voltage is increased to decrease the accumulation of lead sulfate crystals on the electrode plates.

Even with proper charging, batteries have a finite life expectancy. As such, NFPA 110 appendix section A5. recommends that batteries:

  • Be tested semi-annually.
  • If not tested, replaced every 24 to 30 months when exposed to temperatures exceeding 81°F for significant periods of time.
  • Replaced every 36 to 60 months for cooler temperatures.

 3. Bad fuel (diesel)

Most emergency generators see relatively little use over their useful service life. It’s not unusual to come across 25-year-old emergency generators with less than a few hundred hours of total runtime. In areas with reasonably reliable utility sources, a generator’s runtime may be limited to only weekly/monthly testing purposes. However, many EPSS applications require 24 hours or more of on-site fuel supply. In those situations, it may take years for a generator system to burn through a full tank of fuel.

The primary problem with this is that diesel fuel is not maintenance-free. As fuel ages, it will oxidize through mechanisms similar to how animal fats become rancid. Partially filled tanks can also attract water through condensation and promote microbial growth. The resulting water, wax, varnish and sludge can clog fuel filters and cause engine damage.

Modern diesel engines are manufactured to much tighter tolerances and have precision components such as high-pressure common rail fuel injectors. As such, modern engines are much more sensitive to fuel contamination. Most manufacturers have minimum specifications for fuel cleanliness based on ISO 4406. ISO 4406 defines contamination by a coding system that quantities the number of particles for difference sizes ranges within a milliliter sample.

The particle sizes referenced in this standard are not visible to the naked eye. If you can see contaminants or discoloration of the fuel (offroad diesel for generators is typically translucent red in color), chances are that fuel quality is less than what the manufacturer allows. If the fuel is contaminated, the fuel filters are the last line of defense for the fuel injection system. The filter’s ability to remove contaminants (efficiency, or number of particles retained by filter versus what is passed through) is in high +98% range. For larger engines with high fuel consumption, significant amount of contamination can still pass through the filter and cause damage over time.

Ultra-low sulfur diesel fuel typically only has a storage life of 6 to 12 months. The reduced sulfur levels, while good for reducing harmful engine emissions, are less effective at controlling microbial growth. It is recommended where fuel is stored for extended periods of time (more than 12 months) that the fuel be periodically pumped out and replaced with fresh fuel.

Current biodiesel blends potentially have an even shorter storage life, though development of hydrotreated vegetable oil biodiesel fuels may address that issue. HVO complying with European Standard EN 15940 is a further refined biodiesel feedstock that has improved oxidation stability and is less prone to bacteria growth, making it better suited to long-term storage such as in generator applications. It is expected that as carbon neutrality requirements increase, use of HVO biodiesel will become more pervasive.

If the fuel cannot be replaced at regular intervals, there are some options that may extend fuel life. Fuel polishing systems can be installed on fuel tanks to filter out contaminates. Although not officially recognized by generator manufacturers, fuel additives and biocides can also help. NFPA 110 appendix section A5.5.3 specifically mentions concerns with oversizing fuel tanks and even goes as far as suggesting that multiple smaller tanks instead of a single large may be preferable from a fuel management standpoint. Appendix section A. also recommends that tanks be kept cooler and relatively full to slow fuel degradation.

4. Generator controls not left in auto

If you have ever stood next to a large generator when it started and quickly accelerated to full speed, one of your first impressions probably was that it’s a surprisingly loud and violent event. Given that generators are often installed in skin-tight weatherproof enclosures or similar confined areas, maintenance work generally requires extremely close proximity to those engine components that could cause severe injury or death if the engine accidentally started. As such, the first step in any generator maintenance procedure is to disable automatic starting.

However, a surprisingly common reason for generators failing to start is that the service technician forgot to return the generator to automatic operation upon completion of maintenance work. While NFPA 110 requires that “not in auto” be included in the generator’s remote annunciator alarms, annunciators are often hidden in fire command centers or in similar out of the way areas that are not subject to proper supervision. As such, alarm conditions often go unnoticed until it’s too late.

Human error is unavoidable. Design solutions should take this into account and provide the means to make sure that qualified personnel can be made aware of these types of conditions in a timely manner. A simple solution such as adding a second remote annunciator in the engineers’ office or similar supervised location with knowledgeable personnel can go a long way in addressing this. With older generators, adding an additional annunciator could be a significant undertaking. Often, they used dedicated conductors for each and every annunciator alarm point which resulted in a complicated spaghetti mess of wiring. However, most modern generators use RS485 serial cabling, which dramatically simplifies wiring requirements and associated costs.

Figure 3: A 2-megawatt diesel generator serving a mission critical facility. Locating generators within conditioned, dedicated generator rooms can significantly extend their service life. Courtesy: McGuire Engineers

5. Lack of testing with load

This is a frustratingly simple problem. How do you know if a generator will work if you don’t test it? As stated before, most generators have very little run time. While NFPA 110 mandates monthly generator testing and an annual two-hour full load bank test, a surprising number do not comply with the load bank requirement.

Also, many generator owners perform monthly tests consisting of running the generator under no-load for 30 minutes. Running a diesel generator excessively with little or no load can result in wet-stacking. Wet-stacking occurs when cylinder pressure and temperature does not reach a sufficiently high level, resulting in incomplete fuel combustion. As a result, deposits form within the cylinders and a black, oily liquid may ooze from exhaust pipe joints. This can lead to reduced power output and accelerated engine wear. Most manufacturers will recommend that diesel generator be run at a minimum of 30% load for 30 minutes for every four hours of light load operation. This will burn off deposits and minimize the chance of wet-stacking.

Author Bio: John Yoon, PE, LEED AP ID+C; is lead electrical engineer at McGuire Engineers, Chicago. He is a member of the Consulting-Specifying Engineer editorial advisory board.