Eduard Pacuku, PE, Concord Engineering, Philadelphia
When designing electrical distribution systems for mission critical facilities, the overriding factor is reliability. These systems are called mission critical for that very reason: It is critical that power remains uninterrupted. But this mission critical label does not apply to every single part of the facility. Some processes are more important to the business than others, and that is where the focus of the design lies. Certainly, designing for a mission critical facility can be challenging. There are many considerations in play. The system has to be very robust with no single point of failure.
When designing electrical systems, maintenance is an important factor, specifically in deciding what battery to implement. Maintenance includes not only the cost of replacement of a battery cell, but also the accessibility of the battery system, the ease of disassembly and reinstallation, the frequency of service, and the conditions needed in the space (temperature, humidity, etc.).
The goal of maintenance is to prolong the life of the batteries and to make sure the batteries perform as designed. Inspection is done to catch any abnormalities that could impact battery performance as well as long-term life expectancy. To this end, it is important to place the batteries in an environment that agrees with the manufacturer's recommendations.
There are several types of batteries used for providing power to electrical systems. The most popular types of batteries being used nowadays are lead-acid and nickel-cadmium (NiCd). NFPA 110: Standard for Emergency and Standby Power Systems defines two types of lead-acid batteries:
Valve-regulated Lead-acid (VRLA): A lead-acid battery consisting of sealed cells furnished with a valve that opens to vent the battery whenever the internal pressure of the battery exceeds the ambient pressure by a set amount.
Vented (or Flooded): A lead-acid battery consisting of cells that have electrodes immersed in liquid electrolyte. Flooded lead-acid batteries may have a provision for the user to add water to the cell and are equipped with a flame-arresting vent, which permits the escape of hydrogen and oxygen gas from the cell in a diffused manner such that a spark, or another ignition source, outside the cell will not ignite the gases inside the cell.
Although NFPA 110-2010 recognizes the usage of NiCd batteries for emergency systems, no definition is provided for such batteries. A definition can be found in IEEE 1106-2015: Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications:
Partially Recombinant Nickel-cadmium Cell: A vented NiCd cell providing an internal means for the recombination of internally generated oxygen and suppression of hydrogen gas evolution to limit water consumption, typically operating with a recombination efficiency of 90% or higher.
Mission critical facilities are not only the buildings that are critical to human life, such as hospitals, but also are facilities that have a great economic impact. In the critical systems (or processes) of these facilities, many methods are employed to mitigate the risk of power outages. But, almost always, the risk mitigation relies upon batteries. Uses include:
Uninterruptible Power Supply (UPS) Systems: Batteries are used to back up power where UPSs are involved. While the power is processed and converted through the UPS to the load, the batteries are kept charged to provide the much-needed power to the critical system when normal power is out. Even though standby generators are most likely used as backup power, the batteries provide the critical power needed until the generators are able to receive load. Standby generators: Batteries are used to provide the initial crank to the generators as well as power the generator controls.
Controls: Batteries also are used to back up controls, especially when medium-voltage distribution is found throughout the facility. Nowadays, the operation of critical systems is more reliant on controls to avoid nuisance tripping and to enable fast-acting protection.
Emergency Systems: Batteries are widely used to back up life safety systems, such as exit lights. Exit lights normally are backed up by a 90-minute battery to allow people to recognize the way out of the building in case of a life-endangering situation. All of the above-mentioned uses (UPS, generators, and controls) could be part of an emergency system as well.
When designing electrical systems, maintenance is an important factor, specifically in deciding what battery to implement.
The maintenance of the batteries in mission critical facilities is especially important. When maintained regularly, batteries will perform according to design to support the critical systems, ensuring the continuance of power. Maintaining the batteries also elongates their life, resulting in lower operation costs. But the first step of maintenance is to inspect. By first inspecting and then assessing, we can develop a plan for remediation.
Let's look at each battery type separately. IEEE has developed separate standards for just this reason.
Vented Lead-acid: The most routine inspection for this type of battery is a visual inspection. IEEE has developed a standard, 450-2010, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications. This standard is a very good guide for the operations staff to create a maintenance procedure.
The monthly inspection, as recommended in the IEEE 450-2010 standard, includes the following:
A visual inspection of:
A measurement of:
The IEEE 450-2010 recommendation for monthly visual inspection is just a general recommendation for any type of facility. NFPA 110 requires weekly visual inspections for battery systems employed in emergency and standby systems.
The recommended quarterly inspection includes the following:
A measurement of:
There is also a yearly inspection that applies the quarterly inspection to all the battery cells, which is, therefore, much more involved.
If abnormalities are observed during these inspections, there are corrective actions that can be taken.
The room temperature and ventilation usually are set during the design. Total cost of ownership analysis is usually done by taking into consideration the optimal conditions so that the battery life is longest. The rule-of-thumb correlation between ambient temperature and lead-acid battery life, be it vented or VRLA, is as follows: battery life decreases by 50% for every 15°F above the normal temperature, which is 77°F.
Determining the battery life is important, especially in mission critical facilities. With proper maintenance, battery life could be predicted accurately, thereby avoiding any downtime. The most important factors affecting battery life are:
VRLA batteries currently are very popular. They are known as "nonmaintenance" batteries, mostly because of the fact that they are sealed and employ a "recombinant technology." The fact that the battery is sealed ensures the preserving of the electrolyte, no matter the position of the battery cell (you can even turn it upside down). Recombinant technology means that the oxygen released from the positive plate ends up in the negative plate, where it recombines with hydrogen and turns into water. This process preserves the battery water
Just as it has for vented batteries, IEEE has developed a standard, 1188-2005, IEEE Recommended Practice for Maintenance, Testing, and Replacement of VRLA Batteries for Stationary Applications.
Even though VRLA batteries are sealed, there are always dangers associated with installing and maintaining batteries. Only trained and knowledgeable personnel should work around batteries. In addition, unauthorized personnel should not have access to the batteries. The battery area should not be used as storage for tools or anything else. All personnel should use personal protective equipment, such as goggles, gloves, and safety shoes, while working on batteries
Just as with vented batteries, inspection is an important part of maintenance. IEEE recommends monthly, quarterly, and yearly inspections. But this recommendation is for general use. For mission critical facilities or processes, there needs to be a weekly inspection as well.
The monthly inspection for VRLA batteries is more or less the same as the one for vented lead-acid batteries. This inspection includes the following areas:
A visual inspection of:
A measurement of:
The quarterly inspection includes measurements of the following:
The yearly inspection is then limited to measuring the following:
Vented Nickel-cadmium Batteries
Just like the name suggests, these batteries are a vented (flooded) type, where the positive plate is made of nickel hydroxide and the negative plate is made of cadmium hydroxide. These batteries are often preferred because they:
However, these batteries have the following drawbacks:
IEEE has developed a standard for this type of battery: IEEE 1106-2015, Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications.
As with other kinds of batteries, personal protective gear, such as goggles, gloves, and aprons, are recommended when servicing the NiCd batteries.
IEEE 1106-2016 recommends a quarterly inspection, which is the same as the vented lead-acid batteries' monthly inspection:
A visual inspection of:
A measurement of:
There is a semiannual inspection as well. This inspection encompasses what the quarterly inspection requires and adds cell voltage measurements. The yearly inspection also requires looking at the condition of the cable connections and measuring the resistance.
For a list of simple corrective actions to a few abnormalities, see Table 3 (see Table 3).
The NiCd batteries can be submitted to a high-rate charge. Normally, they are charged at around 1.4 to 1.47 V per cell, but in a high-rate charge, the voltage can go to 1.55 V for a maximum of 1.8 V.
A NiCd battery is more tolerant of higher temperatures. Its lifespan reduces by 20% when operating at temperatures 50°F above the recommended 77°F, while the lead-acid battery takes a harder hit, around 50%. The NiCd battery life and temperature correlation is shown in Figure 3 (see Table 3).
In Table 3 (see Table 3), a few corrective actions for some simple abnormalities are shown.
NiCd batteries, if operated at 68 to 77°F, can last a long time. Their capacity drops slowly, reaching 80% after 20 years. Even after that, they can be used in less-demanding applications.
The Importance of Design
Maintenance is especially important for mission critical systems because of the importance of the reliability of these systems. Indeed, maintenance does not start when problems with the equipment arise; rather, maintenance starts with the design of the systems. The design professional always needs to keep maintenance in mind when designing electrical systems, and the same goes for battery systems.
Keep the following tips in mind when designing and specifying batteries in mission critical facilities:
After the system (or facility) is up and running, it is critical to make sure that the design conditions mentioned above are enforced. It is also critical to follow IEEE recommendations for weekly, monthly, and yearly inspections (add a semiannual inspection for NiCd batteries). A visual inspection, as simple as it might seem, is very important to catch early what could become a serious malfunction. It is best for the maintenance team to prepare a standard form for each type of inspection. These forms could have boxes for check marks and spaces for notes where certain conditions observed can be explained. Having a log of the inspections helps in tracking certain conditions and compiling a plan of action if the conditions deteriorate.
Eduard Pacuku is a senior electrical engineer at Concord Engineering. He spends most of his time designing electrical systems for universities, health care facilities, mission critical facilities, and high-rise commercial buildings.