Growth is something that every company, regardless of industry sector, has as a stated goal. Increased revenue, greater market presence, and the prestige of leading an industry are appealing to most organizations, so the incentives for growth are strong.
However, as most industrial facilities struggle to maximize production and increase efficiencies, the addition of electrical equipment can actually work to their detriment. This kind of growth – increased loads on existing electrical apparatus, and the addition of new apparatus to existing electrical infrastructure – can have deleterious effects on many aspects of equipment reliability.
This article explores how the growth of equipment, when not properly tracked and managed, can negatively impact overall equipment health, personnel safety, and ultimately, the bottom line of a facility. This impact can be in the form of power quality issues, increased ambient temperature in equipment rooms, and on the work/rest cycle of maintenance and reliability team members.
Turning up the heat
There is a physical condition known as thoracic outlet syndrome that affects mainly the upper limbs of a person's body and that can result from physical over-training. Symptoms include inflammation in the shoulders and arms as well as both pain and numbness. In athletes, it's commonly understood to be the result of too much of a good thing, and can be remedied by a more controlled approach to a person's workout goals.
This condition is a good metaphor for almost any other type of otherwise positive growth, including industrial facilities where the goals can include meeting more aggressive production schedules, increasing throughput, and increasing productivity in the same space, all of which can lead to less than optimum equipment conditions. If the conditions are recognized and corrected in a timely fashion, long-term damage can be avoided. However, when equipment conditions are ignored and growth is not managed safely and effectively, the negative outcomes can be quite serious, and even life threatening.
Let’s look at electrical load first. The National Electric Code (NEC) gives guidance on what typically is referred to as the “80% Rule”. This rule states that for continuous duty, which is defined in NEC Article 100 as maximum current for longer than three hours, a circuit fed from a single source should not exceed 80% of the capacity of the circuit. So, for a 20 amp circuit, 16 amps of continuous duty should not be exceeded. Having spent the greater part of our adult lives performing various types of electrical predictive maintenance (PdM) services, we can say that this rule is broken as often as it is followed. What may not be as readily apparent is the detriment of such conditions.
Perhaps you’ve heard of the joule effect? Heat generated in a circuit or conductor is a product of the resistance of the circuit and the current through the circuit. The amount of heat energy generated goes up as current (i.e., load) increases, and it increases at the square of the current. This phenomenon often is expressed as the equation P=I2R, with P being Power (in the form of heat). What is often not understood by otherwise conscientious electrical professionals is the fact that electrical resistance is a product of temperature. So, as we increase the load on a circuit, we increase the amount of heat generated in the circuit. That heating causes an increase in electrical resistance in electrical components subjected to the increase in ambient temperature where the circuit in question, or the larger electrical system component, is located.
Most electrical apparatus are rated according to an expected ambient air temperature in the environment where the apparatus is installed. When an electrical space is occupied by numerous electrical apparatus, heat is already being generated under normal loading conditions. What then is the impact when we add load, because of growth? New equipment needs to be fed from somewhere, right? We have an electrical room right there, and there’s space in the panels, so problem solved. Actually no, we may have just caused a problem rather than found a solution.
It’s like the old joke about not being out of money because you’ve still got checks in your checkbook. Just because there are open circuit breaker slots in a panel does not mean that capacity is available to accommodate the added equipment. Once we start to over-duty circuits, more heat is generated. NEC Table 310.15(B)(16) gives allowable ampacities of insulated conductors, and includes temperature withstand ratings for the various types of commonly used insulation. The caveat here, and it is mentioned in the table, is that these figures are based on an ambient temperature of 30ºC (86ºF).
Just for the sake of clarity, let’s define “ambient temperature”. Merriam-Webster defines ambient temperature as “interface temperature between a surface and the fluid medium surrounding that surface”. To be technically accurate, “ambient temperature” is not always synonymous with “room temperature” unless, of course, the electrical apparatus is in the open air of the room. Usually, electrical apparatus is inside of an enclosure, and there is typically a difference in temperature between the room where that enclosure is found and the temperature inside of the enclosure. The temperature inside of the enclosure should be considered the ambient temperature for any electrical components internal to that enclosure.