General Applications study guide

What this area covers

General Applications spans more separate topics than any other area and feels most like day-to-day facility and utility work. It gathers load characteristics (demand, diversity and load factor), interior lighting design, grounding, surge protection, power factor at the service, and the engineering economics that decides whether an improvement is worth paying for. This area accounts for 8–12 questions on the exam. The individual topics are not deep, but there are many of them, so breadth beats depth here.

The ideas everything else rests on

Load is described by ratios. Demand factor compares the maximum demand of a group of loads with its total connected load, and is at most one. Diversity factor compares the sum of individual peak demands with the peak of the group, and is at least one, because loads rarely peak together. Load factor compares average demand with peak demand over a period. Each ratio answers a different sizing question, so read carefully which one is given.

The lumen method is a light budget. The light a room needs, illuminance times area, must be delivered by luminaires after losses. The number of luminaires is

$$N = \frac{E \cdot A}{\Phi \cdot CU \cdot LLF}$$

where the coefficient of utilisation accounts for room shape and surface reflectances, and the light loss factor accounts for lamp depreciation and dirt over time.

Grounding is about resistance to remote earth and about bonding. A single driven rod's resistance depends mostly on soil resistivity and rod length, and only weakly on diameter. Adding rods helps only if they are spaced far enough apart that their zones of influence do not overlap much. Measuring that resistance with the fall-of-potential method needs the potential probe placed outside both electrodes' zones of influence.

A surge arrester must survive normal voltage and still protect. Its maximum continuous operating voltage (MCOV) must be at least the highest continuous line-to-ground voltage at its location. On an effectively grounded system, the short rise on unfaulted phases during a ground fault is checked separately, against the arrester's temporary overvoltage (TOV) capability for the time the fault lasts. On ungrounded or high-impedance grounded systems, where a ground fault can stay on, the MCOV must reach line-to-line voltage. Its protective level must sit far enough below the equipment's insulation withstand to leave a healthy protective margin.

Money has a time value. Comparing alternatives means bringing every cost to one point in time. A single future amount converts to present worth as

$$P = \frac{F}{(1 + i)^n}$$

and series of equal annual amounts use the matching uniform-series factors. The alternative with the lowest present worth of cost, or the highest net present worth, wins.

How to study it

1. Work demand, diversity and load factor from a small table of loads you invent, and use the results to size a feeder or a service transformer.
2. Check service-level power with the Three-phase power calculator, then size a capacitor bank to remove a power-factor penalty with the Power-factor correction calculator.
3. Lay out a rectangular room with the lumen method: compute the room cavity ratio, look up a coefficient of utilisation from a manufacturer-style table you build yourself, and count luminaires.
4. Study grounding electrodes and the fall-of-potential test, then work arrester selection: the MCOV for solidly grounded and for ungrounded systems, and the protective margin against a given insulation level.
5. Close with economics: present worth, annual worth and simple payback for a loss-reduction project, and check feeder losses or drop with the Voltage drop calculator.

Mistakes that cost points

• Swapping demand and diversity factor. One is at most one, the other at least one. If your answer lands on the wrong side of one, you divided the wrong way.
• Using the initial lumen rating with no light loss factor. The design should meet the target after depreciation, not on the first day.
• Choosing MCOV from line-to-ground voltage on an ungrounded system. A ground fault can stay on such a system, holding the healthy phases near line-to-line voltage, so the MCOV must cover that. On an effectively grounded system, check the fault-time rise against TOV capability instead.
• Mixing nominal and effective interest rates. Match the rate to the compounding period before using any factor.

References worth having

• IEEE Std 141 (Red Book): the load and power factor chapters
• IEEE Std 142 (Green Book): the grounding electrode chapter
• IEEE Std C62.22 (application of metal-oxide surge arresters)
• IES Lighting Handbook