THE GREEN CASINO: Trench warfare
November 10, 2011
This is my second article dealing with casino power system design issues. In the first article, I described how a casino’s below floor (sealed metal trench-based) AC distribution system is designed to deliver AC power to 2,000 or more slot machines through many hundreds of cables. Additionally, I introduced the reader to some of the design challenges that power engineers face in designing a casino power distribution system. And, finally, I discussed a real-world AC feeder cable overheating problem as well as a cost effective solution to address this problem.
So, all the folks reading that first article were probably left with one question on their collective minds: How come many sophisticated casino installations were experiencing all these cable and cable termination failures? As it turns out, the answer to this question is not as simple as I’d like it to be-so bear with me as I detail some of the suspect reasons for this problem.
• Cable Resistance Losses (expressed as P = I2R with P = Power loss in watts; I = Current in amps; R = Resistance in ohms)
This power loss results in heating along the entire length of each AC feeder cable in the trench. For instance, a casino installation using #12 feeder wires to carry up to 16 amps of total current has a cable loss of (about) .4 watts/ft. Now, you wouldn’t think that this is very much in the way of heat loss. However, a typical 3-foot wide trench can be loaded with up to 400 #12 feeder wires, and the aggregate resistive cable losses can easily exceed 100 watts/ft. In turn, this amount of heat loss can result in (trench) ambient temperatures reaching upwards of 150+ degrees Fahrenheit.
Thermally speaking, since the trenches are fully enclosed, you can’t count on free flowing (convection) air cooling to restrain cable temperatures. With the exception of some small amount of conduction cooling-resulting from perhaps 10 percent of the cabling coming in direct contact with the trench walls-the majority cooling effect is by way of relatively inefficient radiation cooling. Thus, cables and cable terminations fail-always more frequently than expected! And, trouble shooting, removing, replacing, and retesting a failed cable is an expensive and time consuming proposition.
•Power Supply Selection
Each slot machine is equipped with a computer power supply that transforms the incoming 120 VAC line voltage and current into 5 and 12 volts DC power. This low voltage DC power runs the slot machine’s microprocessor based control/communications circuitry, audio circuitry, a bill acceptor and visually appealing LCD/LED displays.
The voltage/current transforming process has two major efficiency penalties associated with it. The first penalty always appears in the power supply specifications as the conversion efficiency. And, depending on input AC voltage and DC power loading of the supply, a typical computer power supply is rated at 75-80 percent efficient. Also, in addition to the power supply conversion efficiency, there is a stiff penalty related to something called the input Power Factor (PF) rating of the supply.
Unfortunately, this specification is not widely published by computer power supply vendors. I suspect that the reason for this oversight has to do with the fact that most of losses associated with PF don’t appear internal to the power supply. But rather, a low PF value induces additional (harmonic current induced) resistive losses in the AC feeder cables leading to the power supply. For a typical computer power supply-one without something called PF Correction (PFC) technology-the PF rating is .70 to .75. And, out of the 16 amps total feeder wire current, about 40 percent of the current is due to the harmonic current effects of power supplies having a PF much lower than an optimal rating of 1.0. (For those intrepid soles wanting more information about the Power Factor ratings, do a browser search on the following text string: power supply harmonic current and Power Factor ratings.)
Practically speaking, this means that instead of 100 watts/ft of waste heat in the cable trenches, there would only be 60 watts/ft waste heat. And, statistically, there would be fewer cable and cable termination failures to deal with throughout the year. Even better, from the prospective of the casino’s recurring operating costs, replacing existing slot machine power supplies for ones that incorporate PFC technology But there’s an art to player tracking, too, especially on table games where the calculation of theoretical win is more dependent on pit observations that in the slots, where systems can track every wager to the penny. A human touch is necessary.
When the numbers and the art mesh, well, that can be special.
“There’s a magic moment when someone is at the tables, is approached and is invited for a comp,” David Schugar, principal partner of RMC Gaming Management said during a session on player tracking at last month’s Global Gaming Expo in Las Vegas. “If it’s timed right, it can be a magic moment.
“There’s also a magic moment when a player asks for something, and you don’t have to ask them for their card or go to the computer. I mean come on, how personal is that? The ability of the supervisor to whip out that comp book and just write them a manual comp, we have drifted so far away from that.”
The need for that personal touch was a recurring theme at the seminar moderated by Luigi Mastropietro, vice president, relationship marketing for Global Cash Access, Inc., on a panel that also included David Patent, chief operating officer for Rush Street Gaming, LLC, and Frank Neborsky, vice president of slot operations at Mohegan Sun.could result in substantial direct power cost savings. For instance, depending on the mix of slot machine types and their existing power supply loading, a typical casino gaming floor with 2,500 slot machines could have a direct cost savings of $25,000 to $75,000 per year (using power at $.10/Kwatt-Hr).
And, this savings doesn’t include any indirect power cost savings due to a reduced casino air conditioning load. After all, some of the waste heat in the cable trenches does make its way through the carpeting into the casino environment.
• The Design Process
The use of cabling in trench based distribution systems is governed by a set of standards and charts published as part of the National Electrical Code (NEC). The design of the casino trench system and the current rating of trench feeder wiring are usually determined by an experienced consultant/electrical engineer. To do a proper job, he must be thoroughly familiar with the NEC 374 specifications relating to the use of sealed trench systems as well as the information included in the many pages of specifications and charts covering wire current ratings and wiring protection requirements in NEC Article 310.
Unfortunately, almost all the applicable charts are based on the knowledge of the operating ambient temperature in the sealed trench system itself. Since the distribution system design has many variables-number of wires, wire gauge, length of feeder wires, type and rating of load, type and installation details of the trenches, concrete and carpeted cover characteristics, etc.-it is very difficult to come up with a high accuracy estimate of the operating ambient temperature. The bottom line is that these system designs incorporate some level of proprietary art mixed in with engineering design methodology. And, so, if the assumption of the operating ambient temperature is too low, cables and cable terminations will fail-always more frequently than expected!
Billions of dollars are spent designing and constructing major casino installations all across the country and a large number of these installations make use of AC distribution cabling sealed trench technology. Hopefully, the items I have described in my last two columns will help casino engineers win the battle against costly slot cable failure.