Power, grounding, and power distribution.
The best reference for power, grounding, and power distribution is FM 5-424, Theater of Operations Electrical Systems, 25 June 1997.
Power, grounding, and power distribution are extremely important to signal planning. Key components to power planning are the selection of a power source, set-up of power generators (use of phases, wiring, loading, physical location, sun shade, etc.), grounding of power sources and electrical components, and the distribution of power. Reliable commercial power is a good choice when the force is at the long halt. Using commercial power allows the force to prepare tactical power for the next mission.
Chapter 10 of FM 5-424 addresses setup, installation and operation of generators. Power generators must be in good working order, serviced on time, placed on level ground, and grounded properly. Services are determined by hours of operation. Signaleers track the hours of operation of each generator, and perform preventative maintenance checks and services before during and after operations. Air and oil filters are replaced based on hours of operation and the physical environment. Generator services must be accomplished routinely. One generator is never enough. Most must be off line to change filters.
Generators must be kept as cool as possible. Sun loading (sun shining directly on a generator) can cause overheating. Shading a generator with sun shades or camouflage can keep its operating temperature within the normal operating range even if the ambient temperature is high.
Chapter 8 of FM 5-424 addresses power load calculations and generator selection. The first step in a power plan is calculating the electrical load requirements. Power (symbol P) is defined by units called Watts (W). Current (symbol I) is defined by units called Amperes (amps or A). Voltage or the potential difference (symbol V or E) is defined by units called volts (v).
Power = (Current) x (voltage) or P = IE or P = IV.
Electronic components have data plates. The data plates provide the power requirements in voltage and current. Determine the power requirements from voltage and current. Soldiers can also refer to system training manuals for power load requirements. Engineers rate each component separately with added power margin. When components are aggregated the margin above the minimum essential power load requirement adds up and may cause your electrical load requirement calculations to exceed good generator loading practice. Conversely, equipment demand factors, power factors, and capacity for growth may have to be accounted for as discussed in the FM. The best way to determine the full requirement for combined electrical components is to use the data plate information as your guide and then measure the load on the power source.
The next step is to separate signal equipment from air conditioners, refrigerators, microwaves, heaters, coffee pots, etc. These electronic components create power fluctuations. For example: air conditioners have compressors and for most models the compressor only engages periodically, each time the compressor engages the power draw increases, creating fluctuation in the current available. Current fluctuations cause electronic devices to react abnormally, and failure is more likely to occur.
To select the right power source you must understand how generators work and set the phases up properly. The generator power is divided by the number of phases. For example a single phase 10kW generator provides
P = IV
P = power in Watts; I = current in Amps; V = voltage in volts
10kW = (120v)(I)
I = 10kW/120v ~ 83amps.
A three phase 15kW generator provides 5kW on each phase or 5kW/120v ~ 41amps. Therefore, if a 50 amp circuit is required a single phase 10kW generator is better than a 15kW generator operating three phases. Bigger is not always better.
Generators must be set up properly and matched to the task. Generators normally operate with single or three phases, and should be loaded to 80 percent of capacity when possible; an under-loaded generator can be as problematic as one that is overloaded. Loads should be distributed evenly to all phases of the generator. Wiring data plates are provided on tactical generators, and in the technical manuals. To better understand single phase, two and three-wire configurations, and three phase, three and four wire configurations, reference FM 5-424, Theater of Operations Electrical Systems, June 25, 1997. This FM is a must have in power planning. Know what is plugged in to your generator. User creep can cause generator loads to become unbalanced.
Grounding is a force protection and leadership issue. PS Magazine, May 2007 has a great article on grounding. Another good document is Communications Electronics Command TR 98-6 Earth Grounding and Bonding Pamphlet, available at http://www.monmouth.army.mil/ cecom/safety/system/ spublication.htm. Power generators require proper grounding to attain the potential difference needed to provide power. Power sources are especially susceptible to faults when electrical grounds are not up to standards. Generators typically have nine foot copper ground rods with copper grounding cables. Use them, and check them (less than 25 ohms of resistance) after installation. Less resistance is better. Use ground resistance meters every time to ensure a proper ground before operation. A ground that looks good is not always good enough. Pour rock salt and water on, and around ground rods. Add ground rods that are spaced apart and link them together with copper cable until you meet the resistance requirement.
Generators that are on a trailer or vehicle must be grounded to the platform. Dual mounted generators must be equally grounded to the platform creating a common ground, and the platform must be grounded to the earth. If the generator set has a switch box, the box must also be grounded to the common platform ground.
Dismounted equipment also requires grounding. Most transient cases have ground terminals and must be grounded. Fixed station racks also require grounding; typically the rack is the ground point and must be connected with continuity to the ground.
Always make sure that equipment grounding conductors are properly connected. This is a critical part of grounding which helps prevent hazardous voltages from developing on equipment enclosures and helps to trip circuit breakers quickly in the event of a fault. Never use equipment that has a ground pin missing or the equipment grounding conductor (green colored or bare) disconnected. Make sure equipment grounding conductors are properly connected at the generator.
Paralleling generators and using switch boxes is science not art. Generators must be cabled properly and the switch box must be rated to perform the job. Even in the best situation a power spike or failure may occur. Do not enable power failure; get the set up right.
Circuit breakers need to be rated for the circuit requirements. For example a 20 amp circuit breaker on a 120 volt circuit provides P = (120)(20) Watts = 2400 W or 2.4kW. If the load exceeds the rating of the circuit then the circuit breaker will trip and disconnect the power source creating a communications outage.
Communications shelters normally have gauges at the terminal. Adjust power at the source while monitoring the gauge at the terminal to provide the right power. Commercial equipment or dismounted equipment does not come with this capability. Know what power is available at plug-in points, under load. Most equipment comes with a power cord, and the cord is rated to provide the needed electricity to the electronic components from the origination source.
If you are extending the length of the provided power cable using extension cords or other cable, you must measure power available at the source of the last plug, or calculate the power loss for the cable, and ensure the original source can provide what is needed over the distance of the extension. For this you need to know the voltage drop or line loss. To calculate the voltage drop you need to know the gauge of the cable or the loss rating normally provided in percentage of input voltage.
Multi-meters are essential for communicators. Clamp around resistance meters are great for testing grounds, and clamp on ammeters are very useful for testing power under load. A non-contact voltage detector allows quick and easy determinations if circuits, terminals, etc., are live. Know how to use test equipment, and ensure you have spare batteries to power them. Another useful tool is a plug-in polarity checker. You can find these at most electrical shops, and they are inexpensive. Most polarity checkers have LED readouts that provide you with information about the source of power, phase, and ground.
Uninterruptible Power Supplies at the end of an extension cord cannot recharge if you don't provide enough power. If the UPS is getting just enough power to provide power to the components it services then when back-up power is needed the UPS will fail. You must measure the power into the UPS, under load, and ensure it has the capacity to provide failsafe power to the components it is in line to protect. In the wrong configuration the UPS may inhibit the components from working properly by taking needed power from the source. Even a fully charged UPS may steal power from a circuit over time.
Not only do generators and power distribution systems have to be reliable, but they must also be installed, operated and maintained IAW all safety requirements. This is critical not only for the safety of the maintainer, but the user as well. Just because the equipment "works" isn't proof that the system is safe or that it will shut down safely in the event of a fault.
Extreme caution must be taken when working around live power circuits. Maintainers must be qualified, which means they need to know how to identify hazards.
A--amps or amperes
CECOM--Communications Electronics Command
CELMC--Communications Electronic Lifecycle Management Command
E or V--voltage
IAW--in accordance with
LED--light emitting diode
PMCS--preventative maintenance checks and services
RFO--reason for outage
UPS--Uninterruptible Power Supplies
V or E--voltage
By COL John K. Dewey and John M. Tobias, PhD, PE
COL Dewey is the Training and Doctrine Command Capability Manager for Tactical Radios at Fort Gordon, Ga. He can be reached by email at email@example.com or phone at 706-791-7982 DSN 780-7982.
Editor's Note: Contributing to this article is Mr. Tobias, PhD, PE with U.S. Army CELCMC Directorate for Safety, Fort Monmouth, N.J.
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|Title Annotation:||TCM-TR; tactical power generation|
|Author:||Dewey, John K.; Tobias, John M.|
|Date:||Sep 22, 2007|
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