Warehouse Electrical Systems
First, I am going to take a quick look at the trouble we make for ourselves, and I will close with some maintenance suggestions. The facility may be relatively new or a number of years old. The problem comes from new and greater demands for refrigeration capacity, changing technology, or replacement equipment installed in the plant. Whatever, the effect is the same: more electrical energy is needed than was originally planned or designed for.
For example, electrical service overload can occur in one quick step with the installation of a 'planned for' 200 horsepower compressor after the two original 100 horsepower compressors have been upgraded to 150 horsepower, making the planned 400 horsepower compressor room actually 500.
Electrical system load can increase without really being noticed when a storage and packing facility is upgraded over several years without a plan. Typically, new packing equipment replaces the old in a piecemeal process that adds a few more electric motors. At some point the lighting is improved, adding a few kilowatts to the total load. Then the controlled atmosphere equipment is replaced causing a slight increase in the total electrical load. Pre-cooling capacity is increased, using more efficient equipment but requiring a bit more refrigeration capacity. Then the office space and lunch room are brought up to date. Suddenly you have unexplained circuit breaker trips and blown fuses.
Then there is the truly sneaky overload that occurs when "nothing has changed." This is frequently caused by maintenance personnel whose only goal is to keep the plant running. A thirty horsepower motor fails and is replaced with a forty because it was available. Hundred watt lamps get replaced with hundred and fifty watt lamps. A failed 20 kW space heater is replaced with a 30 kW unit. In most cases, the intention is to put the original size back as soon as possible, or it is looked upon as a plant improvement. For whatever reason, the electrical load has been increased just a little bit. Add up several replacements like the ones I have listed, and you have branch circuit overloading as well as sub-feeder and maybe service overloading.
Another source of unexplained, total load increases can be changes in operating procedures. Probably the most frustrating overload is caused by variable speed motor drives, AC or DC, and electronic "soft starts." The electronic drives that operate these motors can cause harmonic currents in the electrical distribution system that can reduce the available capacity of service transformers and conductors, and cause hard-to-explain high currents in neutral and grounding conductors. Often harmonic currents cannot be read with a clamp-on ammeter. The effects of the harmonics currents produced by high current electronic switching devices are not limited to circuit overloading. They can degrade the function of other motors and electronic devices.
First, what is an electrical overload? Simply put, an electrical overload means the circuit in question, be it the main service to the facility or the last branch circuit from a sub-feed breaker panel, is continuously loaded above 80% of the breaker rating. Or a given conductor is continuously loaded above the National Electrical Code recommendation for the conductor size and load connected to it. For example, a 100-amp circuit breaker should not carry more than 80 amps continuously unless it is specifically rated for 100% service. Peak loads, such as motor starts, are not fully considered in the 80% figure. Another form of overloading occurs when circuit breakers are up-sized without increasing the wire size going to the load in question. In the case of electric motors that are replaced with larger ones, the motor starter and circuit breaker may not be overloaded, however, the wire from the motor starter to the motor may no longer meet the 125% conductor size requirement for the full load motor current rating.
Second, what is a harmonic? A harmonic is a multiple of the base frequency in use. In music harmonics give the sound depth and body. Harmonics in electrical systems are currents or voltages that appear at frequencies that are multiples of the 60 cycles per second we use for our electrical power or the switching rate which often exceeds 500 cycles per second. Variable speed drives turn the power to a motor off and on at high switching rates. This high switching rate can cause current to flow at odd multiples of 60 cycles per second, such as 180, 300, or 420 cycles per second or higher. Harmonics can affect the controlled motor, other motors in the warehouse, distant electronic equipment on the same service, power transformers, and even a neighboring warehouse.
The most obvious sign of an electrical system or circuit overload includes circuit breakers that trip off frequently, fuses that have to be replaced often, and overheated conduit or switches. Less obvious and more dangerous are things like main disconnects connected ahead of the original main disconnect or sub-feed circuit breakers in a distribution panel that have been up-sized without increasing the wire size to the related sub-panel. The same applies to branch circuits where someone has increased a circuit breaker or fuse rating without increasing the connected wire size to match. Motor circuit overloading can be masked by overload heaters that have been wired around or otherwise defeated. Fuse protected circuits are especially easy to overload...just replace the fuses with bigger ones.
First and simplest: Keep a record of the total power consumption. Keep the circuit cards in your breaker panels up to date and a duplicate record in the maintenance files. Keep a log of circuit breaker trips and apparent causes. Review these records at least once a year. By following your year to year energy consumption and circuit breaker trips you can catch load trends before they become troublesome or cause down time.
Second: When any change or plant upgrade is planned do an electrical power consumption and load survey. If you have been keeping good records this is easy. If you have no records, it involves a lot of time and effort to gather the information and create the record after the fact.
Third: Keep records of what size motors were installed where, and the overload setting of the related motor starter when the equipment was new. Once a year check the machines to see that the correct motors are installed and that the motor starter overload settings are correct.
Fourth: Inspect or hire a competent electrician to inspect all electrical service, distribution panels, motor starters, junction boxes, and conduit runs once a year. You should do this during a low load period when you can turn the equipment off to allow hands-on inspection and connector tightening where prudent and indicated. The major clues to problems are open junction boxes and switches, loose conduit, discolored connectors and insulation, cracked insulation, and taped joints that feel hard.
Fifth: Do an infrared scan of the complete electrical system every three to four years. This should be done in two steps when the load is at the' maximum. The first step is done with the covers in place and only those doors open that are normally opened for day to day operation. The goal is to find circuit breaker boxes, breaker handles, junction boxes and electrical controllers that are running hot. The second step requires the removal of covers so the conductors and components can be scanned. Here you are looking for individual connectors and components that are producing excessive heat.
Infrared scanning can be done in any one of three ways. The first is to hire a firm that specializes in infrared scanning and provides a detailed report of its findings, including pictures of hot spots. The second method involves purchasing the needed infrared scanning equipment and doing the survey in house. Equipment is available that provides anything from an on-scanner display and computer compatible output to spot-type infrared sensors that give the actual temperature of a small target area. A third method involves taking infrared photographs of the equipment. Infrared photography requires specialized film and cameras, calibration of the photo site, and in some cases removal of ambient light. My personal preference is to hire the periodic scanning done and use spot infrared thermometers as trouble prevention and trouble shooting tools.
If you have a new facility or have had extensive remodeling done, an infrared scan should be done during the first harvest and storage season. This will catch any loose connections or faulty equipment before they become a problem.
In summation, the electrical reliability of your warehouse and packing facility is a matter of good design, personnel awareness, regular inspections, and the repair of little problems before they become big trouble.
Grays Electric, Inc.
13th Annual Postharvest Conference