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Switchgear Room Locations at Feed Mill Facilities

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What are the top considerations when deciding where to locate a feed mill switchgear room?
– A.H.

A.  Switchgear rooms within Feed Mills should be sited in locations which will minimize exposure to grain explosion hazards while maximizing personnel safety during operations and maintenance.

Summary

  1. Good facility design should include adequate separation of switchgear from production areas subject to dust atmospheres and accumulation.
  2. Switchgear rooms need to be conditioned areas to prevent degradation of equipment which can lead to improper operation of protective devices.
  3. Proper Lockout/Tagout should be performed at the source of the energy, following review of written procedures and evaluation of the sources of energy.
  4. Consolidating switchgear into fewer, larger switchgear rooms will save in overall costs, initially and during future operation of the facility.
  5. VFD to motor cable lengths are not the significant factor for siting switchgear rooms as they were even one decade ago.

Discussion

Normal switchgear operation produces sparks (arcs) of sufficient energy to ignite hazardous concentrations of grain dust in the atmosphere as well as grain dust which has settled on surfaces. As such, it is imperative that switchgear is not installed where the likelihood exists that it could cause an explosion or create a fire. Ideally a switchgear room would be housed in its own building or a building separate from the production area. Where this is not practical, switchgear rooms in the production facility should be located nearest the areas of lowest dust production.

While methods exist whereby individual pieces of switchgear can be isolated from environmental and dust hazards, these generally increase the original expense of the switchgear as well as subsequent maintenance costs, due to the additional heating, cooling and periodic testing and cleaning requirements. Switchgear is rated for operation in a rather narrow temperature range. Operation in temperatures outside of that range can result in contactors operating slower than optimal, breakers not clearing faults as intended, and changes in the timing of overload operation. In addition, excess heat can degrade components, while excess cold can cause condensation resulting in shorts within the equipment. All of this makes the maintenance of the HVAC, Air Filtering, and Pressurization systems an integral part of any switchgear room design. This maintenance is more likely to be kept current when larger and fewer systems are involved.

Operation of a feed mill, like nearly all other facilities requires attention to safety. Prior to working on any piece of equipment all persons involved should be able to gather in one location, without distractions, to discuss which sources of energy need to be placed into a safe condition. In the case of electrical energy, the Lockout/Tagout should take place at its source in the Switchgear Room. Such safety is often compromised by the location of multiple switchgear rooms throughout a facility. Being able to find the breaker or motor starter needed for Lockout/Tagout is made simpler with fewer locations to search. Lockout/Tagout when several pieces of equipment need to be placed into a safe mode is also better performed when all are in the same switchgear room. Travelling from room to room to either find a breaker/starter, or to lockout several breakers/starters creates the potential for something to be overlooked.

Each switchgear room comes with its own costs. Construction of the room, conduit penetrations from within the room, an HVAC system for the room, filtration of air entering the room, pressurization of the room, entrances and exits to/from the room, and lighting within the room are all requirements for each switchgear room constructed. By splitti ng the electrical equipment into several smaller rooms, the economy of scale is lost.

One concern often expressed by owners is the cost and potential effects of longer conductor runs for primarily motors, but other loads as well. These considerations are only significant during the construction of the facility. The conductors and conduit systems require no regular maintenance and will rarely be modified and almost never replaced once the facility is in operation. The addition of more loads in the future are generally few and can often be anticipated during initial construction. Original construction conduit banks within the facility can be easily added to and underground conduit runs are always installed with spare capacity. The costs for longer and smaller original construction conduit and cable runs, incurred as a result of fewer switchgear rooms, are largely offset by the larger conduit and conductor sizes necessary for the feeders to supply the additional rooms.

The effects of longer conductor runs alluded to in the previous paragraph pertain to motors powered by VFDs. These concerns involve motor damage due to peak voltages damaging the windings. While these concerns were very valid in years past, they have diminished somewhat over the last decade. Several improvements made to both motors and VFDs have lessened the likelihood of such damage. Motors powered by VFDs are now routinely specified for “Inverter Duty”, providing a higher level of winding insulation than general duty motors. Advances in VFD design have also taken aim at minimizing the side effects of the pulse width modulation upon which they are based. In cases of extra long conductor runs (greater than 1000 feet), two fairly inexpensive methods of mitigating the effects of distance from VFD to motor are easy to implement. Distances from VFDs to motors are significantly less of a concern now than they were one or two decades ago.

Conclusion

While many factors will come into play when determining Switchgear Room siting for a given facility, safety during the operation of the facility should be the number one priority. Three key factors which will affect this are potential for dust contamination, ease and accuracy of Lockout-Tagout and maintaining the safe operating condition of the switchgear.

Second is overall cost for the life of the facility. Savings expected by shortening individual branch conduit and cable runs is greatly offset by the larger conduit and cable for feeders to additional switchgear rooms. When the cost of building and maintaining the additional switchgear rooms is added in, the overall cost is generally greater.

Other factors, such as future equipment additions and VFD effects on maintenance can be addressed easily and for minimal cost during construction.

Answered by WL Port-Land Systems Inc.