TECHNOLOGY RECOMMENDED GROUNDING PRACTICES …

1 148 April 2001 Elevator WorldTECHNOLOGYIf you need some excitement in your life, put five qualifiedpeople in an elevator machine room, and ask for theirrecommendations on GROUNDING . You will most likely get fiveor more methods on how to ground the equipment in thatmachine room. Some may even meet the National ElectricalCode (NEC), some may not! Ordinary people can becomequite animated when challenged on GROUNDING methods!It is also important to note that there are two primarymeans of GROUNDING electrical equipment. First is to meetNEC requirements. The second is performance. This arti-cle will discuss GROUNDING methods to maximize elevatordrive Institute of Electrical Engineers and Electricians(IEEE) used to have two color books on powering andgrounding methods.

2 One was the Green Book, IEEE Standard142, RECOMMENDED Practice for GROUNDING of Industrial andCommercial Power Systemsand the other was the EmeraldBook, Standard 1100, Powering and GROUNDING SensitiveElectronic Equipment. Recently, IEEE recognized that inreality, virtually all electrical devices are sensitive and hascombined these two guidelines into Standard is of particular concern when you consider howmany personal computers are in today s modern officebuildings, not to mention how many other building systemsuse microprocessors to control their systems. Additionally,modernized elevator systems put new and significantlydifferent demands on existing electrical systems that mayno longer provide the best performance with new solid-state are some obvious areas where most people canagree.

3 Safety considerations and the NEC require that themetal chassis and cabinet components of all exposedelectrical apparatus be connected to earth ground forprotection of personnel. This, of course, is only commonsense. However, the manner in which GROUNDING wiresare connected and installed can make a difference in theemissions of and sensitivity to electrical noise. For elevator installations with DC drives it is commonthat there is a primary power isolation transformer nearthe drive:a. To provide the correct VAC drive input to match theVDC requirements the secondary side is usually un-grounded, butb. The secondary may be grounded or have a high Zground with ground fault The DSD drive control unit is mounted on a sub-panel,within an electrical The elevator hoist motor is located nearby and sits onbuilding The velocity encoder is mounted on the motor The elevator car controller may or may not be in a sep-arate there is always some capacitive coupling fromelectrical circuits to ground, some Silicon-Controlled Rectifier(SCR) switching noise will want to flow between the drivechassis and the motor, and between the drive chassis andthe transformer frame.

4 The best method to minimize inter-ferenceis to give noise current a direct low resistancepath in which to flow, and a higher resistance path towhere it is unwanted. We recommend that: A GROUNDING bus be provided in the drive enclosurecabinet, welded to the metal frame. A GROUNDING wire be provided directly from the drivechassis to that bus. A GROUNDING wire be provided directly from the drivesub-panel to that bus. A GROUNDING wire be provided directly from the motorframe to that bus. A GROUNDING wire be provided from the transformerframe to that bus. A GROUNDING wire be provided from approved earthground to that bus.

5 A GROUNDING wire be provided from the car controllerenclosure to that bus. Size these GROUNDING wires equal to the power note on using building steel. It is critical to ensurethat the building steel is bonded and appropriate to beused for GROUNDING . Many buildings do not have suitablebuilding steel due to various foundation constructionmethods. A review with a knowledgeable structural engineeris required to evaluate the status of the building a motor armature ripple filter is used, a groundingwire should be provided from the reactor core to the driveenclosure GROUNDING bus. The panel-holding ripple filtercapacitors and the metal enclosure should be ground-wire bonded to the core of the building electrical distribution system ground-bonding wire for primary power should be connected tothe transformer frame- GROUNDING point, and then bondedto the walls of transformer enclosure.

6 All electrical powerRECOMMENDED GROUNDING PRACTICES FOR MODERN ELEVATOR DRIVESby Patrick J. Welch, President, Vertical Transportation ExcellenceApril 2001 Elevator World149 TECHNOLOGY wiring should be encased in metal conduit rather thanopen electrical raceway wire pair (three for three-phase) should be in itsown conduit. Ground-bonding wires must be sized tomeet or exceed NEC fault-current requirements of theequipment size. Gannett Fleming recommends using largerthan the NEC minimum ground wire sizes between thedrive chassis, motor, transformer and building steel. Sizingthem equal to that of the power conductors will provideimproved performance over NEC Fleming also recommends that the tachometer/encoder housing and shaft be insulated from the motorframe and shaft.

7 Use shielded cable for encoder wiringisolated from the analog signal exchange between the drive and carcontroller, it is preferred that differential signaling beused, with the car controller circuit common alsogrounded appropriately. Second best is to ground the carcontroller single-ended analog circuits via a commonwire tie from car controller to drive. Logic signal circuits24V have the negative side grounded at the drive via theconnection above. If 24V signaling is used, rather thanrelay contacts, be sure to provide a tie wire between drivecontroller and car controller circuit commons. If serialcommunications are used, it is preferred that it be RS422with optically isolated receivers.

8 If RS232 is used, circuitcommons of both the drive and car controller must begrounded as indicated key concept to understand is isolated ground. This doesnot mean a separate ground, it means only that the conduit isisolated from the wires. Conduit is not RECOMMENDED forgrounding to earth since can be changed outside the elevatormachine room on its way to a proper earth considerations for VVVF drives for AC motorcontrol are similar, except using input reactors instead ofa transformer. With AC drives, use four wire-shieldedpower cables from drive to motor, with the fourth wireproviding the controller/motor frame bonding. EMI-filteringcomponents may be required on input and output panel layout should carefully consider the locationand wiring of EMI reduction of the important points of this issue is lightningprotection.

9 If all of the elevator equipment is properlywired to a single point earth ground, the voltage of alightning strike can rise equally over all the GROUNDING resulting from improper groundingcan cause fatal the end, this discussion of GROUNDING methods is intendedto open a dialogue and to get installers and elevator manu-facturersto think beyond the main line disconnect. LIGHTNING GROUNDBUILDING GROUND3 PHASEM/LMOTORBEDPLATEBLDG. STEELDRIVECONTROLLERTWISTED PAIR SERIAL LINKGROUND TO CONDUITWATER PIPELIGHTNING GROUNDBUILDINGGROUND3 PHASEM/LMOTORBEDPLATEBLDG. STEELDRIVECONTROLLERTWISTED PAIR SERIAL LINKGROUND TO CONDUITWATER PIPEGROUND BUS BAR