由于现代数控系统的可靠性越来越高，数控系统本身的故障越来越低，而大部分故障的发生则是非系统本身原因引起的。系统外部的故障主要指由于检测开关、液压元件、气动元件、电气执行元件、机械装置等出现问题而引起的。数控设备的外部故障可以分为软故障和外部硬件损坏引起的硬故障。软故障是指由于操作、调整处理不当引起的，这类故障多发生在设备使用前期或设备使用人员调整时期。对于数控系统来说，另一个易出故障的地方为伺服单元。由于各轴的运动是靠伺服单元控制伺服电机带动滚珠丝杠来实现的。用旋转编码器作速度反馈，用光栅尺作位置反馈。一般易出故障的地方为旋转编码器与伺服单元的驱动模块。也有个别的是由于电源原因而引起的系统混乱。特别是对那些带计算机硬盘保存数据的系统。例如，德国西门子系统840C。SquareDSafetySwitchCatalog#D222N(60amp)(240V)SQUAREDS37430S37430SQUAREDS30021-505-51TRANSFORMER***XLNT***SquareDS30021-503-51Transformer50VAPrimary220/48050/60HzSQUAREDS2936-S1-C27A120VCOIL**SquareDS1C26BCoil100-115VSQUAREDS1861S1***BMagnetStarterCoil208/220VS1861S1***Bid6838/3-4SquareDS.S.ChainAccessKitsPart#9049-DCA2SquareDRSD42P14V53Relay10Amp240VSquareDRSD42M1P14V5324VDC10A2PDTRelay&BaseSquareDRSD14M1V53Relayw/RS4V20ThreeNR45RelaysocketsSQUAREDRS-9991CIRCUITBREAKER20A120/240VAC***XLNT***SQUAREDRS44V20RELAY120V50/60HZWITH851TYPENR34BASE(LOTOF2)**XNLT**SquareDRS43MIPI4V20-Class8501,SeriesA,120VSquareDRotaryKeySwitch9001D2L1022mmSizeINBOXSQUAREDRFK10RFK10SQUAREDRFK03LCLASSRFUSEADAPTERKITFOR30A240VACSWITCH(BOXOF20)*NIB*SQUAREDRFK03HRFK03HSQUAREDRFK03HCLASSRFUSEADAPTERKITFOR30A240VACSWITCH(BOXOF20)*NIB*SquareDReplacementPilotLightForClass9012SwitchSQUAREDREPLACEMENTCONTACTSPARTSKIT9998QA-81SQUAREDREPLACEMENTCONTACTS9998RA-81*NIB*SquareDReplacementContactKit9998PC242Lotof2SurplusSQUAREDRelay,OctalBase2DT24SQUAREDRELAYW/CONTACTORLC1D18/LRD1521105591SquareDRelayw/Base,#9050JCK-21V14,SeriesA,,WARRANTYSquareDRelayUnitB3.702UnitsInBoxNOSSquareDRelayUnitB3.302UnitsInBoxNOSSquareDRelayUnitB3.002UnitsInBoxNOSSquareDRelayUnitB2.402UnitsNOSSquareDRelayUnitB1.3SquareDRelayTypeX8501X040SeriesASquareDRelaySockets8501NR51SeriesBwithRelay8501KP12V20DLOTOF2SquareDRelaySocket850NR51withAlliedControlRelayT***D0-2C(Lotof2)SQUAREDRELAYSERFCAT#8501KPD12V5312A24VCOIL*NIB*SquareDRelayMagnetCoil2959S1W33ASquareDRelayLass8501TypeKF-12852201*FREESHIPPING*SquareDRelayJumperStrapKit#SO-31MeltingAlloySQUAREDRELAYCONTACT8501KPD13V518501KPD13V51SQUAREDRelayCoil110/120V3102140060SQUAREDRELAYCLASS8501TYPERS14FORMM1P14SERB120V50/60HZ0214SQUAREDRELAYAND2REEDDECKS8501X0802011ASquareDRelay8910DPA3330A460V120VCoil**Lotof2**SquareDRelay8501RSD42V53PluginGPRelayDPDT10A24V(lotof2)SQUAREDRELAY8501-KUD12M1P14V53SER.D8501KUD12M1P14V53SQUAREDRELAY8501KUD13P14V538501-KUD13P14V53SER.C24V3PDT10ASquareDRelay8501KP-13120VCoilLotof2SurplusSQUAREDRELAY8501KP12V20SER.E8501KP12V20SquareDRelay8501KLD12V51SerBDPDT12VDCwithCustomConneectorQC11-PCSQUAREDRELAY120V8501-CO16V20SER.C**SquareDRed/WhitePushbuttonWithTwo9001KA3ContactBlocksSquareDRed/WhitePushbuttonWith9001-KA1ContactSeriesHSquareDRedPushbuttonW/Contact9001-KA-2SerGSquareDRedPushbuttonw/9001-KA3ContactBlockSquareDRedPilotLightKit9999SP2RforTypeS,Size00-1SurplusSQUAREDREDPILOTLIGHTKIT9999SP-2R*SURPLUSINBOX*SQUAREDREDPILOTLIGHTKIT9999SP-12R*SURPLUSINBOX*SquareDRedPilotLamp120VacP/N:9001-0R120IncandescentSQUAREDREDMUSHROOMHEADPUSHBUTTON9001KH1SQUAREDREDLIGHTMODULELOTOF49001KM1SERHSquareDRedIlluminatedMaintainMushroomPushbuttonActuatorwithKA3blockSQUAREDREDILLUMINATED9001SERIESPUSHBUTTON9001MB-1SUPERFASTSHIPPING)