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。
Square D Safety Switch Catalog #D222N (60 amp)(240V)
SQUARE D S37430 S37430
SQUARE D S30021-505-51 TRANSFORMER ***XLNT***
Square D S30021-503-51 Transformer 50VA Primary 220/480 50/60Hz
SQUARE D S2936-S1-C27A 120V COIL * *
Square D S1C26B Coil 100-115V
SQUARE D S1861 S1***B Magnet Starter Coil 208/220V S1861S1***B id6838/3-4
Square D S.S. Chain Access Kits Part # 9049-DCA2
Square D RSD42P14V53 Relay 10Amp 240V
Square D RSD42M1P14V53 24VDC 10A 2PDT Relay & Base
Square D RSD14M1V53 Relay w/ RS4V20 Three NR45 Relay sockets
SQUARE D RS-9991 CIRCUIT BREAKER 20A 120/240VAC ***XLNT***
SQUARE D RS44V20 RELAY 120V 50/60HZ WITH 851 TYPE NR34 BASE (LOT OF 2) **XNLT**
Square D RS43MIPI4V20 - Class 8501, Series A, 120V
Square D Rotary Key Switch 9001 D2L10 22mm Size IN BOX
SQUARE D RFK10 RFK10
SQUARE D RFK03L CLASS R FUSE ADAPTER KIT FOR 30A 240VAC SWITCH (BOX OF 20) *NIB*
SQUARE D RFK03H RFK03H
SQUARE D RFK03H CLASS R FUSE ADAPTER KIT FOR 30A 240VAC SWITCH (BOX OF 20) *NIB*
Square D Replacement Pilot Light For Class 9012 Switch
SQUARE D REPLACEMENT CONTACTS PARTS KIT 9998 QA-81
SQUARE D REPLACEMENT CONTACTS 9998 RA-81 *NIB*
Square D Replacement Contact Kit 9998PC242 Lot of 2 Surplus
SQUARE D Relay, Octal Base 2DT24
SQUARE D RELAY W/ CONTACTOR LC1 D18 / LRD1521 105591
Square D Relay w/ Base, # 9050 JCK-21V14, Series A, , WARRANTY
Square D Relay Unit B3.70 2 Units In Box NOS
Square D Relay Unit B3.30 2 Units In Box NOS
Square D Relay Unit B3.00 2 Units In Box NOS
Square D Relay Unit B2.40 2 Units NOS
Square D Relay Unit B1.3
Square D Relay Type X 8501 X040 Series A
Square D Relay Sockets 8501NR51 Series B with Relay 8501 KP12V20 D LOT OF 2
Square D Relay Socket 850NR51 with Allied Control Relay T***D0-2C (Lot of 2)
SQUARE D RELAY SER F CAT#8501KPD12V53 12A 24VCOIL *NIB*
Square D Relay Magnet Coil 2959S1W33A
Square D Relay Lass 8501 Type KF-12 852201 *FREE SHIPPING*
Square D Relay Jumper Strap Kit #SO-31 Melting Alloy
SQUARE D RELAY CONTACT 8501KPD13V51 8501 KPD13V51
SQUARE D Relay Coil 110/120V 31021 400 60
SQUARE D RELAY CLASS 8501 TYPE RS14 FORM M1P14 SER B 120V 50/60HZ 0214
SQUARE D RELAY AND 2 REED DECKS 8501 X0 80 20 11 A
Square D Relay 8910 DPA33 30A 460V 120V Coil **Lot of 2**
Square D Relay 8501RSD42V53 Plug in GP Relay DPDT 10A 24V (lot of 2)
SQUARE D RELAY 8501-KUD12M1P14V53 SER. D 8501KUD12M1P14V53
SQUARE D RELAY 8501 KUD13P14V53 8501-KUD13P14V53 SER. C 24V 3 PDT 10A
Square D Relay 8501 KP-13 120V Coil Lot of 2 Surplus
SQUARE D RELAY 8501 KP12V20 SER. E 8501KP12V20
Square D Relay 8501 KLD12V51 Ser B DPDT 12 VDC with Custom Conneector QC11-PC
SQUARE D RELAY 120V 8501-CO16V20 SER. C * *
Square D Red/White Pushbutton With Two 9001KA3 Contact Blocks
Square D Red/White Pushbutton With 9001-KA1 Contact Series H
Square D Red Pushbutton W/ Contact 9001-KA-2 Ser G
Square D Red Pushbutton w/ 9001-KA3 Contact Block
Square D Red Pilot Light Kit 9999SP2R for Type S, Size 00-1 Surplus
SQUARE D RED PILOT LIGHT KIT 9999 SP-2R * SURPLUS IN BOX*
SQUARE D RED PILOT LIGHT KIT 9999 SP-12R * SURPLUS IN BOX*
Square D Red Pilot Lamp 120Vac P/N: 9001-0R120 Incandescent
SQUARE D RED MUSHROOM HEAD PUSHBUTTON 9001 KH1
SQUARE D RED LIGHT MODULE LOT OF 4 9001KM1 SER H
Square D Red Illuminated Maintain Mushroom Pushbutton Actuator with KA3 block
SQUARE D RED ILLUMINATED 9001 SERIES PUSHBUTTON 9001MB-1 SUPER FAST SHIPPING