PLC系統的正常供電電源均由電網供電。由于電網覆蓋范圍廣，它將受到所有空間電磁干擾而在線路上感應電壓和電路。尤其是電網內部的變化，入開關操作浪涌、大型電力設備起停、交直流傳動裝置引起的諧波、電網短路暫態沖擊等，都通過輸電線路傳到電源原邊。PLC電源通常采用隔離電源，但其機構及制造工藝因素使其隔離性并不理想。實際上，由于分布參數特別是分布電容的存在，隔離是不可能的。

b 來自信號線引入的干擾

與PLC控制系統連接的各類信號傳輸線，除了傳輸有效的各類信息之外，總會有外部干擾信號侵入。此干擾主要有兩種途徑：一是通過變送器供電電源或共用信號儀表的供電電源串入的電網干擾，這往往被忽視；二是信號線受空間電磁輻射感應的干擾，即信號線上的外部感應干擾，這是很嚴重的。由信號引入干擾會引起I/O信號工作異常和測量精度大大降低，嚴重時將引起元器件損傷。對于隔離性能差的系統，還將導致信號間互相干擾，引起共地系統總線回流，造成邏輯數據變化、誤動和死機。PLC控制系統因信號引入干擾造成I/O模件損壞數相當嚴重，由此引起系統故障的情況也很多。

c 來自接地系統混亂時的干擾

接地是提高電子設備電磁兼容性（EMC）的有效手段之一。正確的接地，既能抑制電磁干擾的影響，又能抑制設備向外發出干擾；而錯誤的接地，反而會引入嚴重的干擾信號，使PLC系統將無法正常工作。PLC控制系統的地線包括系統地、屏蔽地、交流地和保護地等。接地系統混亂對PLC系統的干擾主要是各個接地點電位分布不均，不同接地點間存在地電位差，引起地環路電流，影響系統正常工作。例如電纜屏蔽層必須一點接地，如果電纜屏蔽層兩端A、B都接地，就存在地電位差，有電流流過屏蔽層，當發生異常狀態如雷擊時，地線電流將更大。此外，屏蔽層、接地線和大地有可能構成閉合環路，在變化磁場的作用下，屏蔽層內有會出現感應電流，通過屏蔽層與芯線之間的耦合，干擾信號回路。若系統地與其它接地處理混亂，所產生的地環流就可能在地線上產生不等電位分布，影響PLC內邏輯電路和模擬電路的正常工作。PLC工作的邏輯電壓干擾容限較低，邏輯地電位的分布干擾容易影響PLC的邏輯運算和數據存貯，造成數據混亂、程序跑飛或死機。模擬地電位的分布將導致測量精度下降，引起對信號測控的嚴重失真和誤動作。（3）來自PLC系統內部的干擾主要由系統內部元器件及電路間的相互電磁輻射產生，如邏輯電路相互輻射及其對模擬電路的影響，模擬地與邏輯地的相互影響及元器件間的相互不匹配使用等。這都屬于PLC制造廠對系統內部進行電磁兼容設計的內容，比較復雜，作為應用部門是無法改變，可不多考慮，但要選擇具有較多應用實績或經過考驗的系統。三、主要抗干擾措施1、采用性能優良的電源，抑制電網引入的干擾在PLC控制系統中，電源占有極重要的地位。電網干擾串入PLC控制系統主要通過PLC系統的供電電源（如CPU 電源、I/O電源等）、變送器供電電源和與PLC系統具有直接電氣連接的儀表供電電源等耦合進入的。現在，對于PLC系統供電的電源，一般都采用隔離性能較好電源，而對于變送器供電的電源和PLC系統有直接電氣連接的儀表的供電電源，并沒受到足夠的重視，雖然采取了一定的隔離措施，但普遍還不夠，主要是使用的隔離變壓器分布參數大，抑制力差，經電源耦合而串入共模干擾、差模干擾。所以，對于變送器和共用信號儀表供電應選擇分布電容小、抑制帶大（如采用多次隔離和屏蔽及漏感技術）的配電器，以減少PLC系統的干擾。此外，位保證電網饋點不中斷，可采用在線式不間斷供電電源the tunnel Programmable logic controller. So that the reliability of the whole automation system is guaranteed. At the same time, PLC can work independently to complete the basic control task, and the application of fieldbus will make the monitoring task of each area become the corresponding distributed monitoring task of PLC, which makes the tunnel wiring more simple and reasonable. The flexible configuration of PLC makes the tunnel monitoring system form a modular structure, the PLC system can be large or small, according to the site requirements for a reasonable distribution. The following is an introduction of the development and application of the tunnel Programmable logic controller in practice, taking the traffic, lighting and ventilation monitoring system of the main tunnel group on the Zhangzhou-Longyan expressway as an example. 2 system requirements, the Zhanglong Expressway has a total of six tunnels, namely the Wansongguan Tunnel, fengshuangling tunnel, da Longtou tunnel, Houjia tunnel, Nanjing County tunnel, shikongmen tunnel (not included in the scope of this project) , the dilongtou tunnel is a two-hole highway tunnel with a length of about 2.6 km. The designed speed is 80 km/h when the tunnel is driven in one direction, and 60 Km/h when the tunnel is driven in two directions. In order to meet the above requirements, the tunnel traffic, lighting and ventilation have corresponding requirements, respectively by the corresponding number of electrical and mechanical equipment, power, installation location to ensure. In order to save energy and drive safely, the lighting and ventilation in the tunnel are optimized and controlled by corresponding control system and monitoring system. For example, the lighting monitoring system ensures the illumination in the cave, and the illumination from the entrance to the cave can be smoothly transitioned during the daytime to avoid the black hole phenomenon The ventilation control system ensures the CO concentration, visibility and wind speed in the tunnel to meet the design requirements. 3 The system consists of 3.1 programmable controllers the tunnel programmable controllers follow the Transport in China Industry Standard JT 608-2004, which was implemented on January 1,2005, beijing Yunxingyu Traffic Engineering Company has developed corresponding products YXY-RTU according to this industry standard. Tunnel programmable controller PCT is an integrated product of fieldbus + PLC + manual screen, which makes full use of PLC's high stability, high efficiency of fieldbus and flexible control of manual screen, the safety and stability control of Highway Tunnel is realized. According to the regulations of GB/T18567-2001, the programmable controller of tunnel can be divided into two parts, i. e. , the programmable controller in the tunnel monitoring station and the programmable controller in the tunnel. 3.1.1 the programmable controller in the tunnel monitoring station refers to: a) the node