Views: 0 Author: Site Editor Publish Time: 2026-10-09 Origin: Site
During the electrical control design of sewage treatment projects, many engineering parties tend to prioritize PLC brands and component prices while overlooking two core indicators: protection rating and I/O points. Sewage plants feature high humidity and corrosive hydrogen sulfide gas, coupled with future process expansion requirements. If the cabinet protection grade is underestimated or insufficient I/O points are reserved, problems such as cabinet failures, incapacity for system expansion and sharply increased renovation costs will occur in later operation. Drawing on years of project experience in municipal sewage, industrial wastewater and lift pumping stations, Shiyou Electric analyzes the two core selection points of sewage control cabinets to help customers avoid selection pitfalls.
The IP protection code directly determines whether the control cabinet can resist moisture, splashing water, dust and corrosive gas. Protection requirements vary greatly for different installation locations, and a unified cabinet type cannot be adopted for all areas. For control cabinets installed in dry sewage power distribution rooms, the minimum recommended rating is IP54. Cabinets are made of thickened cold-rolled steel sheets with electrostatic powder coating to block dust and lateral splashing water. For highly humid areas with high hydrogen sulfide concentration such as bar screen rooms, dosing rooms and pump rooms, 304 stainless steel cabinets with a protection rating no lower than IP55 are recommended. For outdoor lift pumping stations and industrial wastewater treatment projects with harsher environments, 316L stainless steel cabinets with IP65~IP66 protection rating are preferred, equipped with sealed cable glands and elevated bases to prevent accumulated water from entering the cabinet.
Compliance with IP rating alone is not enough. Due to large diurnal temperature variations in sewage plants, condensation easily forms inside cabinets. It is necessary to equip temperature and humidity sensors and anti-condensation heaters during selection. Industrial air conditioners can be fitted for high-power frequency conversion control cabinets as required to suppress condensation and prevent corrosive gas from eroding circuit boards. Corrosion and short-circuit failures of components in many projects are caused by only checking the IP mark while ignoring sealing strips, cabinet materials and anti-condensation accessories. Although the parameters seem qualified, the equipment fails after a period of field operation.
I/O points are core resources of PLC control cabinets, divided into digital DI/DO and analog AI/AO, used to collect field instrument signals and output equipment control commands. DI collects signals of equipment operation, faults and liquid level switches; DO outputs start-stop commands for pumps, blowers and bar screens; AI acquires 4-20mA instrument signals including liquid level, DO, pH and turbidity; AO outputs PID regulation signals to frequency converters or metering pump controllers to realize variable frequency control of blowers and metering pumps.
When calculating points, count all controlled equipment and monitoring instruments completely, including lift pumps, bar screens, aeration blowers, dosing pumps, sludge return pumps, various water quality instruments and fault alarm signals. Many purchasers only configure points matching the current equipment quantity without spare capacity. When adding instruments or reconstructing processes later, the whole PLC control cabinet has to be replaced, resulting in substantial waste. The industry recommends reserving 20% redundancy for total I/O points. Small township sewage stations and integrated sewage equipment have a small total number of I/O points. For large municipal sewage plants and MBR process projects with numerous connected instruments, modular PLC with expandable remote I/O modules is selected to facilitate phased expansion.
Signal types should be distinguished. Analog points cost more, so the quantities of DI, DO, AI and AO need to be counted separately to avoid mismatched point types. Power cables and signal cables are routed in separate trunking inside the cabinet to reduce electromagnetic interference from frequency converters, stabilize I/O signal transmission and mitigate signal fluctuation and acquisition distortion.
Protection grade and I/O points are the foundation, while matching supporting configuration is also required for selection. Component selection, cabinet anti-corrosion coating, communication protocols, drawing documents and factory tests all affect long-term operation. Most sewage projects need to connect with SCADA systems and online environmental monitoring platforms. During selection, confirm that the PLC supports protocols such as Modbus-RTU and Modbus-TCP, and a DTU gateway can be installed for remote monitoring.
In the selection of sewage treatment control cabinets, the protection rating determines whether the equipment can withstand the harsh on-site conditions of sewage plants, and I/O points determine whether the system can meet control requirements and future expansion. Both are the core of sewage electrical control schemes. Procurement selection should not only compare component quotations, but scientifically determine IP rating and I/O redundancy according to installation location and process scale. Shiyou Electric provides customized sewage PLC control cabinets for municipal sewage, industrial wastewater, integrated sewage stations and lift pumping stations. We offer one-stop services from scheme design and cabinet assembly to on-site commissioning to ensure the long-term stable and up-to-standard operation of sewage automatic control systems.
