The "five protections" function can be achieved through interlocking devices. Interlocking devices are devices installed in high-voltage switchgear to prevent electrical misoperation. Interlocking devices ensure that the switchgear can be operated only when the prescribed procedure is followed; otherwise, the switchgear cannot be operated. "Prescribed procedure" refers to a manually defined operating sequence based on the safety requirements of electrical equipment.
Interlocking devices can be divided into two categories: mechanical interlocking devices and electrical interlocking devices. Mechanical interlocking devices are entirely constructed using mechanical components such as transmission levers, connecting rods, stoppers, and sliders. Electrical interlocking devices include electromagnetic locks and interlocking circuits.
In addition to mechanical and electrical interlocking devices, mechanical program locks and high-voltage live indicators can also be used to prevent electrical misoperation. A mechanical program lock is a mechanical lock designed for use with switchgear to meet the prescribed operation requirements. It generally consists of a lock body and a key. Mechanical program locks are installed on the operating handles of disconnectors and earthing switches, as well as on the switchgear door.
Interlocking devices can be divided into mandatory and non-mandatory types. Mandatory interlocking devices ensure that various operations can only be performed in a specified sequence; otherwise, they cannot be performed. Optional interlocking devices are reminders, such as command signs (red and green flip signs) and high-voltage live indicators. Command signs can be either mandatory or optional. Using a three-function key and token constitutes a mandatory method, while using a command sign and a standard control switch constitutes a optional method.
Of the "five preventions" mentioned above, only circuit breakers, contactors, and load switches that can cause accidental opening or closing can use optional interlocking devices; all others must use mandatory interlocking devices.
The interlocking device of high-voltage switchgear must comply with the requirements of CB3906, "3-35kV AC Metal Switchgear," and the former Ministry of Energy standard SD318, "Technical Specifications for Interlocking Devices of High-Voltage Switchgear." The specific requirements are as follows:
- With the exception of warning measures to prevent "incorrect opening and closing of circuit breakers," mandatory interlocking should be used for the other four protections.
- The interlocking device must ensure the prescribed operating procedures and ensure personal safety when operating the switchgear.
- Mechanical interlocking should be used whenever possible and should be simple, reliable, and easy to operate and maintain.
- All components used in the interlocking device must comply with the requirements of GB3906.
- Grounding terminals installed in the switchgear must be clearly marked, and their grounding area must meet the requirements of the switchgear.
- High-voltage live-current display devices must comply with the requirements of SD334, "Technical Specifications for High-Voltage Live-current Display Devices." The post insulator-type sensors and displays must undergo dielectric withstand voltage testing along with the high-voltage switchgear. The display device installed on the high-voltage switchgear should illuminate normally at 65% of the rated phase voltage, and the forced interlocking mechanism should operate reliably.
- The mechanical program lock used in the interlocking device should open flexibly and reliably, with the key inserting and removing easily and without any jamming. The switchgear must not be operated when attempting non-programmed operations.
- The interlocking device must comply with the switchgear's programmable operation requirements. The device's locked position must correspond to the actual position of the interlocked operating mechanism of the switchgear. Operation cannot be continued until the specified programmable operation is completed, and non-programmable operations must be impossible.
- When an electrical interlocking scheme is used, the power supply for the interlocking element must be separate from the relay protection circuit. The interlocking circuit and contacts must meet interlocking requirements, the wiring must be reasonable, and the housing of the interlocking element must be reliably grounded.
- All interlocking devices must have a dedicated unlocking tool to release the interlock in an emergency. However, unlocking without a dedicated tool is prohibited.
- Mechanical testing of interlocking devices directly connected to the operating mechanism should be conducted simultaneously with the mechanical operation test of the switchgear. If there is no direct mechanical connection to the switchgear mechanism, the test can be conducted at the end of or after the mechanical operation test of the switchgear, and programmed and non-programmed operations should be performed the specified number of times. Programmed operation should be smooth, and the operating mechanism and interlocking elements should not become stuck or malfunction. Non-programmed operation should ensure that the device can interlock reliably when normal operating force is applied. After the test, the device should not be deformed or damaged.
