Five Key Points for Ring Main Unit Maintenance

To enhance power supply reliability and enable consumers to receive power from two sources, the distribution network is often connected in a ring configuration. This power supply method is called ring main distribution. The high-voltage switchgeartypically used in such systems is referred to as a ring main unit (RMU). It is commonly used in industrial and mining enterprises, residential complexes, ports, and high-rise buildings.

Although clear procedures exist for RMU maintenance, accidents involving misoperation still occur frequently during maintenance every year. How can we avoid them? Based on experience and lessons learned, domestic experts have identified these five key steps: Grounding, Voltage Verification, Discharge, Tagging, and Isolation. All five steps must be performed completely; taking shortcuts is not an option.

First, Grounding is the most critical step. Its purpose is to provide a discharge path and ensure maintenance personnel work in a 'zero potential' environment. Crucially, dedicated grounding wires must be used with a three-point connection method. The power supply side should be grounded first, and the load side connection removed last. The second step is Voltage Verification. Use a voltage tester to confirm the conductor is de-energized. The tester must be self-inspected and qualified, and verification must be performed on all three phases separately. Any residual voltage must be treated as live voltage. The third step is Discharge, aimed at releasing residual voltage. Using a discharge rod for short-circuit discharge is recommended. The fourth step is Tagging. Tags should be placed on the RMU door handle, operating handles, etc., clearly stating 'DO NOT CLOSE, MEN AT WORK'. The final step is Isolation. Switching off a circuit breaker does not equal isolation. The isolator or disconnector must be opened, locked, and tagged. The isolation point must have a visible break gap – meaning it must be visible, physically open, and capable of being locked.

Failure to follow these steps can lead to a series of accidents. Improper grounding can leave cables energized with residual voltage, potentially causing electric shock or arc flash burns upon contact. Inadequate voltage verification, due to an uncleared upstream power source or cable-induced voltage, can lead to immediate electric shock during misoperation. Failure to discharge properly can result in residual voltage from capacitive loads causing arcs or burns when disconnecting wires. Omitting tags may lead to others being unaware of ongoing maintenance, potentially causing remote re-energization, leading to secondary discharge, equipment burnout, or casualties. Lack of isolation means the circuit breaker is open but the busbar remains connected, allowing remote control from a SCADA system to re-energize the line, potentially causing breakdown to the load side.

RMU maintenance is a frequent task. We must always remember these five points during maintenance work to ensure our own safety and never take chances.