In the power distribution industry, there is an old saying: “Electricity is a tiger you cannot see.” This is no exaggeration — every operation on a distribution system carries the risk of arc flash burns, fatal electric shock, and short‑circuit explosions. And of all safety accidents, more than 90% are caused by human operating errors — not by poor equipment or bad weather, but by operators failing to follow safety procedures. To eliminate these accidents, the distribution system has a safety framework that runs from design through to operation: the “Five‑Prevention” system. These “five preventions” are not slogans on the wall — they are rigid rules embedded in both the equipment structure and the operating procedures. Whether you are an electrical engineer, a maintenance technician, or a manager who occasionally needs to enter the electrical room, this article is worth reading carefully. The “Five Preventions” are a classic summary of safety operation requirements for power distribution systems in China’s electrical industry. Different sources may express the specifics slightly differently, but the core idea is entirely consistent. The first prevention is to prevent incorrect opening or closing of circuit breakers — the circuit breaker is the master switch of the distribution system; closing it by mistake energises equipment under maintenance, endangering the lives of maintenance personnel, while opening it by mistake causes sudden production stoppages with huge economic losses. To prevent such errors, circuit breaker operating handles are typically fitted with “open/close” labels and mechanical locks, and the equipment number and operation task sheet must be verified before any operation. The second prevention is to prevent opening or closing disconnectors under load — disconnectors have no arc‑extinguishing devices and can only be operated when the circuit is already de‑energised (i.e., under no‑load conditions); forcing a disconnector to open or close under load current produces a powerful arc with temperatures reaching tens of thousands of degrees Celsius, capable of instantly melting metal and igniting surrounding equipment, with the operator bearing the brunt — that’s why disconnectors in distribution systems are always preceded by a circuit breaker. The third prevention is to prevent attaching earth (ground) cables to energised lines — earthing is the final safety safeguard before maintenance work, directing residual charge and any possible backfeed into the earth to protect maintenance personnel from electric shock; but attaching an earth cable to a live line creates an artificial three‑phase short circuit with massive fault current that instantly melts the earth cable, destroys equipment, and endangers personnel — which is why an voltage detector must be used to confirm the line is de‑energised before earthing. The fourth prevention is to prevent closing a circuit breaker with an earth cable still attached — if someone mistakenly closes a breaker or disconnector with an earth cable still in place, the result is “energising with ground applied,” effectively creating a metallic three‑phase short circuit directly on the system; the consequences are severe — fault currents can reach tens of kiloamperes, destroying equipment, causing fires, and even dropping the entire substation bus — so the interlocking system must ensure that the breaker cannot close while an earth cable is attached. The fifth prevention is to prevent unauthorised entry into live compartments — distribution cabinets contain multiple compartments, some energised and some de‑energised for maintenance; if maintenance personnel enter a live compartment without confirming it has been de‑energised, they are effectively walking into a cage with a tiger; hence, cabinet doors are equipped with mechanical or key interlocks that only allow access to maintenance compartments after the power is off. The “Five Preventions” are enforced not by goodwill, but by hardware interlocking devices, which fall into three categories. Mechanical interlocking is the most direct and reliable method, using mechanical structures such as links, shutters, and pins to physically block incorrect operations — for example, a mechanical interlock between a disconnector and its associated circuit breaker physically prevents the disconnector handle from turning until the breaker is opened; this type of interlock relies on neither power nor signals — pure “metal against metal” — offering exceptionally high reliability. Electrical interlocking is implemented through auxiliary contacts and control circuits, feeding position signals from breakers, disconnectors, and earthing switches into the interlocking circuit; when conditions are not met, the interlock cuts off the operating power, making the equipment impossible to operate — its advantage is flexibility and remote controllability, but its disadvantage is dependence on power and control wiring, so a loss of control power can disable the interlock. Key interlocking (key locks) requires the operator to follow a specific sequence, with each step releasing the key for the next — the most common configurations are “three locks with two keys” or a “five‑prevention key box” — and because it does not rely on electrical signals, it remains fully functional even during power outages. Beyond hardware, the “operation ticket” system provides the second layer of safety defence at the management level — it is a detailed step‑by‑step instruction sheet listing the complete operation sequence, including each device to be operated, the order of operations, and the verification result for each step. The core principle of the operation ticket is “call and repeat”: one person reads the instruction from the ticket (calling), and the other person repeats the instruction and executes the operation (repeating), ticking each step off as it is completed. This two‑person verification mechanism maximises the prevention of errors caused by individual carelessness. Any electrical switching operation must be covered by an operation ticket — operating without one is a violation of safety rules. On‑site operations are plagued by several common bad habits. Some experienced electricians, relying on “years of experience,” skip the call‑and‑repeat process — but experience is not a substitute for procedure; equipment numbers, cabinet types, and wiring configurations change, and memory can fail; the operation ticket exists not just for novices but for everyone. Another dangerous shortcut is skipping voltage verification before attaching earth cables, assuming that because upstream power is off, the circuit must be dead — but “upstream off” and “this circuit dead” are not the same thing, as lines can be energised through misoperation, backfeed, or induced voltage; the voltage detector must never be omitted — it is your final evidence that the circuit is truly dead. Failing to wear insulating gloves and shoes during operations is another serious issue — these protective items are not decorative;

the risk of accidental electric shock is real, and even a single incident can make the difference between life and death. Operating without gloves is irresponsible to one’s own life. Operating alone without a supervisor is prohibited — distribution operations must be carried out by two people, one operating and one supervising; the supervisor does not participate in the operation but monitors every step for correctness, sequence, and safety compliance; a solo operator may become too focused and miss other risks, while having a supervisor adds another pair of eyes. Using copper wire as a fuse replacement is one of the most dangerous practices — when a fuse blows, some people bypass it with a piece of copper wire rather than investigating the cause, but fuse wire is precisely calculated to melt under overload to protect downstream equipment; copper wire will not melt under overload, so the cable and equipment will burn instead. When a fuse blows, you must identify the cause and replace it with a fuse of the same rating — never “make do” with copper wire. For non‑electrical personnel who need to enter electrical rooms or work near distribution equipment, three safety reminders are essential. First, never touch any equipment marked with “high voltage hazard” or “live” warnings — the yellow warning labels on switchgear are not decorative; they are the final verbal warning, and under no circumstances should you open a cabinet door marked “do not open.” Second, the electrical room is not a storage area — do not store cardboard boxes, oil drums, metal ladders, or other materials there, as they are not only fire hazards but may also block escape routes in an emergency; the floor must be kept dry, free of standing water or oil spills. Third, if you hear unusual sounds or smell unusual odours, leave immediately and report them — distribution equipment should not produce harsh noises or burning smells during normal operation; if you hear sizzling discharge sounds or smell burning, the equipment may already be in a fault condition, so evacuate the electrical room and notify professionals — do not go in to “see what it is” without understanding the situation. The “Five Preventions” are not constraints — they are protection. Behind every rule lie real lessons written in blood and tears. In front of a power distribution system, every moment of impatience or “cutting corners” could make you regret it for the rest of your life.