The most dangerous substation errors are not equipment failures but human errors: closing an earthing switch onto live busbars, or opening a disconnector under load. An interlocking system is the mechanical guard that makes such errors physically impossible — not merely forbidden.
The Core Idea
An Interlocking System is an arrangement that prevents two switches from being operated at the same time if their combined state would be hazardous, protecting the system from faults and accidents. The most famous example is in the ring main unit: preventing the earthing switch from being closed while the load break switch is closed.
The Operating Logic
| State | Load Break Switch | Earthing Switch |
|---|---|---|
| Normal Operation | Closed (feeding loads) | Open |
| Maintenance | Open | Closed (earths the isolated section) |
These two states are opposites and must never coexist: if the earthing switch were closed while the supply remained connected, the result would be a direct earth fault with a violent arc. The interlock is installed to guarantee mechanically that closing one makes it impossible to close the other — the mandatory sequence: disconnect the load first, then close the earthing switch.
Forms of Interlocking in Substations
- Mechanical: physical locks and levers that prevent movement of the prohibited switch — the most reliable, and unaffected by loss of supply.
- Electrical: control circuits that require a certain state of other equipment before allowing a command (common between circuit breakers and disconnectors in large substations).
- Key Interlock: a single key cannot be freed from the first lock unless it is in a safe position, and that same key is required to operate the second lock — enforcing the sequence through the exchange of the key.
An interlock is a safety net, not a substitute for procedure: if you find an interlock that "resists" you, never try to bypass it — it is most likely preventing an error you cannot see. Any bypass of an interlock is a serious event requiring special authorization and an investigation into the cause.
Sample answer: Its function is to prevent two opposing switches from being operated at the same time, protecting against accidents. The most prominent example: preventing the earthing switch from closing while the load break switch is closed — in normal operation the load break switch is closed and the earthing switch is open, and during maintenance the situation is reversed; these two states must never coexist because earthing live busbars causes a violent earth fault. The interlock mechanically guarantees that if one is open, the other cannot be opened.
Bypassing an interlock that is "blocking the work" with a screwdriver or force. An interlock that resists you protects you from a sequencing error you cannot see — unauthorized bypassing is a direct cause of documented fatal substation accidents.
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