Mechanical interlocking physically prevents an operating mechanism from moving (a cam, a rod or a bolt), and electrical interlocking cuts the coil or motor circuit with auxiliary contacts of other devices. The mechanical one does not depend on the control supply and also protects manual operation; the electrical one allows conditions between separate devices. Substations and switchgear use them together.
Two ways to prevent an operation
An interlock can act on the mechanism or on the control circuit. Mechanical interlocking physically blocks the movement: while the condition is not met, the cam, the rod or the bolt does not let the shaft turn or the crank go in. Electrical interlocking leaves the mechanism free but cuts the command: an auxiliary contact of another device, in series with the closing coil or the motor contactor, opens the circuit and the command does not get through.
At low voltage the terms are associated mainly with contactors, but the principle is the same on a high-voltage disconnector.
Mechanical interlocking
It is built with parts of the device itself, or between two devices mounted together:
- Cam or disc between disconnector and earthing switch. On a disconnector with an integral earthing switch, a cam on the disconnector shaft prevents the earthing switch shaft from turning while the disconnector is not open, and vice versa.
- Rods or transmission cables between two nearby mechanisms.
- Bolts and shutters in medium-voltage switchgear: the withdrawable truck cannot move with the breaker closed, and the shutter covers the fixed contacts when it is racked out.
- Blocking access to the crank on motor-operated mechanisms.
- Padlocking provision, which is part of locking out a work area.
Its strength is that it depends on nothing external: it works without control supply, with the device in manual and with the controller out of service. Its limitation is distance: it only interlocks devices that are physically joined or very close together.
The 2018 edition of IEC 62271-102, the standard for disconnectors and earthing switches above 1 kV, added strength requirements for interlocking devices and a specific test of mechanical interlocks, so that the lock withstands an attempt to operate against it. In metal-enclosed switchgear, IEC 62271-200 requires interlocks between the components of the switchgear and calls for an earthing switch whose short-circuit making capacity is lower than the peak current of the circuit to be interlocked with its disconnectors.
Electrical interlocking
It uses the auxiliary contacts of the other devices as permissives in series:
- 52b in the disconnector control circuit: the contact that is closed with the breaker open enables the disconnector motor. With the breaker closed, the circuit is open.
- “Disconnector open” contact in the earthing switch control circuit.
- “Earthing switch open” contacts in the disconnector control circuit and in the breaker closing coil circuit.
- Contacts from other bays when the condition requires it, for example the bus disconnectors of every bay before earthing a bus.
Its advantage is that it links separate devices, even in different bays. Its weak point is the dependence on contacts: if an auxiliary contact indicates “open” before the disconnector has finished opening, the permissive arrives too early. That is why mechanisms use early-make or late-break contacts and the positions are read as double points, as explained in breaker auxiliary contacts.
- 01Close commandComes from the cabinet pushbutton or from the controller
- 02Breaker 52b contactClosed only with the breaker open
- 03Earthing switch open contactClosed only with the earthing switch open
- 04Local/remote switch and crank accessThe circuit is cut in local or with the crank inserted
- 05Motor contactorIf every contact is closed, the disconnector operates
When each one is used
| Situation | Usual interlocking |
|---|---|
| Disconnector with its earthing switch on the same device | Mechanical, plus electrical on motor-operated mechanisms |
| Breaker and disconnectors of the same bay | Electrical, or logic in the bay controller |
| Bus earth against the disconnectors of every bay | Electrical between bays, logic in the station controller or over GOOSE |
| Medium-voltage switchgear: breaker, truck, earth and door | Mechanical, required by the switchgear standard |
| Distant equipment in industry (transformer, switchgear, access door) | Key |
In substations with numerical control, much of the electrical interlocking moves into the logic of the bay controller, which reads positions as double points and calculates the permissives. Even so, many specifications keep the mechanical interlock between the disconnector and its earthing switch and a minimum hardwired interlock in the mechanism, so that safety does not depend on a single system.
Low-voltage contactors
In a reversing motor starter, two contactors (one per direction of rotation) must never close at the same time, because they would produce a phase-to-phase short circuit. Both methods protect them: a mechanical interlock, a part that joins the two contactors and prevents both from closing, and an electrical interlock, the normally closed contact of each contactor in series with the coil of the other. It is the same principle as in the substation, at the scale of a panel, and low-voltage transfer schemes apply it the same way between source breakers.
What goes wrong in practice
- Mechanism out of adjustment after maintenance: the cam releases too early or never releases.
- Auxiliary contacts that change before the end of travel, bringing the permissive forward.
- Temporary jumpers in the control circuit left in place after testing.
- Manual operation without interlocking, when the design relied on the electrical interlock alone.
- Padlocks and keys without control, which defeat key interlocking.
A device that will not operate should almost never be forced: first identify which condition is missing. The motor-operated disconnector mechanism and the earthing switch have their own typical causes.
How it is verified
In general, each mechanical interlock is checked to prevent the operation in every forbidden position, crank included, and each electrical interlock contact to cut the command when the other device changes position. The test confirms that the permissive does not arrive before the interlocked device finishes its travel, and that padlocks and keys are released only in the intended position. The type test is defined by the standard of each device; on site it is part of interlock testing during commissioning.
Are you modernizing the control of a substation and need to decide what stays hardwired and what moves to the controller? At Tiza Group we design and configure interlocking logic, hardwired or in bay controllers, for substations and industry at medium, high and extra-high voltage. See our secondary engineering service or contact us.
Related guides
- Substation interlocking: types, levels and logic
- Interlock testing: what is checked at FAT and SAT
- Kirk and Castell key interlocking: how it works
- Interlocking at EHV, HV, MV and LV: what changes
- Low-voltage breaker and tie interlocking
References
- IEC 62271-102:2018, High-voltage switchgear and controlgear – Alternating current disconnectors and earthing switches.
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (a and b contacts).
- IEC 61936-1:2021, Power installations exceeding 1 kV AC and 1,5 kV DC – AC.
Frequently asked questions
What happens to electrical interlocking if the control supply is lost?
The electrical operating mechanism does not work either, so there is no unsafe electrical operation. The risk appears in manual operation with a crank or handle: if the only interlock is electrical, manual operation is left unprotected. That is why devices that allow manual operation have a mechanical interlock or a lock that blocks access to the crank.
What is the 52b contact and what is it for in an interlock?
It is the breaker auxiliary contact that is closed when the breaker is open. Wired in series with the control circuit of a disconnector, it only lets the disconnector operate with the breaker open. The 52a does the opposite: it is closed when the breaker is closed.
Why will a mechanically interlocked disconnector not operate even though everything is open?
It is usually an adjustment problem: the cam or bolt does not release because the earthing switch did not finish its travel, or the linkage went out of adjustment after maintenance. Never force it: check the travel of both devices and the adjustment of the mechanism.