What is an electrical interlocking and what is an electrical switch?

What is an electrical equipment interlock and why interlock is required?

The electrical interlocking is done to make sure that the equipment is safe, the electrical interlock is mostly used for substations and they are also used in many industries for the safety of industrial purposes. The isolators and switch gears are the protective devices that would provide proper and smooth operation of the power. By doing the interlock in a system we are making the functions of a system dependent on each other so one mechanism would take place after the other without interrupting the mechanism of the other.

The interlocks are used in an electrical system so that there won’t be any improper switching, we can see interlocks with the isolators and the switch gears so that the electrical circuit operation would take place properly. The electrical interlock is done in a substation or in an industrial process to make the operator and the machine safe. We can also prevent hazardous operations by interlocking. In case if there is no interlock in a system and if the operator closes the earth switch without opening the isolator and this could create the phase to earth fault. This is a wrong operation or it can also be described as a human error and this will be hazardous to the operator and also for the equipment. If an isolator is opened without opening the circuit breaker then it could badly affect the isolator so interlocks are used.

What is an interlock switch?

The interlock switch utilizes a guard door in case if there is any hazard and to prevent the hazard power source. So if the guard door is opened then the machine will be isolated from the power, there are different kinds of interlock switches all of them have their own characteristics so we should select an interlock switch that is suitable for our application. The interlock switch should be capable to work properly even in extreme conditions.

How to install the interlock switch?

  • Only an experienced person must be allowed to do the installation and it should be done according to the instructions
  • In order to prevent the switch from loosening after the installation, the self-locking nuts can be used with the mounting bolts
  • Choose the correct mounting bolts to fix the switch and the actuator
  • Use the proper size gland for the conduit entry
  • A mechanical stop can be used with the guard so that the switch will be protected from damages
  • The switch’s head position should be chosen properly
  • Make sure that the actuator enters the switch properly
  • There should be access to the manual release point
  • After the installation check the operations of all the control circuits and also the locking functions

What is the purpose of interlock?

The interlock would make the state of the two mechanisms mutually dependent, the major purpose of an interlock system is to protect the machine and the operator. The interlock would protect the machine and this would be done by preventing one element from changing state because of the state of the other element. An example of interlock operation would be the elevator, the elevators are designed in a way that they won’t open when it is in motion.

Why should we use interlock in generators?

Generators can be seen in many industries and some people use this for their home or for business, and this will be useful during the power cut. We can use an interlock to safely transfer power from the generator. The interlock would prevent in case if the main power and the generator power from powering the electrical system at the same time. In case if there is no interlock if the main power comes online so both these two power sources would create an overload condition. It could also make the generator power back-feed into the main and this would also cause certain problems.

By using an interlock in a generator, we could prevent the main and the generator power to connect at the same time. The circuit breakers would operate normally without interference in case of an overload condition. Most of the interlock devices for the electrical system would use a mechanical device so that the circuit breaker movement can be managed. Certain types of interlock devices would use padlocks so that no one would accidentally activate the main power system without any authorization.   

What is the importance of the interlock system in a substation?

Interlocks are important in a substation by this we could ensure that all the disconnectors, earthing switches, and circuit breakers are operating properly. Because of this all the equipment and the people around it would be safe too.

Where should we do electrical interlocking and how to do the interlocking in a substation?

  • Interlocking must be done with the circuit breaker and the disconnectors so that the disconnector won’t make or break the load currents
  • Disconnector and the earth switch must be interlocked, so that there won’t be any closing of earthing switch for locally energized circuit and it can’t be energized, during the operation of the disconnectors
  • The interlocking of the switching sequence that uses the power-operated switchgear must be done electrically.
  • The switching sequence with a manually operated switchgear can be electrical or mechanical.
  • The interlocking system should be fail-safe, it must operate properly so that the safety of the substation or the industrial plant can be ensured.
  • The partial interlocking of the earthing switches at the circuit entry to the substation is allowable, but it should not be done to the extent of the interlocking of the remote end disconnectors.

What are mechanical interlock and electrical interlock and how are they different?

Mechanical interlocking

The operation of the mechanical interlocking is achieved by the lever frames which are connected by the wire to the signals and points. This type of interlocking is composed of locking frames, plungers, lock bars, etc. So the lever movement is the major process of this interlocking and certain steps of this interlocking will be done manually.

Electrical interlocking

In this type of interlocking, there won’t be any manual operation all the interlocking processes will be carried out electrically and automatically. This interlock is done only by an electrical circuit and there won’t be any mechanical means for this type of interlocking. Relays and switches are mostly used to do this process.

What are the different types of electrical interlocking?

Power interlocking

In this type of interlocking the hazard power will be interrupted by direct switching.  This type of interlocking will be suitable for the equipment that operates in low voltage and power. But this is not possible in industrial machinery because it would operate mostly in high voltage and power. So in order to do the interruption specially designed power interlocking system with the power interrupting switch will be needed. This switch should be able to handle and break the load effectively. In the power interlocking, the stop command from the interlocking device would directly interrupt the energy supply to the machine actuators, in this type of interlocking there is no role for the control system.

Control interlocking

In this type of interlocking the hazard power source will be prevented by the switching of the circuit that controls the power switching device. This is the most commonly used interlocking method. The guard will be connected to an interlock switch so that the movement can be determined and also the switch contact can be opened if the guard is not closed properly. In this type of interlocking the stop command will be sent from the interlocking device to the control system. So because of this command, the power supply will be interrupted to the machine actuators and this will be done by the control system.

How to select an interlocking device?

  • The device must be selected according to the conditions it will be used
  • It should be selected according to the hazards that should be handled by the device
  • The probability of the interlock device failure should be considered
  • Stopping and access time must be considered
  • Frequency of the access
  • It must be selected according to the duration and also the exposure of the hazard

How to do the interlocking between the LT and HT breakers?

This is done to prevent the backflow from the LT to the HT side. Interlocking should be done according to certain rules. If both the CB’s are closed in case if the A is open then the B would open automatically. If the B is in a closed position then the A must not be in the open position. If both the CB’s are closed and if B is opened, then A can be either closed or opened. If both the CB’s are in an open state first A should be closed then B.


post-graduate in Electronics & communication.

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