Interlocks and conditioned operations

Mis-operations in electrical installations may expose operating personnel to danger and lead to electrical incidents.

As a measure of protection against incorrect sequences of manoeuvres by operating personnel, mechanical and electrical interlocks are included in the mechanisms and in control circuits of electrical apparatus.

The interlocks may be classified in two categories:

• Functional interlocks incorporated in MV functional units and dedicated to the operation of the apparatus located in the units only. These interlocks are generally realized by means of specific mechanical devices linked with the mechanisms of the apparatus
• Interlocks between MV functional units or between a functional unit and another equipment such as a MV/LV transformer. Most of these interlocks are realized by means of keys transferred from one equipment to another when they are made free. They may be improved or by additional electrical interlocks.

Functional interlocks

Some interlocks are mandatory in MV functional units according to IEC 62271-200, dedicated to metal enclosed switchgear, for example to prevent from:

• closing a switch or circuit breaker on a closed earthing switch;
• closing an earthing switch while the associated switching function is closed

Specific additional interlocks may be specified by the users when required by their operational rules, for example:

• Allowing the opening of a MV cable connection compartment only if the earthing switch associated to the remote end of the MV cable is closed.

The access to a MV compartment requires a certain number of operations which shall be carried out in a pre-determined order. To restore the system to its former condition it is necessary to carry out operations in the reverse order.

Dedicated procedures and instructions may also ensure that the operations are performed in the right sequence.

Hence, the accessibility to an MV compartment can be either interlock controlled or based on procedure. A compartment can also be accessible only by means of tools if its access is not necessary for normal operation or maintenance of the switchgear, or "not accessible", access being either forbidden or impossible (see Fig. B28).

Type of accessibility to a compartment	Access features	Type of construction
Interlock-controlled	Opening for normal operation and maintenance, e.g. , fuse replacement.	Access is controlled by the construction of the switchgear, i.e. , integrated interlocks prevent impermissible opening.
Procedure-based	Opening for normal operation or maintenance, e.g. , fuse replacement.	Access control via a suitable procedure (work instruction of the operator) combined with a locking device (lock).
Tool-based	Opening not for normal operation and maintenance, e.g. , cable testing.	Access only with tool for opening; special access procedure (instruction of the operator).
Not accessible	Opening not possible not intended for operator; opening can destroy the compartment. This applies generally to the gas-filled compartments of gas insulated switchgear. Because the switchgear is maintenance-free and climate-independent, access is neither required nor possible.

Fig. B28 – Type of accessibility to a compartment

Key interlocking

The interlocks between devices located in separate MV functional units or between a functional unit and access to a MV/LV transformer for example are performed by means of keys.

The principle is based on the possibility of freeing or trapping one or several keys, according to whether or not the required conditions of operation are satisfied. These conditions ensure the safety of the personnel by the avoidance of incorrect operations.

Note: Concerning the MV/LV substations, the interlocks shall be specified during the design stage. Hence, the apparatuses concerned by the interlocks will be equipped during the manufacturing with the appropriate keys and locking devices.

Service continuity

The notion of Loss of Service Continuity: "LSC" (see Fig. B29 and Fig. B30) defines the conditions of access to any high voltage accessible compartment of a given high voltage functional unit.

Fig. B29 – Example of functional unit architecture with compartments, favoring service continuity

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IEC 62271-200 defines four categories of Loss of Service Continuity: LSC1, LSC2, LSC2A, LSC2B.

Each category defines which other high voltage compartments and /or other functional units can be kept energized when opening an accessible high-voltage compartment in a given functional unit.

For the single busbar architectures the following definitions are applicable:

LSC1 functional unit = Functional unit having one or several high-voltage accessible compartments, such that, when any of these accessible high-voltage compartments is open, the busbar and one or several other functional units of the switchgear must be de-energized
LSC2 functional unit = Functional unit having at least an accessible compartment for the high-voltage connection (called connection compartment), such that, when this compartment is open the busbar can remain energized. All the other functional units of the switchgear can continue to be operated normally.

Note: When LSC2 functional units have accessible compartments other than the connection compartment, further subdivisions into LSC2A and LSC2B are defined.

LSC2A functional unit = Functional unit having several high-voltage accessible compartments, such that, the busbar can remain energized when any other accessible high voltage compartment is open. All the other functional units of the switchgear can continue to be operated normally
LSC2B functional unit = Functional unit having several high-voltage accessible compartments, such that, the high-voltage connections compartment and the busbar can remain energized when any other accessible high voltage compartment is open. All the other functional units of the switchgear can continue to be operated normally.

		Applies when
LSC1	When any compartment of the FU is open the busbar and one or several other FUs of the switchgear must be de-energised	One or several compartments in the considered FU are accessible
LSC2	When the cable compartment is open the busbar can remain energized and all the other FUs of the switchgear can be operated normally	Only the connection compartment in the considered FU is accessible
LSC2A	The busbar can remain energized when any other accessible high voltage compartment is open. All the other functional units of the switchgear can continue to be operated normally	Several compartments in the considered FU are accessible
LSC2B	The high-voltage connections compartment and the busbar can remain energized when any other accessible high voltage compartment is open. All the other functional units of the switchgear can continue to be operated normally	Several compartments in the considered FU are accessible

Fig. B30 – Loss of Service Continuity definitions

Interlocks in substations

Example of functional interlocks, embedded in single functional units

• Load break switch closing: the door must be closed and the earthing switch open
• Earthing switch closing: the door must be closed and associated circuit breaker, switch and/or isolating apparatus open
• Access to an accessible compartment: the associated circuit breaker, switch and/or isolating apparatus must be open and the earthing switch closed.

Example of functional interlocks involving several functional units or separate equipment

(see Fig. B31):

Lets consider a MV/LV transformer supplied by a MV functional unit including:

• A load break switch
• A set of MV fuses
• An earthing switch

The transformer is installed in a dedicated cubicle.

The access to the MV/LV transformer is authorized when the following conditions are satisfied:

• MV load break switch open
• MV earthing switch closed and locked in close position
• LV circuit breaker open and locked in open position

The required sequence of operations to meet these conditions in full safety is the following:

• Step 1: Open the LV CB and lock it open with key "O". Key "O" is then free
• Step 2: Open the MV load break switch. Check that the "voltage presence" indicators are extinguished, unlock earthing switch with key O, key O is now trapped
• Step 3: Close the MV earthing switch and lock it in close position with key S. Key S is now free
• Step 4: Key S allows to open the door of the transformer cubicle. When the door is open, key S is trapped.

The restoration of the supply to the LV switchboard is performed with the execution of the reverse sequence of operation:

• Step 1: Door of the transformer cubicle closing
• Step 2: MV earthing switch opening
• Step 3: MV load break switch closing
• Step 4: LV circuit breaker closing.

Due to LV production, some national regulations require an earthing system as temporary or permanent device to operate on the transformer under full safety, and the earthing connection shall be integrated within the interlock procedure.

Fig. B31 – Example of MV/LV interlocking system

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  Initial configuration: LV Switchboard energized. MV Load break switch closed. LV circuit breaker closed. Earthing switch open and locked in open position. Key O trapped. Key S trapped.

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  Step 1: Load break switch closed. LV circuit breaker open and locked. Earthing switch open, locked in open position. Key O free, Key S trapped.

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  Step 2: Load break switch open, LV circuit breaker open and locked in open position. Earthing switch unlocked, Key O trapped, Key S trapped.

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  Step 3: Load break switch open, LV circuit breaker open and locked in open position. Earthing switch closed and locked, Key O trapped, Key S free.

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  Step 4: Load break switch open, LV circuit breaker open and locked in open position. Earthing switch closed and locked, door of transformer cubicle open, Key O trapped, Key S trapped.

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  Legend