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 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 it 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 remplacement.	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 remplacement.	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: Service continuity

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

In a MV/LV distribution substation which includes:

• A single incoming MV panel or two incoming panels (from parallel feeders) or two incoming/outgoing ring-main panels
• A transformer switchgear-and-protection panel, which can include a load-break/disconnecting switch with MV fuses and an earthing switch, or a circuit-breaker and line disconnecting switch together with an earthing switch
• A transformer compartment

Interlocks allow manœuvres and access to different panels in the following conditions:

Basic interlocks, embedded in single functionnal units

• Operation of the load-break/isolating switch

  - If the panel door is closed and the associated earthing switch is open

• Operation of the line-disconnecting switch of the transformer switchgear - and - protection panel

  - If the door of the panel is closed, and

  - If the circuit-breaker is open, and the earthing switch(es) is (are) open

• Closure of an earthing switch

  - If the associated isolating switch(es) is (are) open⁽¹⁾

• Access to an accessible compartment of each panel, if interlocks have been specified

  - If the isolating switch for the compartment is open and the earthing switch(es) for the compartment is (are) closed

• Closure of the door of each accessible compartment, if interlocks have been specified

  - If the earthing switch(es) for the compartment is (are) closed

(1) If the earthing switch is on an incoming circuit, the associated isolating switches are those at both ends of the circuit, and these should be suitably interlocked. In such situation, the interlocking function becomes a multi-units key interlock.

Functional interlocks involving several functional units or separate equipment

• Access to the terminals of a MV/LV transformer

  - If the tee-off functional unit has its switch open and its earthing switch closed. According to the possibility of back-feed from the LV
    side, a condition on the LV main breaker can be necessary.

Practical example

In a consumer-type substation with LV metering, the interlocking scheme most commonly used is MV/LV/TR (high voltage/ low voltage/transformer).
The aim of the interlocking is:

• To prevent access to the transformer compartment if the earthing switch has not been previously closed
• To prevent the closure of the earthing switch in a transformer switchgear-and-protection panel, if the LV circuit-breaker of the transformer has not been previously locked “open” or “withdrawn”

Access to the MV or LV terminals of a transformer, (protected upstream by a MV switchgear-and-protection panel, containing a MV load-break / isolating switch, MV fuses, and a MV earthing switch) must comply with the strict procedure described below, and is illustrated by the diagrams of Figure B9.
Note: The transformer in this example is provided with plug-in type MV terminal connectors which can only be removed by unlocking a retaining device common to all three phase connectors⁽¹⁾.
The MV load-break / disconnecting switch is mechanically linked with the MV earthing switch such that only one of the switches can be closed, i.e. closure of one switch automatically locks the closure of the other.

(1) Or may be provided with a common protective cover over the three terminals.

Procedure for the isolation and earthing of the power transformer, and removal of the MV plug-type shrouded terminal connections (or protective cover)
Initial conditions

• MV load-break/disconnection switch and LV circuit-breaker are closed
• MV earthing switch locked in the open position by key “O”
• Key “O” is trapped in the LV circuit-breaker as long as that circuit-breaker is closed

Step 1

• Open LV CB and lock it open with key “O”
• Key “O” is then released

Step 2

• Open the MV switch
• Check that the “voltage presence” indicators extinguish when the MV switch is opened

Step 3

• Unlock the MV earthing switch with key “O” and close the earthing switch
• Key “O” is now trapped

Step 4

The access panel to the MV fuses can now be removed (i.e. is released by closure of the MV earthing switch). Key “S” is located in this panel, and is trapped when the MV switch is closed

• Turn key “S” to lock the MV switch in the open position
• Key “S” is now released

Step 5

Key “S” allows removal of the common locking device of the plug-type MV terminal connectors on the transformer or of the common protective cover over the terminals, as the case may be. In either case, exposure of one or more terminals will trap key “S” in the interlock.
The result of the foregoing procedure is that:

• The MV switch is locked in the open position by key “S”.

Key “S” is trapped at the transformer terminals interlock as long as the terminals are exposed.

• The MV earthing switch is in the closed position but not locked, i.e. may be opened or closed. When carrying out maintenance work, a padlock is generally used to lock the earthing switch in the closed position, the key of the padlock being held by the engineer supervizing the work.
• The LV CB is locked open by key “O”, which is trapped by the closed MV earthing switch. The transformer is therefore safely isolated and earthed.

It may be noted that the upstream terminal of the load-break disconnecting switch may remain live in the procedure described as the terminals in question are located in a separate non accessible compartment in the particular switchgear under discussion. Any other technical solution with exposed terminals in the accessed compartment would need further de-energisation and interlocks.

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Fig. B9: Example of MV/LV/TR interlocking