Interlocks and conditioned operations

Mechanical and electrical interlocks are included on mechanisms and in the control circuits of apparatus installed in substations, as a measure of protection against an incorrect sequence of manœuvres by operating personnel.
Mechanical protection between functions located on separate equipment (e.g. switchboard and transformer) is provided by key-transfer interlocking.
An interlocking scheme is intended to prevent any abnormal operational manœuvre.
Some of such operations would expose operating personnel to danger, some others would only lead to an electrical incident.

Basic interlocking

Basic interlocking functions can be introduced in one given functionnal unit; some of these functions are made mandatory by the IEC 62271‑200, for metal-enclosed MV switchgear, but some others are the result of a choice from the user.
Considering access to a MV panel, it requires a certain number of operations which shall be carried out in a pre-determined order. It is necessary to carry out operations in the reverse order to restore the system to its former condition. Either proper procedures, or dedicated interlocks, can ensure that the required operations are performed in the right sequence. Then such accessible compartment will be classified as “accessible and interlocked” or “accessible by procedure”. Even for users with proper rigorous procedures, use of interlocks can provide a further help for safety of the operators.

Key interlocking

Beyond the interlocks available within a given functionnal unit ( see also "Choosing MV equipment"), the most widely-used form of locking/interlocking depends on the principle of key transfer.
The principle is based on the possibility of freeing or trapping one or several keys, according to whether or not the required conditions are satisfied.
These conditions can be combined in unique and obligatory sequences, thereby guaranteeing the safety of personnel and installation by the avoidance of an incorrect operational procedure.
Non-observance of the correct sequence of operations in either case may have extremely serious consequences for the operating personnel, as well as for the equipment concerned.
Note: It is important to provide for a scheme of interlocking in the basic design stage of planning a MV/LV substation. In this way, the apparatuses concerned will be equipped during manufacture in a coherent manner, with assured compatibility of keys and locking devices.

Service continuity

For a given MV switchboard, the definition of the accessible compartments as well as their access conditions provide the basis of the “Loss of Service Continuity” classification defined in the standard IEC 62271‑200. Use of interlocks or only proper procedure does not have any influence on the service continuity. Only the request for accessing a given part of the switchboard, under normal operation conditions, results in limiting conditions which can be more or less severe regarding the continuity of the electrical distribution process.

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