MV/LV Transformer Door interlocking Example
MV/LV transformer supplied by a MV functional unit including:
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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:
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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:
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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:
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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.
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.
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.
Step 2: Load break switch open, LV circuit breaker open and locked in open position. Earthing switch unlocked, Key O trapped,
Key S trapped.
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.
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.