Electrical Installation Guide 2009 - the global specialist in energy ...

B - Connection to the MV publicdistribution network3 Protection aspectSMV switch and LV CB closedOSMV fuses accessibleSFig. B20 : Example of MV/LV/TR interlockingOOTransformer MV terminals accessibleLegendKey absentKey freeKey trappedPanel or doorSSSOAccess to the MV or LV terminals of a transformer, (protected upstream by aMV switchgear-and-protection panel, containing a MV load-break / isolatingswitch, MV fuses, and a MV earthing switch) must comply with the strict proceduredescribed below, and is illustrated by the diagrams of Figure B20.Note: The transformer in this example is provided with plug-in type MV terminalconnectors which can only be removed by unlocking a retaining device common toall three phase connectors (1) .The MV load-break / disconnecting switch is mechanically linked with theMV earthing switch such that only one of the switches can be closed, i.e. closureof one switch automatically locks the closure of the other.Procedure for the isolation and earthing of the power transformer, and removalof the MV plug-type shrouded terminal connections (or protective cover)Initial conditionsb MV load-break/disconnection switch and LV circuit-breaker are closedb MV earthing switch locked in the open position by key “O”b Key “O” is trapped in the LV circuit-breaker as long as that circuit-breaker is closedStep 1b Open LV CB and lock it open with key “O”b Key “O” is then releasedStep 2b Open the MV switchb Check that the “voltage presence” indicators extinguish when the MV switch isopenedStep 3b Unlock the MV earthing switch with key “O” and close the earthing switchb Key “O” is now trappedStep 4The access panel to the MV fuses can now be removed (i.e. is released by closure ofthe MV earthing switch). Key “S” is located in this panel, and is trapped when the MVswitch is closedb Turn key “S” to lock the MV switch in the open positionb Key “S” is now releasedStep 5Key “S” allows removal of the common locking device of the plug-type MV terminalconnectors 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:b 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 areexposed.b The MV earthing switch is in the closed position but not locked, i.e. may be openedor closed. When carrying out maintenance work, a padlock is generally used to lockthe earthing switch in the closed position, the key of the padlock being held by theengineer supervizing the work.b The LV CB is locked open by key “O”, which is trapped by the closed MV earthingswitch. The transformer is therefore safely isolated and earthed.It may be noted that the upstream terminal of the load-break disconnecting switchmay remain live in the procedure described as the terminals in question are locatedin a separate non accessible compartment in the particular switchgear underdiscussion. Any other technical solution with exposed terminals in the accessedcompartment would need further de-energisation and interlocks.B21(1) Or may be provided with a common protective cover overthe three terminals.Schneider Electric - Electrical installation guide 2009© Schneider Electric - all rights reserved