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5 good examples of locking out HV/MV/LV structures and equipment

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Locking and safety reasons

In all cases the choice of locks and safety positions requires prior analysis of the locking sequence to be applied in order to correctly define the requirement and clearly identify the related risks. Remember that “electric” locking systems are never considered to be adequate.

5 good examples of locking out HV/MV/LV structures and equipment
5 good examples of locking out HV/MV/LV structures and equipment (photo credit: mardix.com)

In principle, only “mechanical” locking systems are capable of ensuring safety (as long as they themselves are reliable).

There are various possible graphic representations of locking mechanisms. Some representations give the status of the lock (bolt pushed in or not pushed in) and the key (not captive or captive).

Diagrammatic symbols are also used, but it is advisable to explain complex sequences in words. However, first let’s see the symbols we will use in locking diagram and then five different locking examples:

  1. Locking between earthing switch, HV switch and cell door
  2. Locking cells on HV loop system
  3. Locking on supply inversion and on HV station
  4. HV/TR/LV locking (functional symbols)
  5. Locking on LV supply inversion

Example of diagrammatic symbols

Example of diagrammatic symbols (source APAVE-france)
Example of diagrammatic symbols (source APAVE-france)

Functional locking symbols

Functional locking symbols
Functional locking symbols

Locking examples

#1 – Locking between earthing switch, HV switch and cell door

Locking between earthing switch, HV switch and cell door
Locking between earthing switch, HV switch and cell door

Locking sequence:

  1. Opening of switch I.
  2. The key is released.
  3. Transfer of key a to isolating switch S.
  4. Closing of isolating switch S.
  5. Key B is released.
  6. Opening of the cell door with key B.
  7. Key B remains captive.

Go back to locking examples ↑


#2 – Locking cells on HV loop system

Locking cells on HV loop system
Locking cells on HV loop system

The purpose of this procedure is to prevent the earthing switches closing when the cell is supplied upstream or downstream (loop-back).

Installation:
NB: Switches I and isolating switches T are designed to be mechanically controlled.

Immobilisation sequence:

  1. Opening of switch I1.
  2. Immobilisation of switch I1 and release of key A.
  3. Opening of switch I2.
  4. Immobilisation of switch I2 and release of key B.
  5. Unlocking of earthing switch T2 with key A.
  6. Closing of earthing switch T2.
  7. Key A is captive.
  8. Unlocking of earthing switch T1 with key B.
  9. Closing of earthing switch T1.
  10. Key B is captive.

Go back to locking examples ↑


#3 – Locking on supply inversion and on HV station

Locking on supply inversion and on HV station
Locking on supply inversion and on HV station

The draw-out circuit breaker is fitted with two locks. In normal operation, the circuit breaker I is closed, and keys A and B are captive. Opening the circuit breaker releases keys A and B. key a is transferred to the HV cell upstream (see example 2). Key B is transferred to the standby supply (see example 4).

Locking between the standby supply (circuit breaker g) and the HV cell may also be specified (second lock).

Go back to locking examples ↑


#4 – HV/TR/LV locking (functional symbols)

HV/TR/LV locking (functional symbols)
HV/TR/LV locking (functional symbols)

Used in supply stations with LV metering, this sequence, which is one of the most common, is used to access the terminals of the transformer after:

  1. Opening and locking of the LV circuit breaker.
  2. Opening and locking of the HV cell.
  3. Earthing of the separate HV supply.

Service state:

  1. The LV circuit breaker is closed.
  2. Key O is captive.
  3. The HV cell is closed.
  4. Key S is captive.
  5. The transformer terminals are not accessible.

Locking sequence:

  1. Opening and drawing out of the LV circuit breaker.
  2. Key O is released.
  3. Transfer of key O to the lock on the HV cell.
  4. Opening of the HV switch and closing of the earthing switch by mechanical control. Operation is possible by key transfer, as in example 1.
  5. Key O becomes captive.
  6. The cell panel can be opened.
  7. Key S can be removed.
  8. Unlocking of the immobilisation cover of the plug-in terminals.
  9. Key S becomes captive.

Go back to locking examples ↑


#5 – Locking on LV supply inversion

Locking on LV supply inversion
Locking on LV supply inversion

A standby power supply must only be coupled on an installation when it is certain that the main power supply is disconnected.

Likewise, when devices cannot be installed side by side (supply inverter plate with integrated interlocking mechanism) or they are different types (for example, lower protected power), interlocking by key must be provided.

In normal operation:

  1. Supply via transformer.
  2. Circuit breaker I is closed.
  3. Key A is captive.

In standby operation:

  1. Circuit breaker I is open.
  2. The associated lock is unlocked and key A is released.
  3. Key A is transferred to the lock on circuit breaker G, which is closed.
  4. Key A is captive.

Instruction video Switching ON cable earthing


Instruction video Switching OFF cable earthing

Reference // Power book; Operation functions by Legrand

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About Author

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

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