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 (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)
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 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.
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#2 – 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.
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#3 – 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)
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
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.
