High Voltage Safe Working Procedures
3.12 Trapped Key and Key Safe Systems
Chapter 3
High Voltage Safe Working Procedures
Slot for Racking handle
Racking in
Racking out
Safe Key Interlock
Slot for Earthing Switch
Figure 3.21 – HV Switch Gear Key Exchange in the Front Panel 3.12.1 Key Exchange Systems
These systems work on the principle that the master key is designed to allow an operation to be carried out and it cannot be released until all the necessary isolations have taken place.
In the case of switchgear, this could be a transformer isolation switch where parallel feeds are used.
Figure 3.22 – A Key Exchange box
Chapter 3
A1
A1
A1
A1 A1 A1
S1
Machine stopped, Supply locked off Access doors open, Keys trapped
Machine running, Supply on
Access doors locked Access keys locked,
Switch Keys free
Supply On,
Switch Key Trapped
Supply locked off, Switch Key free
Access keys released Switch Key locked
Figure 3.23 – A Simple Key Exchange System 3.12.2 Use of Key Safes
The unique keys for padlocks used for the purpose should preferably be themselves secured in a Key Safe which can itself be locked with more than one lock. To ensure that neither the issuer, recipient or any other party can alter those safety precautions until the Permit to Work is cleared, it is suggested that the key safe be locked with a unique lock as well as a lock by the issuer and the key to that lock is given to the recipient with the signed copy of the Permit to Work. It is to be returned only on clearance of that Permit to Work.
High Voltage Safe Working Procedures
Connect
1K1
1K2 1K3
Lock with key to open the
rear door Lock to draw
CB Out / In
Lock for earthing switch
operation CB Position
Indicator
ES Operation Handle Hole CB Draw-out / In
Handle Hole
Key Interlocks in the front door of the Circuit Breaker
a) 1K1 To lock the VCB in the Out position.
b) IK2 This key is only available after having completed procedure a) above.
c) IK3 The Earthing Switch can be c ed e O .
Figure 3.24 – The Key Interlock System 3.12.3 Key Interlock for a Generator Cable Compartment
The generator breaker (52) and generator de-excitation switch is to be opened and the earthing switch is to be closed before the compartment door can be opened (Refer Figure 9.13).
Proceed as follows to open the compartment door:
1. O a a (52) and draw- a D
position, then turn key (GM31) to lock the circuit a O . T is now removable.
2. T K (GM32) D - ( -excitation switch - DES). The key is now removable.
3. Insert both keys (GM31) and (GM32) into the door interlock of the earthing switch and turn them to unlock the door. The keys are now held in place and the door can be opened;
the earthing switch can now be closed.
4. When the earthing switch has been closed, turn the key (GM 33) to lock the earthing
C . T a . I (GM 13)
the door interlock of the cable compartment and turn it to unlock the door. The key is now
Chapter 3
Gen Circuit Breaker
G MC
GM 32
GM 31
GM 32 GM
33
GM 33
LO
52 LO
LO
GM 31
LO 6.6 kV AC Bus
Earthing Switch
Cable Compartment Generator
De-excitation Switch
Generator Field
Earthing Switch Door
Figure 3.25 – Generator Key Interlock System 3.12.4 Key Interlock for a Motor Starter
This interlock is applied to the motor starter cable compartment and component compartment.
Motor starter breaker (52) is to be opened and the earthing switch is to be closed, before the compartment door can be opened.
High Voltage Safe Working Procedures
Circuit Breaker
BT 12
BT 11
LO
6.6 kV AC Bus
Cable
Compartment
52 88 42 M
6
BT 12
LO BT
13
BT 13
BT 13 BT
13
LO
BT 13
LO
BT 11
LO
BT 13
LO
Front Rear
ATR
Component Door
AC Motor
Figure 3.26 – Motor Starter Circuits
With reference to Figure 3.26, proceed as follows to open the compartment door:
1. Open circuit breaker (52) and draw- a D ,
(BT11) a O . T a .
2. Insert key (BT11) into the door interlock of the earthing switch and turn them to unlock the door of the earthing switch. The key is now trapped and the door of the earthing switch can be opened.
3. When the earthing switch is to be closed, turn key (BT12) to lock the earthing switch in
C .
4. The key is now removable.
Chapter 3
7. Insert the key (BT13) into the door interlock of the cable compartment and turn it to unlock the door. The key is now held in position and the door can be opened.
8. Reverse the process to restore service.
3.12.5 Key Safe Arrangement for a Generator
C
DG
UGS
A
AS
To ICC
To bus riser or bus tie panel
Protection Circuits
Measuring Instruments
To Terminal Panel AVR
R,S,T No. 1 or No. 2 AC 6.6 kV Bus
M
CT
TE-CL TE-OP CB-OP
VT
4-20 mA 4-20 mA
I DC I AC
I DC I AC
FU 52
CB-OP
Key Free with CB in drawn-out
position
TE-OP
Key Free with Earthing Switch in open position TE-CL
Key Free with Earthing Switch in closed position
Resistor
Figure 3.27 – Key Safe System for a Generator
High Voltage Safe Working Procedures
3.12.6 Bus Bar Earthing Key Safe Interlocking Arrangement
The bus bar earthing switch is operated by key K1 which can be unlocked only when Keys K2, K3, K4, K5 and K6 are put in key exchange box and turned.
These keys are only available only when all the incoming and outgoing breakers in the bus bar are isolated.
Figure 3.28 – Key Safes for Bus Bar Earthing in a HV System
C
DG4C
DG3C
STB CM1 - 10 CM1 - 9 CM1 - 8
No. 1 Bus No. 2 Main
Trans Panel
No. 4 DG Panel
No. 3 DG Panel
Bus Riser
& Synchro Panel
Bus Tie Panel
Earthing Truck
VCB
VCB VCB VCB
Chapter 3
1. Open and draw out the VCB of #2 Main Transformer panel.
2. Isolate the VCBs of DG#3 &DG#4 and Bus Tie Panel so that keys QM-1 8,9,10 are free to be removed.
3. Insert the key QM1-8, QM1-9, QM1-10 in the key exchange box and turn it.
4. Turn and remove the key STB. Other keys will be locked.
5. Insert the key STB into the Earthing Truck and turn it.
6. Insert the earthing truck into the #2 main transformer panel and close it by relevant push button.
7. #2 Bus bar is now earthed and the Key STB is now locked.
8. Repeat the operation reverse way to return to service condition.
3.12.7 VFD Safety Key Exchange System
Control
Cubicles Medium Voltage
Cubicles
Grounding Switch
No. of Locks N
Inverter Front
Other Customer Locks Other Locations Locks of Customer Supply
Box 2
Box 3
No. of Locks N
Customer Scope of Supply Main Breaker
Key Locking
Supply Gr. Switch
Pre-charge Supply
Box 1 Z X
Order of Operations
Symbols Key Positions
Captive Key Free Key - Present
Free Key - Absent Bolt Positions
Bolt In Bolt Out
High Voltage Safe Working Procedures
1. Open the incoming breaker, close the grounding switch of the supply => get Key X from Box 1.
2. Open the pre-charge supply 400V breaker => get Key A.
3. Put Keys X and A into Box 2.
4. Wait for 10 minutes to allow the capacitors to discharge.
5. Get Key B from Box 2 to close the MV7000 grounding switch.
6. Get Key C from the grounding switch that was closed to release Keys D from Box 3 to open each medium voltage cubicle.
3.13 Procedure for Isolation in High Voltage Switch Gear