Medium Voltage Application Guide

13679.A
4.3 Safety Considerations
Switchgear interlocking systems
SWITCHGEAR
Interlocking between different switchgear apparatus and enclosure access covers and doors enhances personnel
safety, as well as improving operational convenience. If a switching device can cause serious damage in an incorrect
position, this must also have a locking facility.
Interlocking uses electrical and mechanical methods or a combination of both.
IEC 62271-200 states mandatory rules for switchgear interlocking:
For metal-enclosed switchgear with removable switching apparatus:
the switching device must be in the open position before it can be withdrawn
the switching device can only be operated in the positive service or test position
the switching device cannot be closed unless the auxiliary control circuits required to open the switch are
connected. Auxiliary control circuits cannot be disconnected with the switching device closed in the
service position
For metal-enclosed switchgear with disconnectors:
Methods
a disconnector cannot be operated under conditions other than those for which it is intended to be used
a disconnector cannot be operated unless the main switching device is open
operation of a main switching device is prevented unless its associated disconnector is in a positive service,
test or earth position
disconnectors providing isolation for maintenance and servicing must have a locking facility
The illustration shows a common switchgear arrangement for a medium voltage power distribution system. This
switchgear arrangement uses three separate interlocking methods.
Typical MV power distribution switchgear arrangement with interlocks
TXR_L
TXR_R
A
B
C
D
Incomer panel
Left bus
Right bus
Feeder panel
Interlock scheme 1 (typical)
B
Q-FL
A
Q-IL
3
(LOCK)
(KEY)
E-IL
2
1
Q-BC
(LOCK)
Q-IR
A
E-IR
2
(LOCK)
(KEY)
Q-FR
C
3
Q-IL
E-IL
TXR_L
Q-IR
E-IR
TXR_R
Q-BC
Q-FL
E-FL
Q-FR
E-FR
Interlock scheme 2 (typical)
Interlock scheme 3 (typical)
Circuit breaker - left incomer
Earth switch - left incomer
Supply transformer - left bus
Circuit breaker - right incomer
Earth switch - right incomer
Supply transformer - right bus
Circuit breaker - bus coupler
Circuit breaker - left feeder
Earth switch - left feeder
Circuit breaker - right feeder
Earth switch - right feeder
E_FL
E_FR
D
D
710-12280-00A Medium Voltage Application Guide Page 83