4.3.3 Power connections from the isolation switch to the main power fuses shall be made using
bus bar. Connections from the contactor through the current transformers to the outgoing load
terminals shall be made using power cable furnished by others.
4.3.4 The contactor shall be available with an optional mechanical latch with provisions for
manual or electrical trip.
4.3.5 The contactor shall be mechanically and electrically interlocked with the isolation switch as
described in the section on interlocking (4.5).
4.4
Power Fuses
4.4.1 The power fuses shall be arranged horizontally at the top of the contactor unit. In this
position, the blown fuse indicators on the fuses shall be visible, through the viewing window, when
the medium voltage door is closed.
4.4.2 The fuse unit shall include either single or double barrel power fuses, R-rated or E-rated, 3"
diameter.
4.5
Interlocking
4.5.1 A mechanical interlock shall be provided between the vacuum contactor and the isolation
switch. The interlock shall prevent the isolation switch from being opened or closed unless the
main contacts of the contactor are opened.
4.5.2 The interlock described in 4.5.1 shall also operate in a manner such that it will prevent the
closing of the main contacts of the vacuum contactor unless the isolation switch is either fully
opened or fully closed.
4.5.3 An electrical interlock shall be provided as a backup to the mechanical interlock in 4.5.1.
This interlock shall switch off control power to the vacuum contactor at any isolating switch
position other than fully opened or fully closed.
4.5.4 A mechanical interlock shall be provided between the isolation switch and the medium
voltage compartment door. This interlock shall prevent the door from being opened unless the
isolating switch is in the fully open position.
4.5.5 The interlock described in 4.5.4 shall be capable of being circumvented in the event
emergency entrance to the controller is required. Circumventing the interlock shall require two
separate and distinct operations and shall require the use of a tool.
4.5.6 A mechanical interlock between the isolation switch and the medium voltage door shall be
provided to prevent the isolating switch from being closed when the door is open. This interlock
shall be capable of being circumvented only by the use of a tool.
4.5.7 An electrical interlock shall be provided to ensure the isolation switch does not open with
the control power transformer under load. This interlock shall be designed such that during
switch opening, the transformer secondary is disconnected prior to the opening of the switch
blades. Also, the isolating switch blades must close before the secondary load is reconnected.
4.5.8 An electrical interlock shall be provided to prevent the vacuum contactor from being closed
using external test power unless the isolation switch is fully opened and the secondary of the
normal control power transformer is disconnected from the control circuit to prevent backfeeding
the transformer.
