insulating material and have direct-acting
in the operating mechanism. Then move the
automatic tripping devices. They are used to
handle to the ON position.
protect single-load circuits and feeder circuits
The AQB-A250 circuit breaker may have
coming from a load center or distribution panel.
auxiliary switches or shunt trip (for remote
tripping) attachments. Figure 9-13, view C, shows
The outside dimensions of these breakers are
a shunt trip. The instantaneous trip setting of the
the same for both the two-pole dc CB and three-
AQB-A250 trip units may be adjusted by the
pole ac CB. They are designed for front or rear
instantaneous trip adjusting wheels. These trip
connections. They may be mounted so they can
adjusting wheels can be adjusted to five positions,
be removed from the front without removing the
LO-2-3-4-HI. The trip unit label will list the
instantaneous trip value obtainable for each
The frame size of the CB is designated by the
marked position. The settings must be the same
250. In a 250-ampere frame size CB, the current-
on each pole of the CB.
carrying parts of the breaker have a continuous
Terminal mounting block assemblies (fig.
rating of 250 amperes. Trip units for this breaker
9-14) consist of terminal studs in terminal
(fig. 9-13, view B) are available with current
mounting blocks of insulating material. The
ratings of 125, 150, 175, 225, and 250 amperes.
terminals of the CB have slip-type connectors,
The trip unit houses the electrical tripping
which engage the terminal studs as shown in figure
mechanisms, the thermal element for tripping the
9-14. Two mounting blocks are required for each
CB on overload conditions, and the instantaneous
CB. This method of connecting a CB to a bus or
trip for tripping on short-circuit conditions.
circuit is known as a back-connected CB. Some
The automatic trip devices of the AQB-A250
AQB circuit breakers have solderless connectors
attached to their terminals for front-connections.
circuit breaker are trip-free. In other words, the
CB cannot be held closed by the operating handle
if an overload exists. When the CB has tripped
AQB-LF.--The AQB-LF250 circuit breaker
due to overload or short circuit, the handle rests
(fig. 9-15, view A) combines the AQB circuit
approximately in a center position. To reclose the
breaker features with a current-limiting fuse unit,
CB after automatic tripping, move the handle to
which interrupts the circuit when the current is
the extreme OFF position. This resets the latch
more than the interrupting rating of the breaker.
Figure 9-14.--AQB-A250 circuit breaker, rear view, with terminal mounting block.