ASCO Series 300
Operator’s Manual
Page 8
ASCO Power Technologies
381333-453
Block Load Control
Each load (except priority 1) has an estimated KW
value and step time delay associated with it. As
each block of load is allowed to transfer to
emergency power, the individual loads will begin
their step time delay. As the time delay for each
load completes, the load will receive a permissive
signal to transfer to emergency power. If no delay
is desired, the individual step times can be set to
zero seconds, and the loads will transfer
immediately when the load block is permitted to
transfer to emergency.
Note: Any automatic transfer switch without the
30B accessory (Load Shed) will automatically
transfer to emergency on its own regardless of the
permissive signal.
Priority 1 loads must not have a
30B accessory.
If a generator fails while operating in the automatic
mode, it is disconnected from the bus and
shutdown. Audible and visual alarms will be
activated to indicate the condition. System loads
will not shed unless a bus overload or a bus under
frequency occurs. This feature is referred to as
“Load-Latch”.
A soft push-button on the operator interface
terminal (OIT) permits override of the load-shed
circuits for supervised operation (one for each
priority except priority 1) refer to (Figure 9).
Loads that have been block shed can be manually
re-added using the priority-# Load Shed
Bypass/Reset push-button located on the Master
control section OIT. By pressing the Load Shed
Bypass/Reset push-button, the system will transfer
the selected load block to the emergency bus. By
pressing the Load Shed Bypass/Reset push-button
a second time, the ENTIRE selected load block
will shed. The OIT System Status screen will
annunciate the status of each Priority load block,
indicating if it is shed or manually bypassed. Refer
to (Figure 8). If the operator inadvertently
bypasses the load shed and overloads the
Generator(s) resulting in a bus under-frequency, all
manually bypassed loads will be automatically
shed along with blocks of load such that the
remaining load blocks shall be one less than the
number of generators remaining on the bus. The
“Bus Under Frequency” annunciator light will
indicate this condition.
If the online load exceeds 105% of rated generator
capacity, unsupported load blocks (load block
priorities greater than the number of generators
online) will be step shed in reverse priority at one
second intervals. No load blocks equal to or less
than the number of generators online will be shed.
(Ex. If you have two generators online, load blocks
1 and 2 will stay online, priority 3 and above will
be shed). The Bus Overload light will light to
indicate that the bus is overloaded automatically
reset as the overload is corrected. When the online
load no longer exceeds 105% of the generator
capacity, step shedding will cease, and the system
will continue to power the loads that are left
online. Indicating lights will annunciate the status
of each Priority load block, indicating if any loads
in that block are shed or if the load block is
manually bypassed. If the operator bypasses the
load shed to bring those shed loads back online,
and the online load exceeds 105% of capacity
again, the system will again step shed loads in
reverse priority until the load no longer exceeds
105% of capacity.
Note: Block loading can be disabled so all loads
(except priority 1) can be added to the emergency
bus only through the bus optimization feature.
