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From the 2006 IPC, Section 1107.2 Separate systems required.
Secondary roof drain systems shall have the end point of discharge separate
from the primary system.........Discharge shall be above grade, in a location
that would normally be observed by the building occupants or maintenance
personnel.
Also, from the 2006 IPC, 1107.3 Sizing of secondary drains.
Secondary (emergency) roof drain systems shall be sized in accordance
with Section 1106 based on the rainfall rate for which the primary system
is sized in Tables 1106.2, 1106.3 and 1106.6...........Scuppers shall be sized
to prevent the depth of ponding water from exceeding that for which the
roof was designed as determined by Section 1101.7...........Scuppers shall
not have an opening dimension of less than 4 inches (102 mm).........The
flow through the primary system shall not be considered when sizing the
secondary roof drain system.
Also, see Section 1110 - CONTROLLED FLOW ROOF DRAIN
SYSTEMS.
FWIW, I am not a fan of [ more ] electrical devices to monitor /
control anything... That said though, ...are you planning to get your
secondary roof drainage systems engineered with the electrical device
you posted?......Recommend that you get input / approval from the AHJ
before going to the expense of getting a design engineered that they
may not approve.
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