APTech AP74B - Manual (Page 4)

Operation Manual Flow Switches
Manual 15000002 Rev 2 Page 4 of 4
4. Wiring capacitance – when wiring a load and reed switch over a long distance,
electrostatic capacitance arising from the cable can influence the reed switch contact and
an inductor L
S
should be installed. The inductance (L
S
) value depends on the load
current, but should be in the range of 0.5 to 5 mH. The figure below shows a typical
protection circuit.
D. Operation
The AP74 or AP74B flow switch does not require any specific action by the system operator for
correct operation once installed.
When system flow exceeds the trip point, a magnet inside the device will move upward causing
the reed switch normally open contacts to close and normally closed contacts to open. When flow
drops slightly below the trip point, the magnet will move downward and the reed switch contacts
will revert to the previous position.
Note: Initial pressurization of system lines can cause a flow surge that exceed the system
maximum design flow rate and trips the flow switch. If this occurs, first check the system for
leaks or open vent valves and, if no problems are found, reset the controller and reinitiate system
pressurization.
Please contact the factory or your local representative to answer questions or for further
information.