When designing an application that uses a Megablock for short circuit protection, allow an additional 60 mA in the design to give the power supply the additional current capability to support a short circuit in one device (should it occur) and continue to power the segment without interruption. Normally a 0.5 voltage drop (assuming a 20 mA device) occurs across the Megablock to the device. An additional voltage drop occurs during a short circuit condition. To prevent devices from dropping off the segment because of reduced voltage, be sure that the application design allows for the voltage drop in both the normal and short circuit condition.
Use the following calculation to calculate the short circuit voltage drop to the farthest device:
.060A X (44 ohms/km) X distance in km
The following example calculates the voltage drops on a 0.5 km segment to the farthest device:
Normal voltage drop = 0.5 Volts
Short circuit voltage drop =.060A X (44 ohms/km) X.5 km = 1.32 Volts
Total voltage drop = normal voltage drop + short circuit voltage drop = 1.82 volts
This calculation is based on a design that allows for one short circuit in a running segment. An example would be a situation in which an inadvertent shorting of a device occurs during routine replacement in an operational system. The calculation does not allow for multiple short circuit conditions in a new installation that has not been verified with the segment checkout procedure.