The lamp is placed in "parallel" to the EB3 so that when the circuit
breaker trips there is an another path for the current to the track, in
this case through the lamp. The C/B is open circuit now and the current
to the track which has a short on it but the current is limited by the
resistance of the lamp thus reducing it to a safe current for loco
pickups and wheels causing the short, and in this case to either .3 amp
or .5 amp depending on 3 watt or 5 watt lamp being used.
The beauty of this arrangement is that it automatically resets the
circuit breaker, no need to operate a switch, also gives great visual
indication of someone running against a switch. With circuit breakers
installed this creates power zones, so when we have a short not all the
layout shuts down. The automatic resetting of the circuit was not
happening with more than 2 sound locos in the same zone, so this what I
have done.
Attached is photo of EB3 and lamps on the test bench, with the 2 watt
lamp connected and the 5 watt lamp sitting on bench. One side of lamp
connected to EB3 input + and the other side connected to EB3 output
channel 2 +, thus making a parallel path. In normal operation with no
short the power comes from command station to input of EB3 where it will
divide to go in 2 direction, some will go via the lamp but most of the
track required current will go via the EB3 because the resistance of
this path is much less than the 5 ohms resistance of the lamp. Ohms law
will determine how much goes in either direction. Current= Voltage
divided by resistance. EB3 will have the less resistance. In this
scenario the lamp does not impact on the track current at all UNTIL the
EB3 opens, now all current will flow via lamp, and with a short we could
supply up to 5 amps but the lamp restricts it to .3 of an amp. Note: the
cold resistance is 5 ohms but when current flows through lamp and it
heats up the filament, it's resistance increases. This is true for all
incandescent lamps.
