The problem can be solved either by measurement of actual signal loads, or perhaps more easily using some circuit theory with Kirchoff’s Current Law along with Ohm’s Law. First, we need to simplify the schematic to a network of resistors as shown in Figure #3. Our goal is to calculate the equivalent resistance of the circuit so that we can derive the total current flowing from the Red load switch output.
Figure #3
Since the Red Field Terminal is energized and the Yellow and Green are open circuit (load switch output is Off), the circuit can be carefully redrawn as shown in Figure #4.
Figure #4
The voltage across R1 is easily determined as V (typically 120 Vac). To determine the current through the remaining network we need to calculate the equivalent resistance of the circuit. In this problem all signal loads are of resistance R. Careful use of Ohm’s law breaks it down as shown in Figure #5.
Figure #5
Knowing that the equivalent resistance is 1.25R, the current I1 through the resistor network is thus V/1.25R.
Since all resistors are the same value the total current through the network divides symetrically. The current through R2 and R3 is 0.5(V/1.25R) each. Thus the voltage across R2 and R3 is 0.5(V/1.25R)*R or 0.4V. Similarly one half of the current I1 divides equally through Y2, Y3 and G2, G3. Thus the respective voltages are 0.25(V/1.25R)*R, or 0.2V. Finally the voltage across Y1 and G1 is each 0.5(V/1.25R)*R or 0.4V.
R1 = V, R2 = 0.4V, R3 = 0.4V
Y1 = 0.4V, Y2 = 0.2V, Y3 = 0.2V
G1 = 0.4V, G2 = 0.2V, G3 = 0.2V
The MMU senses the voltage at the Field Terminals which are labeled as MMU-R, MMU-Y, and MMU-G in Figure 3. Thus the voltages reported by the MMU will be as follows:
MMU-R = V
MMU-Y = 0.4V
MMU-G = 0.4V
Depending on where the Neutral is broken and how many signals are “isolated” from a direct Neutral connection, the voltages across the signal heads will be different, resulting in many different potential solutions to the problem of a bad Neutral splice. Since the resulting voltages are primarily well below full voltage (V), in this scenario the incandescent style lamps would “glow”. Since the Red, Yellow, and Green signals were all glowing to some extent because of these sneak paths, it reminded some of Christmas Lights.
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TRAFFIC
