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| Technical Brief - AC Reverse Polarity False Indicators | |
In the Technical brief on Reverse Polarity the
flowing diagram of a Reverse Polarity sensing circuit is shown.
Diagram 1
It is possible for such a circuit to faintly
illuminate the Reverse Polarity light even though the circuit is
properly wired. It is useful for boaters to understand how this
can occur.
Imagine that one small change is made to Diagram 1
that results in the circuit shown in Diagram 2.
These two circuits are, absent a ground fault,
electrically equivalent. All that has been done in Diagram 2 is to
represent the green safety ground wire as an extension of the one
leg of the Reverse Polarity sensing LED. We now have an LED with
one leg on each end of the neutral wire. Because voltage is always
consumed pushing amperage through a resistance (wire), the voltage
is different at points A and B on the Neutral wire when there is
current flowing through it. This is called “voltage drop”.
When high amperage loads are operated in the
circuit, enough voltage drop in the length of the Neutral wire can
be created to overcome the resistance in the 25K resistor required
by ABYC that sufficient current is driven through the LED to cause
faint illumination. This situation is not inherently dangerous,
however, it can indicate undersized wiring in the dock, shore power
cord or ship’s wiring portion of the AC circuit.
Diagram 2
Courtesy Of Blue Sea Systems