Arkansas HVACR NewsMagazine November 2019

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contactor may chatter, circuit board lights dim etc… I have heard all of these situations called “ghost” Voltage, but they are actually just voltage drop and these symptoms are caused by additional resistance in the circuit OTHER than the designed load. Quick Note: there are also “induced” voltages that can appear as ghost voltage due to conductors running in parallel with other current carrying conductors. This is more common in Commercial and industrial applications where many wires are bundled or in close proximity over long distances. These charges are usually small and often “disappear” under load. Rarely do we want more than one electrical load (resistance point) in a single circuit. When this does occur it is usually undesigned and caused by of long wire lengths, improperly sized wire and poor connections. Now to CLARIFY, when referring to a circuit we mean one complete path between electrically different points (say L1 and L2 in single phase 240 or 24v hot to 24v common on a control transformer). Some think of parallel circuits as a single circuit, but while they may share conductors they have an individual load path. To cut to the chase, whenever wire is undersized, runs of wire are too long or the circuit contains poor connections

there will be additional resistance introduced to the circuit. When there is more resistance added in places other than the load (in this case a contactor coil) there will be a voltage drop and therefore the voltage applied to the load will be decreased. When a wire isn’t connected to the load this drop will be invisible because the load isn’t in the circuit and therefore you are simply reading across the OTHER, unintended load (resistance) which will often be the full voltage depending upon the exact issue and when you are making the measurement. In every complete and independent circuit, including a series circuit, the amperage is the same no matter where in the circuit you measure it. Before the load, between loads, after the loads… it doesn’t matter. The amperage is dictated by the total applied voltage and the resistance (or more accurately the impedance) of the entire circuit. The voltage applied to each load is dependent on the resistance of the load in comparison to the total resistance of the circuit. In the example below, you can see that the amperage is the same on each load and is dictated to be 500 micro-amps because the total circuit ohms is 18,000. The voltage drop of each load in series is equal to it’s percentage of the total circuit resistance. Since load R1 is 16.5% of the total resistance in the circuit, the voltage drop across R1 is 1.5V because 1.5 is 16.5% (0.165) of 9V.