Arkansas HVACR NewsMagazine November 2019

S tate, National, Chapter News Tech News

BEWARE OF “GHOST” VOLTAGE

So what is meant by ghost voltage?

Bryan Orr , HVACR School.com

In some cases, you will be diagnosing an electrical issue, usually controls / low voltage issue. You will be measuring potential on a circuit and then when the circuit is connected to the load the voltage will disappear … like a “ghost”. For example, you make be measuring 24v at a condensing unit on the “Y” contactor circuit when the conductor (wire) is disconnected, but as soon as you connect it to the contactor/control board the voltage “disappears” when measured across the load (across the contactor coil) or more simply from Y to C. In other cases the voltage may not disappear completely, it may just drop way down, or in other cases the

Disclaimer: Ghost voltage is a term used by techs to explain a phenomenon where they measure voltage they don’t expect or when the voltage they see doesn’t do the work they expect. More advanced techs know how to use the Lo-Z (Low impedance) mode on their volt meter if it has one to help eliminate this. The vast majority of what techs call “ghost voltage” is just a circuit with high voltage drop under load and not stray inductance from other conductors. I write this first so that more experienced techs understand the context of this article.