Polarity indicates the relative electrical potential of terminals in a circuit, defining which is positive and which is negative. In direct current systems, polarity is fixed and dictates the direction of current flow. In alternating current systems or transformers, polarity refers to the relative instantaneous phase relationship between primary and secondary windings, which is critical when paralleling equipment.
Heard on the job
Before you parallel those two distribution transformers, you must verify their phase polarity or you'll create a dead short.
Insider tip
Reversing the polarity on a DC instrument can destroy it instantly, while reversing transformer polarity in an AC system usually results in a catastrophic short circuit.