NEIEP Machine Room Maintenance (550) Practice Test

Capacitors don’t have a fixed DC resistance. When you connect a low-ohm meter to an electrolytic capacitor, the meter applies a voltage to charge it. The uncharged capacitor looks like a short, so the initial reading is a low resistance. As the capacitor charges, the charging current drops and the reading rises toward a very high resistance (essentially open circuit). So you won’t see a sustained high resistance during the test; you typically see a brief low reading that increases quickly. This charging behavior is why describing a healthy electrolytic as showing high resistance on a low-ohm meter is not accurate. For meaningful cap health checks, use leakage-current testing at rated voltage or an ESR test rather than a simple resistance check.