Overheating is suspected in a high voltage bolted bus-bar joint. If the local continuity resistance is to be checked off-line after the necessary safety precautions have been taken, what instrument would be used for the resistance test?
• Difference between insulation resistance testing and continuity/connection resistance testing • Typical resistance values of a bolted bus-bar joint (very low, close to zero ohms) • Which instruments are designed to measure very low resistance versus very high resistance
• Is a bus-bar joint more like checking the insulation between conductors, or checking how good a metal-to-metal connection is? • Would you expect the resistance of a good bolted joint to be in ohms, milliohms, or megohms? Which type of meter is accurate in that range? • Look at the word "overheating" – what does excess resistance at a current-carrying joint do, and which instrument can detect such small added resistance?
• Identify which instrument is specifically designed for very low resistance (micro- or milliohm) measurements • Recognize that a megohmmeter is primarily for very high insulation resistance values, not metallic joint resistance • Decide whether a conventional ohmmeter has enough resolution and accuracy to detect tiny changes in a bolted bus-bar joint’s resistance
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