🔍 Key Concepts
• How an insulation resistance tester reading changes when a ground fault is present (very low resistance vs very high resistance in megaohms)
• Which conductors are actually connected together in Figure 3 vs Figure 4 (u‑v‑w side vs x‑y‑z side, lamps removed, switch position)
• How opening the switch and moving the test leads isolates whether the ground fault is on the supply side (u,v,w) or the load side (x,y,z)
💭 Think About
• In Figure 3, when the tester shows the low reading, exactly which conductors are tied to the tester’s positive lead via closed contacts and jumpers? Trace the path from the tester lead through the switch and lamp sockets.
• In Figure 4, when the tester shows the high reading, what part of the circuit has now been isolated from the test? Which group of conductors is no longer connected to the tester?
• Compare which elements are common to the low‑reading test and which are common to the high‑reading test. From that comparison, which specific group of conductors must contain the ground fault?
✅ Before You Answer
• Identify clearly which conductors belong to the supply side (u, v, w) and which belong to the load side (x, y, z) in the illustration.
• Verify for each figure whether the switch is open or closed and whether any “wires connected/disconnected” notes change which conductors are in the test circuit.
• Before choosing, confirm: in the test that shows low resistance (kΩ), that entire path includes the grounded conductor; in the test that shows high resistance (MΩ), the grounded section has been isolated and is no longer in the measurement.