- Joined Oct 3, 2010
I work at a manufacturing facility that makes cables. We will make a reel of single conductor insulated wire maybe 40,000ft long, then submerge it in a water tank and megger it to the water. This is to check for pinholes in the insulation (faults) and get an insulation resistance reading. The (-) of the megger is connected to earth ground, and into the water at one end of a 20 ft pit; the (+) is connected to both ends of the conductor, elevated above the surface of the water. I have beef with this. I was sent out there to troubleshoot their megger not working. I tried several different things with my megger (working) and noticed that I was getting high resistance readings on the water alone. I had always assumed that water had very low resistance, but seemingly not so. I also believe that the resistance of water is going to vary depending on how far of distance between measuring points. My tests with the megger seemed to prove this at the time. So, my theory is that if you were to megger the reel at one end of the tank, near the ground wire, you would read Insulation resistance of the wire, plus resistance of ~1ft of water; then if you were to move that same reel to the far end of the pit, you would read Insulation resistance of the wire, plus resistance of ~20ft of water, which might be double the first reading. This could cause product which should have failed, to pass with flying colors due to being on the far end of the tank (the operator is only looking for a resistance value). I brought my concerns up to an electrical engineer and he defended the principle, saying that resistance of water is negligible and that it is constant throughout any body of water, no matter how far between measuring points. He can't be right, can he? If he were right, then we would be using water pipes to transmit electricity!