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Electrical Troubleshooting Quiz

You're a maintenance electrician for a Midwestern company that provides a range of digital services from a five-story office building it owns. This past summer, people complained about getting shocks from various equipment located throughout the building. Given how dry it was this summer, the complaints were chalked up to static electricity due to low humidity. You'd gotten a few shocks yourself, after walking across the carpet.

But it's no longer summer, and yesterday you got a shock of a different sort. Using your DMM, you confirmed steady current flow between the AC receptacle ground and a connector on a scanner. What could be causing this?

You've found a difference of potential between two ground systems, namely the network ground and the AC power ground. During this summer's drought, severe soil movement may have destroyed the bonding connection between these systems. But was there a connection in the first place?

Soil resistivity rose dramatically during the drought. So, if the bonding jumper between these was earth instead of wire, the impedance between these systems also rose dramatically. When you lack a low-impedance path between two grounding systems, you'll have a difference of potential between them.

The immediate solution is to bond these systems (and any others) together per Art. 250, Part V. Longer term, you want to gradually move the network connections away from the users — so those connections exist only on the wired side of wireless routers. As you replace scanners, etc., ensure they are wireless-enabled and connect them to the building-wide wireless network.

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