Electrical Troubleshooting Quiz - July 21, 2015

Electrical Troubleshooting Quiz - July 21, 2015

A process line controlled by a Distributed Control System (DCS) has frequent failures.

A process line controlled by a Distributed Control System (DCS) has frequent failures. These aren’t isolated to any particular type of equipment. It seems everything associated with this line has a higher failure rate than similar equipment elsewhere in the plant. Consequently, the plant engineer ruled out a systemic power quality problem.

The DCS installation instructions require this line to have its own isolated grounding system to prevent undesired current from entering the system. Everything appears to comply with those instructions. Where do you start?

Start with that isolated ground, which violates the NEC and creates a difference of potential between this line and the plant’s other equipment (and distribution system). The equipment grounding path must be back to the source (e.g., service), not to some load side ground rod.

You can use a variety of electrical tests to verify that differences of potential exist between this system and the utility ground, so conduct two or three such tests to document what’s going on. Then properly bond this system per Art. 250, Part V of the 2014 NEC.

Use isolation transformers, not “isolated” ground rods, to keep undesired currents from entering this system.

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