When equipment fails catastrophically, a cause can be hidden in the rat’s nest of the equipment’s electrical control wiring such as that found in a failed gas fired boiler, press brake (used for bending sheet metal), or an industrial air compressor, for example.
To help sort out the issues, preparing as-found electrical control drawings should be considered at the onset. The as-found control drawings might include the following types of drawings:
- Legend Drawing
- Equipment Location Drawing also known as a Flow or Process Flow Drawing
- Ladder Drawing also known as a Control or Schematic Drawing
The ladder drawing is organized in the shape of a vertical ladder using straight lines to clearly illustrate the logic of how the electrical control components were wired together. The ladder drawing straightens out the tangled-up rat’s nest of the control wiring so the control system logic can be analyzed. Wiring changes, inappropriate wiring connections, and electrical jumpers that bypass safety shutdown devices, for example, will become readily apparent.
Without as-found electrical control drawings, the cause of the failure may not converge to an answer independent of how voluminous the discovery and the number of depositions.
One method of preparing a set of as-found electrical control drawings is with a team of two people. One person is the scribe, and the other person physically traces each control wire relaying information to the scribe. When completed, the two people may trade positions to check the scribe’s sketches. The sketches are converted into CAD drawings for analysis and possibly exhibits for deposition and/or court.
This method of preparing as-found control drawings also works with programmable logic controller (PLC) and/or other solid state control devices.
In some industries, a horizontal ladder control drawing is standard rather than a vertical ladder format.
Image accredited to By David Lippincott for Chassis Plans