Grounding Survey and Power Quality Monitoring After Repeated Drive Failures at a Brewery, South Central Texas (2021-2022)

Variable frequency drives kept burning out at a brewery in South Central Texas. In September 2021 the brewery asked us for two things: a survey of its electrical grounding and a period of power quality monitoring, with a plan of action at the end.

196 ground readings in two days

On October 12 and 13, 2021 we ran a point-to-point grounding survey. From reference points such as the main ground rod, the ground bus in the electrical room and the motor control centers in the brewhouse and the cellars, we measured the resistance to panels, motors, drives, disconnects, transformers, tanks and structural posts, 196 readings in all, and subtracted the test lead resistance from each. The median was about 0.6 ohm. Thirty-two readings were above 1 ohm, and three were effectively open: a battery charger with no ground and rust at the connection, a motor with no ground at all, and a drive that showed a high residual voltage instead of a resistance.

Twelve items to fix

The report singled out twelve locations. Besides the three open readings, they included disconnects and motors reading from about 2.8 to 4.3 ohms, a loose ground wire on a motor, an improper ground clamp, a broken ground with signs of high heat, a pump control box with mold that needed cleaning and a heater, and corrosion and standing liquid at a yeast to beer interface. Some of these are ongoing corrosion problems, and left alone they raise shock risk as well as damage equipment.

Power quality: not the main cause

Voltage harmonic distortion on the primary feeders of the bottling plant ran about 2% to 3% depending on the running load, under the usual 5% guideline. At the control panels it held near 3%. No power quality events were recorded during the monitoring. Moderate harmonics still add heat inside control cabinets, and that heat has to be removed by fans, heat sinks or air conditioning. The full data set went to the brewery in a format their staff could open and sort.

What we recommended

Fix the twelve grounding issues. Return the cabinet air conditioners to working order, because constant heat and moisture at ambient conditions will destroy the drives. Fit line filters on any replacement drive, use higher pulse drives on motors of 50 hp and above, and install space heaters in control cabinets located where it falls below about 45 °F. The report was issued in February 2022.

Related services

The monitoring side is described on the page for power quality and harmonic studies.

The testing side is described on our page for grounding system tests. Loose and corroded connections like these also show up in infrared inspections.