It was an unassuming late January day in Southern California. The weather was fantastic with clear skies and reasonable temperatures. It was a perfect weekend for skiing, hiking, or even surfing at local beaches. Unfortunately, the weather was about to take a turn for the worst, and no one was prepared.
On Jan. 31, 2015, at 2:30 PM a severe storm hit the inland area and local mountains. The storm brought high winds, hail, tremendous rainfall to the lower elevations, and blizzard conditions at higher elevations. The storm caused widespread damage and electrical outages. One of the powerlines in the path of the storm was damaged by winds strong enough to snap a down-guy wire supporting a pole. When the down-guy came loose, it made contact with a 12kV electric utility distribution line, causing a transient voltage for approximately 30 cycles.
On this day of extreme weather, a farm that grows flowers and house plants experienced damage to two 125-hp variable frequency drives (VFDs). The two VFDs operated two 125-hp pump motors. Due to local environmental regulations, no water runoff from irrigation was allowed. These two pumps moved runoff water from a lower retention pond to the upper retention pond. The farm used the upper retention pond water to irrigate the plants. Without the pumps operating properly, the owners were faced with potential fines. It was imperative to identify the cause of the damage and place the VFDs online.
Upon inspection, multiple National Electrical Code (NEC) grounding and bonding violations were identified at the VDF cabinet and electrical service. VFDs require a low impedance ground path and proper bonding for the protection settings to operate properly. All the necessary protection values require the low impedance ground to calculate high and low voltage, as well as voltage and current imbalance. Without the low impedance ground, the VFD is exposed to potentially damaging voltage ranges. The VFD manufacturer analyzed one of the damaged drives and found the final voltage reading before the failure was 553V.
At the service, the grounding electrode system (GES) was properly established. The main disconnect had a supplemental ground rod and was lacking the grounding electrode conductor (GEC) from the service GES. The GEC also did not extend to the VFD cabinet. At the VFD cabinet, another supplemental ground rod was installed and connected to the metal backboard. The VFD was only grounded to the motor housing.
This improper grounding and bonding installation allowed the VFDs to be exposed to “backdoor” transient voltage during the storm. Had the installation been properly grounded and bonded per the NEC, the drive protection settings would have safeguarded it.
The minimum requirements of the NEC are often all that is required to properly protect electrical equipment. Additional protection measures can be taken to install properly sized SPDs at the service and VFD cabinet. The manufacturer's installation requirements will provide additional guidance. However, it’s your job to ensure that the NEC, state, or local requirements are met.
