A metal manufacturing company complained of alleged frequent “power fluctuation”, “spikes”, and “high voltage” conditions that damage their CNC (Computer Numerical Control) machines. According to the company’s engineer, they are incurring a lot of losses due to the frequent replacement of their CNC machine’s power supply and operation stoppages specially during weekends since they operate fully automatic (without the need for operator intervention).

In addition, the company has experienced an incident wherein one of their CNC machines was severely damaged including their TVSS (transient voltage surge suppressor). Figures 1 and 2 show the damaged CNC machine controller.

Figure 1. CNC Machine Controller

Figure 2. Inside the CNC Machine ControllerSituation Analysis

The company is located in a lightning prone area and we suspected that it is likely that a lightning surge caused the damaged. The engineer however informed us that there was no clear indication of direct lighting hit in the area and that they have good system grounding.

To further assess the problem, a site visit and inspection was conducted. It was confirmed that indeed there was good system grounding but one important element was missing – the absence of the EGC (Equipment Grounding Conductor) that connects the Main Bonding Jumper and the company’s system grounding. Figure 3 shows the first private pole with the secondary feeder conductors and no EGC.

Figure 3. First Private Pole with the Secondary Feeder Conductors

Notice in Figure 3, four wires came into the service entrance but only three wires came out and extended into the company’s facility.

The VSOP board is a motor drive that is air-cooled, transistorized, PWM (Pulse Width Modulation) inverter in a very compact package. The chillers use it as a replacement of the old mechanical valve system used in the past to regulate oil pressure.

According to the company’s engineer, they often experience a VSOP failure/burnout after a power disturbance. He added that they have already replaced several VSOP boards in the past.Solution

Absence of the interconnection (in this case the EGC) between the company’s own grounding system and the transformer grounding may lead to malfunction/failure of sensitive electronic equipment specially those that utilize the ground as the reference point.

Whenever there are lightning strikes or faults (utility or end-users’ electrical faults) anywhere near the vicinity, the company’s system ground may have a potential that is different to that of the transformer’s grounding electrode. In cases where this potential difference is considerably high, current will be impressed upon the connected equipment and could cause significant damage. This is what may have happened to both the TVSS and the CNC machines.

To prevent this harmful current from damaging equipment, it was recommended to install the EGC and bond it to both the Main Bonding Jumper and the company’s own system grounding.

The company was pleased with the findings and recommendations. There were no other reported failures since 2008.