REGENCY PLASTICS SELECTS I-GARD HIGH RESISTANCE GROUNDING

System Information

The Regency Plastics facility's main distribution system is a 600V 34>3W delta configured system (i.e. ungrounded). The system supplies numerous drives and other manufacturing equipment throughout facility. The system is equipment with three ground fault indicating lights located on the west wall of the plant.

The distribution system is approximately 35 years old and very little data regarding the system's layout and equipment was available (i.e. no single line diagrams, as built shop drawing, etc.).

The plant has experienced several failures within their drives over the past several months.

A Dranetz PP-4300 Power Quality and Disturbance Analyzer was set-up at the 100A fused disconnect switch that feeds the "2M" drive. The Dranetz was set to monitor the three phase voltages with reference to ground.

Although the system is "ungrounded", that is a system of conductors with no intentional connection to ground, in reality the system is coupled to ground through the capacitance of the conductors, and transformers and motor windings. Under normal operating conditions (i.e. no ground faults) the three phase-ground voltages are roughly equal, in this case nominally 347 V.

However, when one phase of the system experiences a solid ground fault, that phase and ground are at the same potential and the remaining two phases experience an increase in voltage t0 ground of 73% (in this case nominally 600 V). Although under these ground fault conditions the phase-ground voltages change, the phase to phase voltages remain unchanged and the system continues in service as normal.

Unless left for long periods of time or frequent re-occurrences, these ground faults generally do not result in equipment damage or failure. The exceptions are the scenarios noted below.

 When one phase of the system becomes grounded, the occurrence of a second ground fault on a different phase results in a phase-phase fault via ground. These faults will result in equipment damage and operation of phase overcurrent devices (i.e. fuses, breakers).

Ungrounded systems are susceptible to severe overvoltages due to a couple of possible ground fault conditions. The first scenario involves a ground fault through an inductive reactance (i.e. operating coil, transformer winding, etc.). When the inductive reactance of the ground fault closely matches the system capacitance to ground, overvoltages as much as ten times normal can be experienced.

The second scenario involves an intermittent, sputtering or arcing ground fault (i.e. in vibrating equipment etc.). In this case, it is possible that the recurring ground fault can generate overvoltages in the 6-8 times normal range?

Findings

On-site observations, inspections of the ground fault indicating lights and discussions with the plant electrician revealed that the plant is frequently experiencing ground faults within the distribution system. The ground faults are identified by the ground fault indicating lights and the plant electrician traces and eliminates the source as soon as possible, although they often exist for days.

The on site monitoring confirmed the existence of the ground faults within the system and helped to illustrate the effects on the phase to ground voltages. On one day of activity in which an unidentified piece of equipment had a ground fault on the “A” phase, it was cycling on for 13 minutes, and off for 5 minutes.

During the ground fault condition, the A phase voltage to ground was approximately 190 V while the remaining two phases were approximately 410 V. The cycling of the equipment effectively created a repeating ground fault condition. There was no reported equipment damage on that day.

The failures which occurred to the equipment are likely a result of circumstances which exist that expose the distribution system at the plant to frequent ground fault conditions. The frequency of the ground faults increases the likelihood that conditions will be established that will result in severe overvoltages and/or multiple ground faults.

When these severe overvoltages are created by the ground fault, insulation breakdowns occur and phase-ground-phase faults develop resulting in equipment damage and lost production.

Recommendations

In order to eliminate the severe overvoltages due to ground faults, it is recommended that an artificial neutral (i.e. a zig-zag grounding transformer) and resistance ground be installed along with the appropriate ground fault monitoring equipment and protective fusing.

The artificial neutral should be installed between the main breaker and the main power transformer secondary so that the artificial neutral is always on line. IPC Resistors Inc. in Mississauga (I-GARD) specializes in the design and manufacture of zig-zag grounding transformers, ground resistors and ground fault monitoring equipment for ungrounded systems.

Given the frequent nature of the ground faults within the facility and the amount of time (and associated costs) spent tracing and eliminating these ground faults, it is recommended that in addition to the artificial neutral and ground resistor, a fault locating system be installed as well. The fault locating system is an add-on to the artificial neutral and resistor package.

The fault locating system will significantly reduce the time required to locate and eliminate the ground faults and hence reduce the likelihood of double ground faults resulting in failures.

In order to properly specify the necessary equipment, updated system information should be obtained including an up to date and accurate single line diagram detailing the on site equipment (i.e. cables, transformers, protective devices, etc.).