Should I install an ungrounded, solid, or high-resistance grounding system? That is the question asked by many designers and installers. The answer to this question depends on many factors. To make the correct decision, you must completely understand the pros and cons of each type of system. But first, you must also understand the different types of faults that can occur on your system and in what frequency they may appear.
Faults and Failures. Faults can damage equipment and facilities, drive up costs due to lost production time, and lead to employee injuries, and even fatalities. The four types of faults include:
Line-to-ground faults, which represent about 98% of all failures.
Phase-to-phase faults, which account for about 1.5% of all failures.
3-phase faults, which make up less than 0.5% of all faults and are often caused by human error. Failure to remove a grounding breaker, leaving ground clusters on systems, and lifting a truck bed into an open wire system can cause this type of fault.
Arcing faults are intermittent failures between phases or phase-to-ground. They’re discontinuous currents that alternately strike, extinguish, and strike again.
Now that we’ve addressed the different types of faults that can appear on an electrical system, it’s time to provide an overview on the three main types of grounding systems you may encounter in the field.
Grounding systems.
1. Ungrounded. Electrical power systems that are operated with no intentional connection to earth ground are described as ungrounded. Although these systems were standard in the ’40s and ’50s, they’re still in use today. The main advantage of this type of grounding system is that it offers a low value of current flow and reliability during a fault. Unfortunately, this type of system also offers some big disadvantages. One major disadvantage to an ungrounded system is in the difficulty in locating a line-to-ground fault. Finding the fault is a time consuming process. For that reason, it’s often done on the weekends so a company doesn’t have to shut down its normal production processes. In addition, the fault must be located and repaired quickly because if a second fault occurs, the fault acts like a phase-to-phase fault extending the repair process.
Advantages
Offers a low value of current flow for line-to-line ground fault (5A or less).
Presents no flash hazard to personnel for accidental line-to-ground fault.
Assures continued operation of processes on the first occurrence of a line-to-ground fault.
Low probability of line-to-ground arcing fault escalating to phase-to-phase or 3-phase fault.
Disadvantages
Doesn’t control transient overvoltages.
Cost of system maintenance is higher due to labor involved in locating ground faults.
A second ground fault on another phase will result in a phase-to-phase short circuit.
2. Solidly grounded. This type of grounding system is most commonly used in industrial and commercial power systems, where grounding conductors are connected to earth ground with no intentional added impedance in the circuit. A main secondary circuit breaker is a vital component required in this system, although it has no bearing in other grounding systems. This component is large in size because it has to carry the full load current of the transformer. Back-up generators are frequently used in this type of grounding system in case a fault shuts down a production process. When this happens, the generators become solidly grounded. However, it’s important to note that the generators aren’t designed for the larger short circuit current associated with solidly grounded systems.
A solidly grounded system has high values of current ranging between 10kA and 20kA. This current flows through grounding wires, building steel, conduit, and water pipes, which can cause major damage to equipment and shut down production processes. When a line-to-ground fault occurs, arcing can create flashes–generally in the terminating box. In this enclosed area, water is turned to steam, causing the terminating box. To locate the fault, all you need to do is follow the smoke.
Advantages
Good control of transient overvoltage from neutral to ground.
Allows user to easily locate faults.
Can supply line-neutral loads.
Disadvantages
Poses severe arc flash hazards.
Requires the purchase and installation of an expensive main breaker.
Unplanned interruption of production process.
Potential for severe equipment damage during a fault.
High values of fault current.
Likely escalation of single-phase fault to 3-phase fault.
Creates problems on the primary system.
3. High-resistance grounding. High-resistance grounding (HRG) systems are commonly used in plants and mills where continued operation of processes is paramount in the event of a fault. High-resistance grounding is normally accomplished by connecting the high side of a single-phase distribution transformer between the system neutral and ground, and connecting a resistor across the low-voltage secondary to provide the desired lower value of high side ground current. With an HRG system, service is maintained even during a ground fault condition. If a fault does occur, alarm indications and lights help the user quickly locate and correct the problem or allow for an orderly shutdown of the process. An HRG system limits ground fault current to between 1A and 10A.
Advantages
Limits the ground fault current to a low level.
Reduces electric shock hazards.
Controls transient overvoltages.
Reduces the mechanical stresses in circuits and equipment.
Maintains continuity of service.
Reduces the line voltage drop caused by the occurrence and clearing of a ground fault.
Disadvantages
High frequencies can appear as nuisance alarms.
Ground fault may be left on system for an extended period of time.
Grounding of an electrical system is a decision many of us face on a daily basis. As we’ve seen, several methods exist to accomplish this task, each offering its own advantages and disadvantages. As an electrical designer or installation professional its up to you to make the final decision as to when best to install the most appropriate system.
Jack Woodham, P.E., is a senior electrical engineer for Jedson Engineering, Inc.
Editors note: The information presented in this article was based on a presentation given at a grounding symposium in October 2002 and hosted by Post Glover Resistors.