Choosing Between Grounded and Ungrounded Systems

When designing and installing electrical systems, electrical contractors must decide whether the system needs grounding or if it is better left ungrounded.  While most electrical systems require proper grounding, there are certain instances where the National Electric Code (NEC) has determined that the benefits of grounding do not outweigh the risks that can come with it.  In order to make this decision, electrical contractors must know the basics of grounding, and the advantages and disadvantages of each system.

The Importance of Proper Grounding

Grounding plays a critical role in many electrical systems because it helps protect both people and their electrical equipment from dangerously high voltages caused by lightning storms or line surges.  Proper grounding provides a current with an alternate path in an electrical distribution system, allowing it to flow to the earth in case problems occur.

While it is a common procedure in residential, commercial, and industrial applications, mistakes can be made, and it is important that electrical engineers adhere strictly to NEC rules for safe installation.  By ensuring that your installations are NEC-compliant, you can avoid serious mistakes, such as failing to install ground fault circuit interrupters where they are required.

Fault Types

Before you choose a grounding system, it is essential that you first know the different types of faults that can possibly affect your system.  The four main types of faults include the following:

  • Line-to-ground faults
  • Phase-to-ground faults
  • 3-phase faults
  • Arcing faults

While the most common type of fault in a three-phase power system is a line-to-ground fault, electrical contractors must be aware of all types in order to reduce electrical hazards that can damage both personnel and equipment.

Grounding Systems

There are three main types of grounding systems:  solidly grounded, high-resistance grounding, and ungrounded systems.  Each system has its pros and cons that should be carefully considered for better performance, practicality, and compliance with the NEC.

Solidly Grounded Systems

Most of the electrical systems today are required by Article 250 of the NEC to be grounded, and the most commonly used system is the solidly grounded system.  This type of grounding system is where the neutral is connected directly to the ground without intentional added resistance in the ground circuit.  Most often seen in industrial or commercial applications, a solidly grounded system can come in two different arrangements:  the wye arrangement and the delta arrangement.

Advantages

There are many reasons why a solidly grounded system can be advantageous for an electrical system.  This system allows users to easily detect faults and therefore isolate them quicker.  In addition, solidly grounded systems offer more control of transient over-voltages and can support line-neutral loads.

Disadvantages

The biggest disadvantage of a solidly grounded system is that it can cause extremely high value fault currents when used in high voltage distribution systems and possibly result in equipment damage.  Because of this danger, solidly grounded systems are used in low voltage systems.

High-Resistance Grounding Systems

High-resistance grounding systems are typically used in applications where continuous operation is essential.  This type of system can be achieved by grounding the neutral in order to limit the ground fault current to a lower value level.

Advantages

High-resistance grounding systems can be designed as stand-alone units or integrated into low voltage switchgear and medium voltage switchgear, and helps reduce electric shock hazards.  This system also limits the damage of equipment and provides plant facilities with uninterrupted process operations in the event of a ground fault.

Disadvantages

The problem that some may have with these systems is that there is the potential to leave a ground fault on the system indefinitely and the loss of neutral path.

Ungrounded Systems

Electrical systems that are ungrounded operate without a grounded conductor and only in certain systems does the NEC allow an electrical system to be ungrounded (such as isolated power systems commonly used in healthcare facilities).  They are usually required to have ground-detection equipment that is also clearly marked.

Advantages

The main benefit of ungrounded systems is that they allow continued operations of processes even when a single line-to-ground fault occurs.  Additionally, the chances of a line-to-ground fault evolving into a phase-to-phase or 3-phase fault is low.

Disadvantages

On the other hand, an ungrounded system makes it difficult to find line-to-line ground faults, and additional labor to find these ground faults will result in higher cost of maintenance. 

Conclusion

When it comes to choosing the type of grounding method for a power system application, electrical engineers must carefully weigh the pros and cons of each method, and consider the factors that can affect each system.  By doing so, safety and optimal performance of the electrical distribution system can be achieved.

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