Grounding electrical systems properly is crucial for safety, yet many of the requirements in electrical codes are obscure or overlooked. As an electrical engineer specializing in power systems, I often see issues that stem from inadequate grounding during inspections. In this article, I will provide an overview of key grounding requirements, explain why they matter, and share real-world examples where lack of proper grounding led to dangerous situations.
Why Proper Grounding is Essential
Grounding provides a low impedance path for fault currents to flow to the ground. This prevents the current from flowing through equipment or people, where it could cause damage, fire, electrocution, or death.
In a properly grounded system, when a hot conductor accidentally contacts a grounded enclosure or other conductive surface, it will trip the overcurrent protection device quickly by creating a short circuit to ground. Without adequate grounding, a fault might go undetected and pose a dangerous electrical shock or fire hazard.
While most electrical engineers understand the importance of grounding, many specific code requirements remain murky. Unclear, buried grounding rules in the National Electrical Code (NEC) and confusion regarding how and when to apply them means improper or missing grounding often slips through in the real world.
I've seen the consequences of poor grounding at both new and old facilities I've assessed. Below I will summarize key grounding guidelines, explain their rationale, and share cautionary tales from the field.
The Multiple Layers of Grounding Systems
Grounding and bonding involves connections at multiple levels:
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Equipment grounding provides a path from enclosures, motors, vessels, and other conductive surfaces back to the system ground.
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The system grounding electrode system connects the power source to earth via conductors and electrodes.
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Bonding ties these grounding systems together.
Each layer works together to shunt wayward currents directly to the earth. Problems arise when one or more elements are undersized, disconnected, or missing entirely.
Key NEC Rules for System Grounding Electrode Systems
The NEC contains directives for proper grounding electrode system installation. Here are some of the major requirements that are commonly overlooked:
Article 250.50 - Grounding Electrode System and Grounding Electrode Conductor Materials
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Copper, aluminum, copper-clad aluminum, steel, and stainless steel are acceptable grounding electrode materials. However, aluminum and copper-clad aluminum cannot be used where they are in direct contact with the earth.
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Grounding electrode conductors can be bare or insulated but must be sized appropriately per NEC table 250.66.
Article 250.52(A) - Concrete-Encased Electrode (Ufer)
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In new construction, a concrete-encased electrode consisting of at least 20ft of steel reinforcing bar or conductive wire must be installed. This provides a grounding path utilizing the rebar grid in the building foundation.
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When present, the concrete-encased electrode must be bonded to the grounding electrode system. Failing to connect to the Ufer ground where available is one of the most common code violations I see.
Article 250.53 - Grounding Electrode System Installation
This section lays out the required elements of a proper grounding electrode system:
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A concrete-encased electrode per 250.52(A)
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A ground ring encircling the building/structure with at least 20ft of bare copper conductor buried at least 2.5ft below grade
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Two or more ground rods at least 8ft long spaced at least 6ft apart
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A metal water pipe in direct contact with the earth for 10ft or more
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Other available electrodes such as metal building frames connected to the earth
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All electrodes must be bonded together to form the grounding electrode system.
Article 250.56 - Resistance Requirements
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The maximum resistance between the grounding electrode system and earth ground should be no more than 25 ohms for commercial/industrial systems or 5 ohms for transmission towers.
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If this resistance cannot be achieved with a single electrode, additional electrodes must be installed at least 6ft apart until the requirement is met.
Let's look at some examples of how lack of adherence to these rules created hazards in real electrical systems.
Real-World Cases of Improper Grounding
Missing Ufer Connection in New Construction
I conducted an inspection at a new industrial facility and discovered that the concrete-encased Ufer ground was present but had never been connected to the grounding electrode system.
This violated NEC Article 250.52(A) and created a potentially dangerous situation, especially considering that the other electrodes alone measured 50 ohms to ground.
I advised them to immediately bond the rebar inside the foundation to the main ground ring to reduce the overall system resistance before energizing the new equipment.
Corroded Ground Rods and Conductors
At an older facility, I measured extremely high resistance between the main service panel ground bus and the ground rods and ring conductors outside the building.
Investigating further, I found that the bonding connections to the external ground electrodes were severely corroded.
Exposed copper ground wires were so badly oxidized that they crumbled apart by hand, while the rods themselves had wasted away to a fraction of their original size after decades in acidic soil.
These findings explained why parts of the building's steel frame and some exterior metal equipment emitted a tingle when touched - they were energized from a ground fault due to the high-resistance path back to the panel.
To correct this, I had the ruined conductors and ground rods replaced and used irreversible compression connectors everywhere to ensure good continuity. The ground resistance immediately dropped back to an acceptable level after repairs were made.
Missing Main Bonding Jumper in Service Panel
I spotted a serious red flag at a restaurant's main panel: there was no bonding jumper between the neutral and ground bars inside.
With the neutral-to-ground bond missing, neutral current couldn't return safely to the transformer. Any fault would likely energize the building frame and metal equipment housings.
The owner was shocked when I explained the hazard since the original installer hadn't connected this critical link. We promptly added heavy bonding jumpers between the bars according to code 250.24(A) to restore a safe ground path.
Following Obscure Rules Prevents Danger
These examples illustrate how obscure NEC grounding rules are easily overlooked, even though violating them can leave systems vulnerable to fires, equipment damage, or serious injuries.
Staying up-to-date on the latest grounding codes, thoroughly inspecting installations, and correcting any long-standing ground deficiencies are the keys to ensuring public and worker safety. Though robust protective grounding doesn't get much attention, following the nuances of the code can prevent catastrophic consequences down the line.
As an electrical engineer, keeping obscure grounding requirements on my radar has helped me avert disaster and improve safety at facilities that need upgrades. While proper grounding practices may not be exciting, skipping the details can put infrastructure and lives at risk.