You're right to question the need for grounding those underground metal storage tanks. Here's a breakdown of why you likely don't need dedicated grounding for them, and some considerations

Why Grounding is Typically Not Required for Underground Storage Tanks (USTs) containing collected fire suppression chemicals:
No Electrical Equipment Inside the Tank

The primary reason for grounding tanks in many industrial applications is to prevent the buildup of static electricity which could cause sparks and ignite flammable vapors inside the tank. If there's no electrical equipment inside the tank (pumps, mixers, level sensors, etc.) and the tank itself contains non-flammable chemicals, the risk of ignition from static is extremely low and not the primary concern of the NEC.

Inherent Grounding

The buried metal tank is already in contact with the earth, providing an inherent path for any static charge to dissipate. The large surface area in contact with the soil acts as a very effective natural ground.

Chemical Properties

SFFF (Synthetic Fluorinated Film Forming Foam) is designed to suppress fires and not be flammable itself. The vapor inside the tank after activation is unlikely to be flammable and create an explosive vapor in the tank.

Reasons why people sometimes think it needs to be grounded/bonded



Misunderstanding of Grounding Principles

Many people conflate "grounding" and "bonding." Grounding is typically done to provide a path for fault current to trip a breaker and protect electrical equipment and personnel. Bonding is done to equalize electrical potential between metallic objects to prevent voltage differences and potential shocks.

Inherited Practices

It's possible someone saw this practice somewhere else and assumed it was always necessary without understanding the underlying reason.

Overly Conservative Application of Codes

Sometimes, people will take a "better safe than sorry" approach, even if it's not specifically required by code.

Following the NEC by the book

If the tank is part of a sprinkler system then following NEC 250.104(B) allows for bonding of other metal piping systems.

When You Might Consider Bonding (Not necessarily grounding to a ground rod)



Lightning Protection System

If the building or nearby structures have a lightning protection system, bonding the tank to that system might be considered to equalize potential during a lightning strike. This is about mitigating the risk of arcing outside the tank, potentially through the soil to the building, not about internal ignition.

Proximity to Electrical Equipment

If the tank is very close to underground electrical conduits or other buried metallic structures carrying electricity, bonding the tank to those structures could help equalize potential and minimize stray currents.

Local Code Requirements

Always check with your local Authority Having Jurisdiction (AHJ). They may have specific interpretations or requirements that differ from national standards.

Important Considerations and Best Practices



Consult with an Electrical Engineer

This is the most important step. An experienced electrical engineer can assess the specific situation, review the relevant codes (NEC, NFPA, and local regulations), and provide a definitive recommendation.

Risk Assessment

Conduct a thorough risk assessment, considering the chemical properties, the location of the tank, and the presence of any electrical equipment or potential ignition sources nearby.

Documentation

Document your risk assessment and the rationale for your grounding/bonding decisions. This will be helpful for future inspections and modifications.

Corrosion Protection

Ensure the tank has proper corrosion protection. Grounding can sometimes accelerate corrosion under certain soil conditions.*

Review Fire Suppression System Design

Understand the overall system design and how the tank fits into the system.

In Summary:

While it's unlikely you need dedicated grounding for the underground SFFF storage tanks, consult with a qualified electrical engineer to ensure your design meets all applicable codes and best practices. They can properly assess the risks and determine the appropriate grounding and bonding requirements for your specific situation. Don't rely solely on anecdotal evidence or what you've seen in other situations without understanding the rationale. Flag for review