Static electricity creates a fire risk for flammable liquid containers subject to pumping, pouring, transferring or bulking of the flammable liquids into or out of the container. This risk is mitigated through the proper bonding or connecting of these containers to an electrical grounding system.
Bonding and grounding at University of Wisconsin System campuses should be installed in accordance with NFPA Chapter 77 “Recommended Practice on Static Electricity,” NFPA Chapter 45 “Standard of Fire Protection for Laboratories Using Chemicals,” and any local codes. This page will address some typical problems, and provide some points for safety professionals at UW System campuses to consider when addressing this issue.
Frequently noted problems with bonding and grounding systems for flammable liquid containers and piping include:
(1) inappropriate choice of what is used as a pathway to ground,
(2) poor connections of bonding and/or grounding conductors to flammable liquid system components,
(3) damaged conductors, and
(4) multiple containers in series.
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This is risky because conduit often becomes separated at joints due to vibration, collision, etc. Once the connection is broken, there is no pathway to ground and whatever is connected to the conduit is not grounded. Also, threaded connections can rust in damp or corrosive environments, which may result in loss of electrical conductivity and grounding.
This also is risky. Most often, the threaded connections of such piping systems are wrapped with Teflon tape to ensure an airtight seal. Unless each piping connection is tested for electrical continuity and the overall system is tested to confirm effective connection to ground, there is no assurance that anything connected to the piping is rounded.
Unless the point of connection is electrically tested and confirmed to be connected to ground, there is no assurance that the items connected to it are grounded.
Even though these clips and clamps usually have small teeth or a chisel point, they are seldom applied to the items with sufficient force or diligence to break through the painted surface and ensure a metal-to-metal contact. The paint acts as an insulator and prevents flow of electricity. Consequently, the connection is ineffective.
The bonding and/or grounding conductors are connected to containers with bucket-style handles by closing the clamp or clip around the handle so that it simply encircles the handle without making solid metal-to-metal contact. There may be an electrical connection at times if the handle is unpainted, but the connection is intermittent and unreliable. Most likely, there would be no connection on painted handles.
Most often, the ring, the lid and the drum body are painted. While the bolt is in electrical contact with the ring via its threaded connection, there is no assurance the ring is in electrical contact with the other drum components due to the paint barrier.
Occasionally, bonding and/or grounding conductors become damaged due to crushing by heavy drums being set upon them, or they become rusted and broken near the wire and clip connections where there is no protective coating over the wire. Also, spring-loaded clips and clamps sometimes lose their elasticity and no longer hold onto a surface well enough.
Multiple containers sometimes are bonded to one and other in series, with just one container connected to ground. When done correctly, this can be effective. However, it is an unnecessarily risky arrangement because each container is subject to any problem that occurs between it and the grounding connection. A poor connection on one container can result in loss of grounding to a number of other containers. In this system, one poor connection creates more risk than the alternative of connecting each container to ground individually.
Good practice principles
Whether the situation consists of multiple pressurized spray containers at a spray booth, or a series of drums connected to a dispensing system in a paint locker, or just a single container of flammable liquid or contaminated rags, the same principles of bonding and grounding apply.
- Confirm the ground. Confirm that whatever the grounding conductor will be attached to actually is grounded (preferably tied into the building grounding electrode system).
- Create a reliable electrical connection. Attach the grounding conductor to the source of ground permanently with a reliable means that will ensure continued metal-to-metal contact (e.g., mechanical, bare-metal connection via bolt and nut; welded connection; etc.).
- Chisel-point clamps. If the grounding conductor will be used to connect a single item to ground, ensure that the connecting end of the conductor is equipped with the type of clamp that has a chisel point bolt and requires tightening with a wrench. This type of clamp will break through the paint on drums and other items that may need to be grounded. Do not use weak spring-loaded clips and clamps.
- Conductor wire. Use a conductor that is sufficient for the application such as No.4 copper wire for grounding and No.6 copper wire for bonding. (NOTE: This is for connecting to an existing grounding system, not for creating the grounding grid. The grounding conductors in the grid must be sized per the NEC Article 250.)
- Clamp to conductor connection. Make sure the clamp at the end of the grounding conductor is attached to the conductor in a reliable fashion that will ensure continued metal-to-metal contact (e.g., compression fitting, welded connection, etc.).
- Grounding multiple items at one location. If multiple items need to be grounded at one location, install a main grounding conductor with lateral conductors attached to it. In most cases, the main conductor will be run horizontally next to the items that need to be grounded. The laterals should be attached to the main conductor at locations nearest the respective items to be connected. Install the main conductor so that it is solid and taut. Keep the lateral conductors as short as feasible. Connect lateral conductors to a main conductor in a fashion that will ensure continued metal-to-metal contact (e.g., compression fitting, welded connection, etc.). Do not use spring-loaded clips or clamps.
- Bonding straps. When constructing a bonding strap, use a suitable conductor (e.g., No.6 copper wire) and two chisel point clamps that require tightening with a wrench. Make sure the clamps are attached to the ends of the conductor in a fashion that will ensure continued metal-to-metal contact (e.g., compression fitting, welded connection, etc.).
- Paint and other coatings. When applying bonding or grounding straps to containers or other items, make sure the chisel point clamps are tightened sufficiently to break through any layers of paint or other coatings so a metal-to-metal connection is achieved.
Reference material and information
- How Do I Work Safely with Flammable and Combustible Liquids? (Static Electricity). Canadian Centre for Occupational Health and Safety.
This publication was prepared for environmental, health and safety staff at University of Wisconsin System campuses, to assist in finding resources and information for regulatory compliance. It is not intended to render legal advice.