What You Don't Know About Hoses Can Kill You...Because electrical conductivity or non-conductivity is not a consideration for many applications, hose manufacturers do not commonly publicize hose electrical resistance ratings. That being the case, most people do not think about conductivity when selecting a hose; some just assume that all rubber hoses are electrically non-conductive. After all, rubber is non-conductive material, right? Wrong.
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CSB Lead Investigator Randy McClure said, "The CSB investigation found the nozzle and hose were not intended for use in transferring flammable liquids. Furthermore, we found the steel parts of the plastic fill nozzle and hose assembly were not bonded and grounded…”
It turns out that rubber hoses can be either electrically conductive, non-conductive, or partially conductive. Some rubber hoses can be non-conductive at low voltage, but conductive at high voltage. It may be surprising to some, but many materials are used in the production of “rubber” hoses. For example, reinforcement layers are commonly made of metal. In addition, some rubber compounds are electrically conductive.
Unless the hose is described as, or specifically and clearly branded as, either electrically conductive or non-conductive, you must assume that the electrical properties are not controlled in the manufacturing process. The hose’s electrical properties may change with each production run without notice.
Static Electricity in Rubber Hoses
As material moves/flows through a hose, molecules collide with each other. During the molecular collisions, some electrons are “knocked off” and create a tiny amount of electrical charge (or electrical potential, measured in Volts). The electrical charge increases with the length of the hose assembly, velocity of material transfer, material volume transferred, and coarseness of the transferred material and hose’s inner tube.
If a non-conductive hose is used, the electrical charge will accumulate in the hose fitting at the delivery end of the hose. If the fitting is not grounded, the charge will keep accumulating, until the potential difference between the charged fitting and the nearest conductive grounded object becomes high enough for the electrons to “jump” over to the object, creating an arc.
Depending on circumstances, the electric arc may ignite volatile materials in the vicinity of the hose, or inside of the hose.
Conductive Rubber Hose
Metal wire reinforcement layers and conductive rubber components are used to manufacture electrically conductive hoses. Electrically conductive materials prevent static electricity buildup and arc discharge.
Some types of conductive hoses use helical or static wires. This approach can be used for electrical continuity, as long as a solid contact is made and maintained between the wire(s) and the hose end.
Nonconductive Rubber Hose
In some situations, such as proximity to high voltage wires, it is necessary to use a non-conductive hose. In these situations, do not assume that the hose is non-conductive. Many black compound rubber hoses are inherently conductive. Unless the hose is specifically designed to be non-conductive, do not use it. None-conductive hoses are usually manufactured to a qualifying standard that requires it to be tested to verify its electrical properties. Non-conductive hoses often feature color sleeve (non-black), but not always.
SAE standard includes a requirement for non-conductive high-pressure hoses to have an orange cover. Parker industrial hose (found in Parker catalog 4800) generally uses Alcoa Aluminum standard for non-conductivity, which calls out a minimum resistance of one mega ohm per inch at 1,000 volts DC.
Do not use non-conductive hoses for any applications calling for conductive, static dissipating, anti-static, or similar requirement.
No Standard Practice for Use of Conductive Hose
Electrical engineers, hold different views on the effects of static electricity and suitable ways of dissipating it. There is no consensus among recognized national associations, laboratories, and users, on what a standard practice should be for using conductive hose.
Until such standards are established, it is necessary for users to examine the intended use of the hose, consult the hose manufacturer, and review the laws governing the conveyance of material in their location, in order to determine if a conductive hose should be used.
More on Hose Selection…
We like to use the acronym STAMP, to remind people of the most important hose features to consider when selecting any pneumatic or hydraulic hose. STAMP stands for Size – Temperature – Application – Media – Pressure.
The following catalogs include hoses designated as conductive and non-conductive:
- Catalog 4400 – Parker Hose, Fittings and Equipment
- Catalog 4660 – Parker Parflex Thermoplastic Hose and Fittings
- Catalog 4800 – Parker Industrial Hose and Couplings
Posted by Marek Bobik