In the United States OSHA Safety Standards dictate that all products designed to be used in hazardous locations must be approved by a nationally recognised testing laboratory (NRTL).
These requirements are designed to protect employees who work in hazardous areas, by ensuring products are designed for safe use in hazardous workplaces. In the US, OSHA Safety Standards contain general requirements for workplace safety, while a NRTL generally certifies products for a manufacturer to standards recognised by OSHA.
The only products covered under the NRTL Program are those for which OSHA regulations require certification by an NRTL. In order to be recognised as an NRTL, an organisation must have the necessary capability to act in the capacity of both product safety testing laboratory and product certification body.
OSHA's recognition is not a government license, but rather acknowledgment that an organisation is adequately able and appropriately qualified to perform safety tests on equipment manufactured for use in explosive atmospheres.
The two largest NRTLs are Underwriter’s Laboratories (UL) based in Illinois and FM Approvals LLC based in Massachusetts.