NFPA 86 requires ovens, furnaces and related equipment to be designed to minimize the fire hazard inherent from equipment operating at elevated or high temperatures. Limit controllers are an integral tool in complying with the safety standard.

This socket-style high-limit controller meets the requirements of NFPA 86. Limit controllers that can be installed inside a control panel also satisfy the safety standard.


End-user safety must be of the highest priority for OEMs of process heating equipment, including ovens and furnaces. To ensure safety, the United States government established a system of expert safety guidelines, which includes standards from the National Fire Protection Agency (NFPA), as well as the nationally recognized testing laboratories Underwriter’s Laboratory (UL) and Factory Mutual Research Corp. (FM) agencies. Regulations or standards set by NFPA ensure that OEMs and remodelers of existing equipment meet minimum safety requirements. OEMs submit products for testing to UL or FM, which list products for use after they pass testing under certain requirements. Final approval must be given by an “authority having jurisdiction” to ensure that minimum standards have been met.

One key component that a furnace or oven needs for approval is an excess temperature limit controller. Based on portions of the 2003 NFPA 86 Standard as it relates to excess temperature limit controllers, this article will define limit controllers, discuss certain requirements and offer solutions for meeting safety standards.

NFPA 86 requires ovens, furnaces and related equipment to be designed to minimize the fire hazard inherent from equipment operating at elevated or high temperatures. As part of this safety requirement, NFPA 86 mandates that excess temperature limit controllers be installed on certain classifications of ovens and furnaces. An excess temperature limit controller is defined by the standard as “a device designed to cut off the source of heat if the operating temperature exceeds a predetermined temperature setpoint.” What this means is that the excess temperature limit controller must be a separate device with the purpose of turning off the source of heat to an apparatus if the process exceeds a predetermined temperature. This differs from the definition of a temperature controller, which is “a device that measures the temperature and automatically controls the input of heat” into the oven or furnace.

All limit controllers should have the ability to latch off on a fault condition and at power up, as well as the ability to be manually reset, according to NFPA 86.

Electrical Wiring Requirements. As integral parts of the process, limit controllers have a list of requirements to meet. Electrical wiring is an important NFPA 86 requirement. The standard references The National Electrical Code, NFPA 70 Standard, and Electrical Standard for Industrial Machinery, NFPA 79 Standard, to ensure that minimum requirements for safe installation of electrical wiring and equipment are being met.

In addition to the NFPA 70 and NFPA 79 standards, NFPA 86 requires OEMs and remodelers of existing equipment to submit complete plans, sequence of operation, and specifications for approval by the authority having jurisdiction. It further requires electrical wiring diagrams and sequence of operation for all safety equipment, including excess temperature limit controllers, to be provided with this submittal.

Installation Specifications. In gas-fired and fluid-heated applications, “an excess temperature limit controller shall be provided and interlocked into the combustion safety circuit.” As part of the combustion-safety circuit, the contacts of limit controllers “are arranged in series ahead of the safety shutoff valve(s) holding medium.” Electrical heating systems have a similar requirement for providing a limit controller and interlocking it into the safety circuit. The intention here is that the limit controller be installed so that the resultant control is an “and” function, to shut down the heat source, in an effort to minimize the chance of an actual fire occurring.

There are exceptions for Class B, C and D ovens and furnaces for excluding limit controllers. The exclusion requires a demonstration “that the maximum temperature limit specified by the [oven or] furnace manufacturer cannot be exceeded.” OEMs and remodelers of existing equipment should use caution when taking this exception and weigh the cost of designing and producing an oven that would allow for this exception vs. the cost of including a limit controller. It is the authority having jurisdiction that has the final approval on whether or not a limit controller should be included.

In addition, all instrumentation and control equipment must be installed in a common location for “ease of operation, adjustment and maintenance, and be protected from physical and temperature damage and ambient hazards.” When selecting an excess temperature limit controller, some consideration should be given not only for the installation location, but also for the physical size of the controller as it relates to the purpose of operation and clearance around the device, and for wiring.

Operational Mandates. Operation of the controller also is key to NFPA 86 requirements. As previously stated, the operation or purpose of limit controllers is to shut off the source of heat prior to reaching the maximum operating temperature of the oven or furnace, and it pertains to gas-fired as well as electric- and fluid-heated furnaces and ovens. Including the sequence of control in the submittal process helps ensure that this requirement is met, and proper installation and use of the limit controller determines whether the oven or furnace performs this action. Resetting of the excess limit temperature controller must be manual prior to the restart of the oven or furnace and must return the operation to the “safe” condition upon startup, according to NFPA 86. In other words, end users need to reset the limit controller upon power up, or if power is momentarily lost. Also, the excess limit temperature controller is required to have temperature indication, and the setpoint must be displayed in units of degrees Fahrenheit or Celsius. Excess temperature limit controllers must also be separate from the operating temperature controller.

Listing Agencies. NFPA 86 mandates that the safety equipment “shall be listed.” Factory Mutual is a nationally recognized testing laboratory with an approval standard for excess temperature limit controllers. Limit controller manufacturers should secure the FM mark of approval, indicating that the product meets or exceeds the requirements of the standard set forth by Factory Mutual. OEMs should be sure the excess limit temperature controller being installed has been tested and approved for this purpose by Factory Mutual, indicated by the “FM” mark.

This article is intended to serve only as a guide. It is the responsibility of each OEM and end user to check with the authority having jurisdiction to ensure its products and equipment comply with all applicable codes and standards.

Sidebar:
Standards, They Are A-Changing

OEMs need to keep in mind that in 2007, revisions to NFPA 86 will be released. According to the Industrial Heating Equipment Association (IHEA), the NFPA 86 Standard for Ovens and Furnaces, which incorporates NFPA 86C, Standard for Industrial Furnaces Using a Special Processing Atmosphere, and NFPA 86D, Standard for Industrial Furnaces Using Vacuum as an Atmosphere, will be published as one comprehensive standard in 2007.

The requirements for the use of PLCs for burner management systems are among the significant changes in this new edition. Other changes include:
  • Confirmation of the removal of the infrared equipment requirement.
  • The change to safety shutoff valves for mixing machines.
  • Time delays of all types for safety devices.
  • The clarification of heating chambers under purge requirements.
  • The added requirements for the exception regarding safety shutoff valves in multiple burner systems for service proven above 1,400oF (760oC).


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