The Siemens LFS1 flame switch works in conjunction with a PLC. When the PLC operates as the combustion safeguard, it powers the shutdown valves, perform burner sequencing, monitors interlocks, and controls purging. The LFS1 flame switch communicates the existence of a flame to the PLC through a contact closure. The LFS1's small form conserves valuable panel space.
Flame safeguards have many names. We'll oversimplify and group all of these names to make a point. These names imply a stand-alone device detecting flame, energizing the shutoff valves, monitoring interlocks, and conducting purge. A flame switch is simply a contact. The contact closes when a flame is present and opens when a flame is not present. This opening and closing of the contact let the PLC know the presence of the flame. That's it. The sequencing, powering of the shutoff valves, monitoring, and conducting purge are all done in the PLC. The heart of the flame switch is the FR1 contact closing between terminals 11 and 12. The contact is normally open and closes upon the presence of the flame. The LFS1 takes on the liability of proving the presence of the flame, so the PLC doesn't have to. HR1 contact is an auxiliary contact used for other Siemens products. For a PLC application, SCC instructs to keep the power on terminal 6, resulting in HR1 being closed all the time.
NFPA 86 has criteria for the PLC to be combustion safety. Combustion safety here means the sequencing, powering shutoff valves, and conducting purge. There are two avenues for the PLCs to be the combustion safety: 1) listed for combustion service and 2) not listed for combustion service. The listed PLC is straightforward and designed for combustion safety. The non-listed PLC to be used for combustion safety must meet the criteria of section 8.4.2 in NFPA 86. Third-party certified SIL-2 and restricted access are a few of the requirements for a non-listed PLC. If the PLC is the combustion safety, all you need is the LFS1 flame switch.
Panel space is premium. The LFS1 flame switch is a compact device being 3.5 inches wide and 2.5 inches tall. The small footprint saves on space, especially for multi-burner applications. A mounting base goes with the LFS1, making it 2.5 inches deep.
The LFS1 can report the flame signal strength via a 0-10VDC output, making it helpful to display flame signal strength for the service technician or the operator. The LFS1 can condition the relationship between the microamp flame signal and the 0 to 10VDC signal.
The LFS1 has global approvals, most recently North American approvals of UL, FM, and CSA. The LFS1 is also SIL-3 capable.