While it’s true that the pre-commissioning and commissioning stages can be complex, it’s exciting to know that when fine-tuning elements like solenoids, beacons, or sounders, any issues with HMI activation can be swiftly overcome. The field-termination-assembly (FTA) models TDOL-0724P, TDOL-0724U, and TDOL-07120, although sometimes overlooked in the engineering phase, offer an insightful cue: they necessitate the installation of a diode for loads with resistance under 400 ohms.
Implementing this small but crucial step can significantly enhance the performance. Even if initial indications suggest faults on the HMI, displayed as “magenta” for discrepancies like short circuits, open circuits, or lead breakages, we’re often pleasantly surprised to find that the components function effectively in the field. This difference can present an opportunity to delve deeper into the system and uncover solutions that ensure the HMI accurately reflects the operational status. Such challenges remind us of the remarkable intricacies of our field and the joy of solving puzzles to achieve seamless functionality.
Honeywell FTA for LMDO
Within the Honeywell system’s architectural framework, the line-monitoring digital output is engineered to offer a variety of state configurations (refer to Figure 1 below).
Based on my findings, the detected fault is classified as a “short-circuit” fault within the Safety Manager logic. Upon inspecting the fuse, its condition appears to be intact, as evidenced by the continued operability of our solenoid. However, the observed load resistance seems unusually low, although the current remains below the 2A fuse rating. This discrepancy contradicts the information outlined in the SM hardware manual (refer to Figure 2 for the schematic and Figure 3 for further explanation). No lead breakage but logic recognizes it as “short-circuit”.
(for other type is typical but not exactly similar)
As per SM hardware reference manual, there are three types of FTA for line-monitored relay contact digital output, as follow:
- TDOL-0724P for standard LMDO card (7 channel, 24 VDC)
- TDOL-0724U for RUSIO card (7 channel, 24 VDC). RUSIO card (by default) all DO are line-monitored.
- TDOL-07120 for standard LMDO card (7 channel, 120 VAC / 120 VDC)
all of those required to install diode for any loads that have resistance below 400ohm (see below Figure 4).
Diode type 1N4004 (or equivalent) at load currents up to 0.7Amp, or of type 1N5404 (or equivalent) for loads up to 2Amp is required to solve the issue.
Problem and Solution
Upon discovering a faulty indication for our solenoid valves, as manifested by a magenta color display on the HMI, we proceeded to assess the resistance of the solenoid at the field level.
We resolved the issue by installing the provided diode 1N4004 in the junction box, as there was insufficient space in the SOV terminal for installation.
Additionally, after further consideration, we devised a solution that involved utilizing terminals equipped with diodes to streamline operations at the site.
Lessons Learned and Best Practices
This type of fault is not typically detectable during the engineering stage unless one possesses prior commissioning experience. The nonlinearity of load resistance means that simply doubling the resistance does not result in a proportional doubling of the power dissipated; instead, it increases by a factor of four, as per Ohm’s law. Therefore, when dealing with outputs that demand higher power (exhibiting low resistance), this issue must be taken into consideration.
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