Mechanical Switch Upgrades Made Easy

When considering upgrading the electro-mechanical switches that have been in service for many years, the task can seem daunting. These 2-wire alarm and shutdown devices have served the plant well for many years. Many of the switches have been in service for more than 15 years without failure. But these switches are blind (lack self diagnostics) and are difficult to calibrate.

Process transmitters seem like the obvious choice to replace these switches. Transmitters include diagnostics and don’t require calibration inspections as often as their mechanical brethren. But replacing a mechanical switch with a transmitter is overkill. The unit cost of a mechanical switch versus a process transmitter has a differential of 1.5 – 2 times the original cost of the switch. On top of that, there is wiring, analog inputs and control schemes to consider. And because the plant was built in the 1970s, there are switches everywhere!

Where does the One Series 2-Wire get its power? Directly from the digital input (DI) of the control system. By using a small amount of leakage current taken from the DI, the One Series can provide the same alarm and shutdown functionality previously provided by the mechanical switch – using the same 2 wires and unmodified control scheme.

2slpm041 (safety transmitter)

By using extremely efficient (e.g., low) power management techniques, the One Series is able to scavenge just enough current from a typical DI without tripping it and provide the same on/off switch signal formerly provided by replaced electro-mechanical switch. This patented technology allows the One Series to replace any mechanical switch that is connected to a PLC or DCS control system input and utilize the same switch control scheme and wiring.

No wiring or programming expenses. No MoC considerations – replace a switch with another switch. The cost of the upgrade is the cost of the One Series.


Is there an alternative? One that provides easy adjustability, self diagnostics and can operate using the existing plant wiring and control scheme? Yes! Consider replacing your electro-mechanical switches with a new breed of switch – the One Series 2-Wire electronic switch. The One Series provides IAWTM self diagnostics, configurable set point and deadband, digital process display and operates on the same 2 wires and control scheme as the old mechanical switch being replaced.


By Rick Frauton ,Senior Product Marketing Manager at United Electric Controls


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