Lightning Strikes Again – Countering Lightning Strike Failures in Instrument & Control I/O Circuits
By Mike, Inside Technical Sales Representative
What does a golden oldie song title and a plumbing jingle have in common with countering lightning strike failures in instrument and control I/O circuits?
Every time I field a call from a customer (most often a utility/power gen plant), after a lightning strike has taken out signal isolators, transmitters, and/or monitoring/control devices; my memory’s ear plays that song by Lou Christie; Lightning Striking Again. In addressing a solution for a customer with reoccurring lightning-induced signal I/O failures, I recall the timeless Roto-Rooter commercial jingle; “there goes trouble down the drain.” (Possibly the longest continuously running advertising jingle). What does a jingle and a famous song have to do with addressing lightning-induced failure events? When considering signal isolators for circuits prone to lightning strikes (or transients/surges/electrical noise), it’s important to know that the signal isolator/transmitter has built-in components. These components “dissuade” the transient energy from continuing down the signal I/O path; therefore encouraging the transient energy away to earth ground through virtue of impedance. This is only part of the picture in protecting instrumentation from transient energy.
Lightning Strike Failures in Instrument & Control I/O Circuits
When lightning’s striking again, it has one purpose; getting home to earth ground by the easiest and quickest path available to it. Therefore, it’s essential to incorporate an effective path of least resistance to earth grounding methodology. In an instrument and control signal input and output circuits, the control signal must get through with as little interference as possible. Acromag signal isolators and transmitters have components including diodes, caps and TVS (transient voltage suppressors) to deter and act to steer away the non-control signal transient energy. The signal I/O transmitters isolation is rated to withstand 1500VAC for one minute; however the isolator is just part of the control signal circuit to consider for transient interference. Quality twisted-pair cable with overall shield, tied to a good earth ground termination and leads kept from being fanned apart; help reduce the entry points of the transient energy.
Be cognizant that these isolator “filter” parts don’t dissipate the transient energy, they only help steer it to ground connections. It’s important to realize that all the caps and TVS’s do is “steer” this energy around the circuit wiring in the absence of an actual destination to ground. Thus, by failing to give it someplace to dissipate means that it will blow along its own (potentially destructive) path. Think of loop isolators as roadblocks along the path to earth ground. The isolators help to redirect/steer this destructive transient energy in another direction; sending transient trouble down the drain “wire” safely home to earth ground.
With over sixty years in signal conditioning design and manufacturing, Acromag has many helpful technical application notes and white papers available. I often recommend these two; relating to signal isolation and interference issues such as ambient noise, ground loops and yes, lightning striking again:
To steer you in the right direction of current loop isolators, our 650T and TT series are available. For signal converter/transmitters that include signal isolation, Acromag has many I/O products to choose from including limit alarms.
Lightning is an equal opportunity act of nature; ever-searching for a path to ground when the atmospheric potential is right. This means instrument controls and monitoring systems, including those incorporated in the information explosion of IIoT, are also susceptible. Acromag’s network I/O (Ethernet & RS485) transmitters incorporate isolation filtering on analog, discrete, power and network ports; keeping the information flowing and helping send trouble down the drain whenever lightning is striking again, and again, and again.