We're in the US midwest and have to deal with lightning from summer thunder storms.

Our corn silos all have lightning arrestors installed and ground rods driven all over the place, but last week we took a hit somewhere that demolished an ultrasonic level sensor, along with a couple of AI cards, the loop power supply and the PLC controller module. The energy apparently went from the silo mounted level transmitter through the 4-20mA wiring to the controller.

I'm prepared to take the loss of the sensor and a power supply, but I was wondering if standard, commercial I/S barriers in the signal loop are likely to act as fast fuses which could prevent the energy from blowing up the AI's and controller.

Any thoughts?

David

 

Nothing will withstand a direct hit by lightening. However, this sounds like a great case for a 4-20mA isolator with built-in surge suppression.

A good 4-20mA isolator w/ built-in surge suppression, versus a simple i/s barrier (ie; energy limiter) or surge suppressor, will give you the added benefit of breaking the ground connection between the input and output. This will help protect your AI cards and controller.

Acromag offers a wide variety of single and multi-channel 4-20mA isolators/splitters that are AC/DC powered (4-wire), Output Loop Powered (2-wire), and Input Loop Powered (2-wire).

Here are links to some datasheets and application notes on products you may find useful:

Model 63xT (4-Wire isolators/splitters):
http://www.acromag.com/pdf/630T.pdf

Model 65xT (2-Wire isolators/splitters, output loop powered):
http://www.acromag.com/pdf/651T_652T_653T.pdf

Acromag's Series 600 4-20mA Isolator/Splitter Application Notes:
http://www.acromag.com/pdf/600T_appl_notes.pdf

--See app notes #5 and #6 for a quick reference to your possible
application.

If you have any questions or I can assist, please email me.

Kind Regards,

Donald Lupo (dlupo [at] acromag.com)

 

No, they aren't intended to be lightning arrestors.

One of the things that could likely have happened is that the power ground for the controller and the ultrasonic transmitter were connected to the lightning ground grid. Lightning might have passed through the ground grid, into the devices from either the power or instrument ground, and caused the damage. Some 20 years ago, I saw the same thing in Hawthorne, Nevada at the Navy Ammunition plant. I still have, somewhere, the plastic enclosure with the flash vaporized circuit board imprinted on the front and back of the inside of the box...and that was the conclusion we reached on the cause of that one.

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes [at] putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff

 

Surge protection devices (SPDs) protecting each signal wire, may have helped - by handling surges up to 20kA per path, which could just give you another option for next time - here is a link which could offer you both isolators / converters and SPDs (belt and braces):
http://www.weidmueller.com/61104/Products/Product-Range/cw_index.aspx

Best regards
Alan Balcombe
Weidmuller Applications Center
Richmond, VA
(abalcombe [at] weidmuller.com)

 

There are a lot of things you can try. I've had good luck with surge suppressors made by folks who know what they are doing. These kept the losses to within reason. But, lightning is not very predictable. You can provide a good low impedance path to ground and 5% of the time the lightning will go elsewhere. It sometimes doesn't do what it is supposed to, it does what it damn well pleases. I put a network cable plant in a site in Florida that got hit several times every day in the summer. It was a disaster at first, but it did make it easy to tell what worked. We used suppressors by a company called
Ditek. I haven't seen them around recently. I wouldn't bet on the I/S
barriers.

Regards
cww

 

Curt's right. And Ditek IS still around. www.ditekcorp.com

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes [at] putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff