Let’s say I have a prox switch and a solenoid valve mounted on a 10’ steel pole in a field. The steel pole is bounded to a good earth ground and the control power for the prox and the solenoid is from an ungrounded 24vdc power supply. The prox is an input for a PLC and the solenoid is an output. Now let’s say the pole is hit by lightning, being that there is on earth ground connected to the prox or the solenoid. Does my PLC live? Does the prox or the solenoid live? Would lightning protection on each I/O point help or give a different outcome?

To me seeing as there is no reference to ground back to my PLC. Why would current flow back trough the wires from the prox or solenoid to the PLC?

The PLC and the 24vdc power supply are powered by 120vac source and they have their own lightning protection.

 

Your PLC might live or it might not. And it depends on far too many factors for anyone to give you an answer here. Let's put it in perspective, you have enough voltage to jump 5000 feet or more. And some thousands of amps if a good path is found. Lightning protection is for induced voltage or current, side effects so to speak.

If the main bang hits your pole and the easiest path to ground is through your PLC, you might not even _find_ your PLC. If the pole is really, really well grounded you might be OK. In between the pole might be raised to a high enough voltage to incinerate various components.

If they happen to be your MOVx, arrestors, whatever, you may still luck out. But there is no way to guarantee such a setup would withstand a direct hit to the pole or wiring.

Regards
cww

 

years ago one of my projects was struck by lightning.

the MCC and control panel were in a small concrete building.

Lightning apparently struck the incoming 480V service traveling to the MCC and destroying it. It ended up being replaced, there was so little left of it that was usable.

The control panel used an STD bus computer and some proprietary software and cards.

One of the cards died during the attack, along with a MOV that was on the 120V line powering the computer's power supply. The MOV literally exploded and only some very small pieces of it were found inside the control cabinet, but there was no other damage inside the cabinet.

 

Hafa adai from guam

i hear ya. my friend installing a isolation transformer will cure your fears of future burnout

I hate to say it but mov's can protect to a certain level but are truely useless my friend

i hope this helps

george ishmael
[email protected]

 

Isolation transformers may help with surges coming from the distribution side but the original question was about a strike to a field mounted device and where purpose designed signla protection devices will be needed to protect the PLC I/O cards

Steve

www.mtlsurge.com

 

Well there is no perfect route for lightning and it passes through the least resistive path. In this case there are many parallel paths. Try to insulate the prox and solenoid valve using some dielectric, that could be one way.