Lightning Strikes faulting SLC

[Bob O 8]

Over the past two weeks one of our customers [ waste water treatment plant] has been getting hit with sever thunderstorms with lightning strikes and it has faulted the SLC three times that I know. The second time I had to travel out there and trouble shoot the faulty I/O cards. The panel does have surge suppression on the incoming power which then goes through an isolation transformer and from there out to the field devices and the rest of the panel. What have you guys/gals used to isolate/protect your systems from a lightning strike coming in on the field device wiring? Thanks, Bob

[BobLfoot 302]

So Bob your theory is that the Lighting is striking the Device I/O Wiring and entering the system that way. And after this you are able to recover the SLC without parts being replaced? I had an experience where we lost a radio transmitter at each thunderstorm. Tuned out to be faulty gound connection. I suspect the Lighting Spike is coming thru your power source despite the isolation.

[Bob O 8]

Two times they were able to recover from the fault but I had to replace one AO and one AI on the expansion rack when I was out there. I assume this is all lightning related since it happen during sever storms with a lot of lightning strikes. Thanks

[Leitmotif 0]

I an not a lightning expert and know maybe just enough to be very dangerous. I think a good resource is www.lightning.org/ They seem to know what they are talking about and refer to contractors and vendors. Dan Bentler

[Crossbow 297]

I had an issue with lightning once in a quarry. I put the PLC on an online UPS, so it was receiving clean filtered power. Nice and smooth. Then one day the system went down, and I went out to troubleshoot. Found out they had lightning strikes on the conveyors sticking up into the air. Apparently the structural steel of the conveyors was connected to the same ground post as my control panel (and everything in the plant). They lost PC power supplies, CB radios, computer monitors, and 3 analog cards in my PLC. So my bet is the voltage came in on the ground wire. Find a good way to put the control panel on its own ground.

[paulengr 44]

You need to separate your signal and equipment grounds, just like it says to do in the book. All your equipment, conduit, etc., gets bonded together. You run separate signal grounds from the PLC cabinet. Then THAT gets grounded separately. If you don't have a good ground, you will never survive lightning hits. Inspect everything. What typically happens is that a cadweld falls off somewhere or the equipment bonding/grounding was garbage in the first place. Since the equipment and power side is not effectively grounded, lightning takes the path of least resistance and uses your sensitive electronics which is usually well grounded as the ground path. Not good at all. It will very much behoove you to resurvey any of your ground grids to make sure that you are down around 10 ohms or less on your ground grids. Often even after initial installation you will need to add ground rods due to soil conditions. If it turns out you need to add more, get the threaded kind. Then instead of driving in more and more and more, simply thread on another piece and keep driving the same one deeper until you hit a sufficient water table. Consider investing in a ground tester from AEMC. They are extremely expensive but invaluable compared to doing it the old way of hauling around 200+ feet of THHN that you've calibrated with a standard ground tester. All of your bonding should leave you with less than 5 ohms (preferably 1 ohm or less) back to the system bonding jumper. Even then, you can get a pretty nasty transient coming in through 4-20mA signal lines and such. If you can't protect them externally, then you need to insert surge suppressors. For low level signals, use a combination of SNAP diodes and GDT's. Joslyn makes a little white unit that is very popular in the telecom business. Recently Allen Bradley has started selling them under the "4983" part number. It's a 5-terminal device. Ground it to your system ground in the PLC cabinet. Run both 4-20mA and "network" cables such as any RS-485 links across these. With Ethernet, you'll need a different lightning arrestor arrangement (get a special purpose Ethernet lightning arrestor or switch to fiber). Finally, and I'm not trying to hock AB here but they have some nice stuff, consider the difference between "type 1" and "type 2" surge suppressors. AB's 4983 lineup also includes both. The type 2 category stuff is the common surge suppressors for industrial use that you see. It's designed to handle motor transients but not a full blown lightning strike. For that, you need a type 1 suppressor. And if you are out at the pole, consider putting in station class arrestors for medium voltage equipment. The price difference these days between a distribution class arrestor and a station class arrestor is often under $200. As a result it's usually not even worth it any more to buy distribution-class arrestors for distribution equipment. This is speaking from working for years in surface mining areas where lightning is a menace because all of the equipment becomes very effective lightning rods during storms.

[tommu56 0]

We had same trouble at our WWTP loosing a few analog cards and a processor It took along time to figure it out but whoever did the original insulation grounded both ends of the signal cables sheilds. I lifted the field end of them and that minimized the trouble. I put a separate ground buss bar in control cabinet and all the shields are tied to that plate tom

[TimWilborne 108]

Lightening does it's share of destruction, but it also gets blamed for a lot of unknown failures. I had an SLC that was loosing it's memory because of "power surges". It was also on a waste water plant being the tallest point around. A UPS and surge suppression had been added but the SLC would still erratically loose it's program. I finally found that the SLCs power supply was the culprit. Upon inspecting it I found that the board was slightly discolored. It could have been a quality issue or perhaps it was a lightening strike...I believe that's what we blamed it on. I believe on initial inspection the SLC had a solid fault light indication that the processor was damaged beyond repair. On cycling power it would flash just as a new processor and indicate that there was no user program I'm not saying that it wasn't lightening, but it might do you good to look at other possible component failures that could be causing your problem.

[Duffanator 1]

We've had a lot of problems with Lightning strikes at our waste water plant taking out the analog cards on our Opto 22 systems we use to trend all of the info out there. We tried insulators and seperating grounds but nothing really worked, it seemed like the lightning was still finding its way through the 4-20mA loops. We put in Banner Wireless 4-20mA transmitters and recievers (They are called Flex Power I think...) and we haven't had a problem since. We had one go bad but it wasn't due to lightning, the box leaked and it got wet :-P We went from spending $5,000 a year on replacing parts for that system to no having to replace anything for 2 years. Maybe you can look into that.