Lightning Protection

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Thread Starter

Gee, Jeffrey

I have a client who keeps roasting their PLC processor which sits in a small building in the middle of a field. The 480VAC 3 phase power to this building is fed underground from switchgear about 1/4 mi away. An MCC and transformers are inside the building to knock the power down to 120/208 which feeds a breaker panel, which then feeds the PLC. There is local digital I/O to the MCC buckets, and remote 4-20 mA signals fed underground to monitor flows, pressures, etc. An RS-422 link (also fed underground) allows the PLC to be networked to others and to the SCADA system. The roasting of PLC's seems to be directly related to inclement weather/thunderstorms, and I am pretty sure that we need some sort of lightning/surge protection. The PLC is the only 'electronic' item in this building, and it is the only thing to have ever failed. The question I have is where should this protection be? On the 480V 3 phase? On the breaker panel? In the control panel? Should I also protect the RS-422 comms? I know that there are various technologies for lightning protection...Any advice on which to use? Recommendations on vendors? Info/tutorials on-line? Thanks in advance! Jeffrey R. Gee
 
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Al Pawlowski

There are lots of possible fixes. But, you can be much more efficient at finding the right one/s if you can get a better handle on exactly what is being damaged: "fried PLC" is not particularly informative. That said, my approach would generally be to keep all interconnected equipment commons at the same (ground) potential and reduce potentials on the other (non-common/ground) connections to levels that can be handled by the equipment safely. First though, carefully install a good surge arrestor (I like Phoenix arrestors) on the PLC's power. The PLC ground should go the equipment terminal on the surge arrestor or to the main building ground separately from other equipment. Do not allow any other PLC connections to ground. This should take care of power line problems from all but the most heavy incomming power line surges. Next, attack signal lines running outside of the building. Disconnect any signal line shields that are connected to ground/PLC chassis inside the building. If possible (and it should be with 4/20mA and normal discrete signals) get rid of the shields and use unshielded twisted pairs. If you need shields, ground them at the far (from the PLC) end only and keep their PLC ends heavily insulated. If the cost is not a problem, install isolators on the lines too. They may be essential anyway if the lines can not be isolated from field grounds. When you are trying to figure where damaging potentials might come from, remember to look at line closeness and not just physical connection. Large, and fast, currents caused by lightning can couple some nasty potentials without physical connection.
 
Hello Make sure the structures are well grounded, per NEC with low resistance paths to embedded ground rods. Would be worth checking resistance if installation is not new, ground resistance can change with weather and water table. Try www.harger.com for lightning air terminals and ground rods, they have useful information. Al Boake P.E. [email protected]
 
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Johan Bengtsson

Well, first of all, there might be several ways in (as you suggest) and each one of them have to be looked at of course. Others will probably suggest a lot of nice things to put at the power input lines (I know protectors exist, but not what would be best for you) On the signal level I would suggest to optically isolate as much as possible, for the 4-20mA signals and other remote I/O I would suggest some kind of fieldbus that could use fiber. This because lightning never could damage something at the other end of the fiber, even with a direct hit. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------
 
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Curt Wuollet

I have solved many lightning problems with various MOV's, GDT's and the like. For the serial link I have had the best luck with some protectors made by Equinox for their serial cards. A MN firm called DITEK made many protectors for coax and various other applications. The analog (4-20 ma) lines are a special problem because of their high impedance and high compliance. A good rule of thumb is that self contained devices that use system grounds are often useless. Devices having a heavy lead that they advise go directly to an earth ground stand a better chance of being effective. The very best is to understand the problem by analyzing the current path through the failed devices and selecting MOV's and circuits that prevent that particular overstress. Bear in mind that it is quite common to see damages caused by the ground system being lifted a couple hundred volts above actual earth potential. I would start by improving grounding at your building. Then you have something to connect protectors to. Without effective grounding the best protectors can't help. Sometimes, effective grounding is all you need. Regards cww
 
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Darold Woodward

I spent about 10 seconds on the web and turned up "lightning.org" that suggested that if you call them that they can help with specific recommendations. I assume that your problems are a result of direct or local lighning strikes, so I suggest providing a locally "shielded" and well grounded environment for the PLC to operate. Find an electrical engineer in your area who is experienced with the lightning conditions locally. He would probably recommend some or all of the following: Beef up your local ground at the site by putting in a ground mat style system consisting of direct burried bare copper wire around the building perimiter cadwelded or clamped to at least two ground rods. Reinforce the equipment case grounding and building grounding by connecting the building steel, water pipes that leave the site, and equipment grounds to this new grounding electrode. Add a lightning rod on a pole or on the building. Provide proper isolation on any circuits, especially communications connected to the PLC that leave your local ground plane. Darold Woodward PE SEL Inc. [email protected]
 
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Chaverri, Luis F

Look at http://www.ch.cutler-hammer.com/ There you can find a lot of information about lightning protection. First of all, and depending on the size of the project, is desirable to have only one point of ground connection; then they have to protect in at least two sites: at the main entrance with a surge protection of 250kA and at the MCC where the PLC is located with another surge suppressor of about 160kA. The last protection shall be located close to the PLC. This one has to be a surge suppressor with an internal filter. This is one proved way to resolve the problem. Test it and then send me the results. Even in a hard area of a high incidence of lightning and switching surges, this mix of suppressors will resolve the problem. Now, don't forget to connect them to the same ground potential; otherwise, no system will work. Regards, Luis Chaverri C.A. CHESS Coordinator Ph (506) 236 8022 FAX (506) 241 1041 Cel (506) 384 2184 Bpr (506) 224 2400
 
M
Where should the protection be? Hard to tell without knowing exactly what 'roasting their PLC processor' means. However, I'd make sure the 120AC common is well connected to a ground rod, and that _ALL_ signals from outside the building, (RS 422, and analogs are protected) along with some surge protection on the AC input. Mark
 
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Curt Wuollet

> From: "Gee, Jeffrey" <[email protected]>
>
> I have a client who keeps roasting their PLC processor which sits in a small
> building in the middle of a field.
>
> The 480VAC 3 phase power to this building is fed underground from switchgear
> about 1/4 mi away. An MCC and transformers are inside the building to knock the power down to
> 120/208 which feeds a breaker panel, which then feeds the PLC.
>
> There is local digital I/O to the MCC buckets, and remote 4-20 mA signals
> fed underground to monitor flows, pressures, etc.
>
> An RS-422 link (also fed underground) allows the PLC to be networked to
> others and to the SCADA system.
>
> The roasting of PLC's seems to be directly related to inclement
> weather/thunderstorms, and I am pretty sure
> that we need some sort of lightning/surge protection. The PLC is the only
> 'electronic' item in this building, and it is the only thing to have ever
> failed.
>
> The question I have is where should this protection be? On the 480V 3
> phase? On the breaker panel? In the control panel? Should I also protect
> the RS-422 comms?

All of the above. I'd start with the longest lines. Distance is your enemy with lightning events amd EMP. Unless you can determine the current path you may have to do them all. Drive a ground rod and clamp (electronically speaking) everything to that earth ground. Then you won't
ever know what fixed it if it doesn't happen again. Direct hits will blow something up no matter what you do.

Regards

cww

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