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4 to 20 ma loop and lightning

4 to 20 ma loop and lightning

4 to 20 ma loop and lightning

I have used this design for many years and have two recent failures. A loop powered pressure sensor is installed in a manhole near a water tank. This device senses the water level in the tank. 4,000 to 5,000 feet away, at the well site is a 24 volt power supply and PLC input. Two of these, about 5 miles apart, were destroyed, probably by lightning, within two days of each other. PLC analog card and pressure sensor. Both ends have lightning protectors designed for loop service. Beldon twisted/shielded in 2-inch PVC  conduit connects the devices. The conduit is run in the same ditch as the water pipe. In one case the water pipe was PVC and the ather was ductile iron. Neither tank was struck directly.

Can anyone recomend some heavy duty lightning protectors? A-B PLC cards and $400 pressure sensors are costing too much.


Steve Wagner

RE: 4 to 20 ma loop and lightning

     Try a three stage hybrid potector. An example is EDCO PC642-036X.

RE: 4 to 20 ma loop and lightning

You say you've used this for years, and only recently had some failures. I'm curious - many areas are experience a drought right now. Is it true in your area? If so, the problem may be a change in ground conditions (soil moisture).

RE: 4 to 20 ma loop and lightning

I don't think the dry conditions have had much to do with it.  I see that the EDCO PC642 has 2-line capability(designed for two ma loops). Can I loop thru the second TVSS and maybe get a little more protection(it would be like putting two in series)?


Steve Wagner

RE: 4 to 20 ma loop and lightning

    In general placing two or more surge suppressors of the same type will not give you more protection. However if you do have an event, and that event takes out the first suppressor, the second might still survive. It has been my experience that with a good hybrid surge suppressor properly installed, the protected device will not be damaged as long as the suppressor is good. The problem is one good hit, and your suppressor is toast, and you probably will not know it. Second hit and your device is toast also.

RE: 4 to 20 ma loop and lightning

Thanks djs;

These are two new installs, in the same area, but different suppliers. I had specified TVSS at each end of both. (Used the same spec.) One vendor removed the suppressor at the far end after the installer had it in, saying HIS pressure transmitter had built-in protection. POOF, $600 gone. He will be installing something Monday. The second install had some sort of protection that has been replaced with an Essex 3800. 12 hours later, the day it was installed, lightning took out some 0.125 ma fuses in the 3800, but everything came alive when the fuses were replaced.
I will be doing an onsite on Monday. I think everything in the vault should be bonded, the pipe, all the enclosures, the tressure transmitter, etc. Do you agree or have any ideas?

Steve Wagner

RE: 4 to 20 ma loop and lightning

     The transmitter should definitely be bonded to ground. However what is more important is that the ground of the surge suppressor be solidly connected to the ground of the transmitter. The idea is that if an event happens, the common mode voltage seen by the transmitter will not raise too much.
     On a further note, unless you can test the surge abilities of the Essex 3800, I would replace it. It is cheap insurance.

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