Surge suppression for 4-20mA loop

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Thread Starter

Ed Bauch

A 4-20mA loop extends via an underground cable about 300 yards to a two wire transmitter at a remote tank site. The loop is protected by a 1/4 A fast blow fuse on the 24v loop power supply conductor. Sometimes during thunder storms the fuse will blow. Can anybody suggest a good surge suppression device or technique to protect a single loop?
 
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Bill Gausman

I think the "surge" problem happens because the thunderstorm induces potentials in the earth, which vary from place to place. When two locations of differing potentials are connected by a piece of wire, current WILL flow. The amount of current is determined by Ohms law, considering the difference in potential, and the resistance of the path. While surge suppression is important, I think isolation is even more important, to eliminate the path.

Bill Gausman
Engineering Manager, System One Control, St. Paul, MN
 
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Marcos Giorgianni

Hi Ed,

There is a good company that has the product to solve your problem.

Try to look at: www.dehn.de

They have a line of product called "yellow line" an you will find exactly what you want.

Regards.

M.Giorgianni
BRASIL
 
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PTS Technology

We had this problem in our oil production facilities and was solved with a three dollar solution, (sorry for the money hunters) just install three 32 volts VDR one from ech side of the 4 to 20 to ground and one in between this should be installed in the sensing device input and at the other side (input of PLC or DAQ)
install a saftety/isolation barrier, this was suggested by other friend today we have been using Stahl barriers with good results

Regards
German
**********************************************************************

German Lopez, [email protected]
Production Testing Services, Technical Support Engineer
TRV 19A 121-11 Ph 57 1 637 2577 FAX 57 1 6373990
Bogota Colombia, South America
In Houston: Production Testing Services Inc.
6226 HWY 6th South, HOU TX 77083
PH 281-493-2500, Fax 281 493 3338
 
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PTS Technology

Adding to my last mail, where I tell you that we just use a simple circuit of 32 Volts VDR at one side and intrinsic safety repeater barriers at the other side of our 4 to 20 mA lines, please take into account that:

1 (first first)use lightning protection devices at the exterior of the operations area not inside.

2 Use excellent grounding nets, with that, the differential of voltage generated by the pass of the lightning hit will be minimum

3 Use shield cable to carry the 4 to 20 mA

4 It is very important not to hook up both sides of the shield of the shield cable, just ground the sensor´s side of the shield to avoid
grounding loops.

5 The reason of failure due to lightning storms is usually induction of relatively low voltages (hundreds of volts)on the 4 to 20 mA lines, not very high voltages (as expected) because everybody uses ionising systems or similar devices to avoid discharges near the operations area. Due to that It is very improbable the "reception" of a lightning hit directly on the operations area, if it happens it is time to fire your electrical engineer, (or the cost control manager that cut the order of an appropriate lightning rod pole or tower) If the lightning hits your installations, You will need to buy all the devices again because a discharge like this manages thousands of amperes that generates thousands of volts that will kill whatever it
finds on his route. Also you need to make NEW grounding system to your lightning system, because now you have "Igneous" rocks without any
channels to conduct the electricity acting as grounding system.

6 This mini circuit must be tested at least once a year or when a lightning hits the surrounding area

7 If your application does not need intrinsically safe devices, then you just need a repeater/isolator in each input, some PLC come
with it already hooked up

By the way, I already sent a letter to ISA and also to companies that produce 4 to 20 mA devices, asking them to add this simple circuit on the hooking bolts, this will not add any important cost to the device and extends the life. We have statistical information that shows the effectivity of the device because our operations area is in a very rainy-stormy area in Colombia.

Of course you can buy expensive devices that have the same circuit inside

None of the companies nor ISA has answered (I am senior member of ISA), may be you have the power to redirect this messages to the right person or bureau.

regards
German Lopez, [email protected]
Production Testing Services, Technical Support Eng.
TRV 19A 121-11 Ph 57 1 637 2577 FAX 57 1 6373990
Bogotá Colombia, South America
In Houston: Production Testing Services Inc.
6226 HWY 6th South, HOU TX 77083
PH 281-493-2500, Fax 281 493 3338
 
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Al Pawlowski

I second German Lopez's remarks about surge supression on 4-20ma. My experience has been the same in that:

1) Most lightning problems are due to inductive voltages that are not too hard to handle and the rare direct hit usually takes out everything.

2) Line to line and line to ground (both lines) zener diodes will very quickly absorb the fast part of a lightning induced transient.

I worked for a company that made such an arrestor product. It included ionization snubbers (neon bulbs),"spark" gap capacitors in all three positions and line fuses in addition to the zeners. It also had board positions to put in a small transformer and/or series capacitor for ac signals like tone telemetry. We must have installed a thousand of these arrestors and having hardly more than a fuse go; never any connected equipment. I remember one plant
installation where a 15kv supply line fell onto the aerial network cable linking about 20 nodes. All of the arrestor line fuses went, but when they were replaced not one RTU had been damaged. The in-line fuses were almost always fast enough to protect the zeners.


Al Pawlowski, PE
[email protected]
dba ALMONT Engineering
Baton Rouge, LA USA=
 
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PTS Technology

Just a comment to add
AL [email protected] comments that:

2) Line to line and line to ground (both lines) zener diodes will very quickly absorb the fast part of a lightning induced transient.

And what I could comment is that VDR (Voltage dependant Resistors) are stronger comparing with same power VDR

1- the Zener semiconductor union blows up too fast
2 - The zener will start dissipating power as soon as the voltage reaches the 0.6 volts of direct polarization (the zener diode is used in the inverse polarization region)

Also, friends, do not forget that we need to test the arrestor devices at least yearly because it would protect the lines and die in its duty and the next time ....? this mostly happens with VDR because Zeners usually die in short circuit

German

German Lopez, [email protected]
Production Testing Services, Technical Support Manager
TRV 19A 121-11 Ph 57 1 637 2577 FAX 57 1 6373990
Bogota Colombia, South America
In Houston: Production Testing Services Inc.
6226 HWY 6th South, HOU TX 77083
PH 281-493-2500, Fax 281 493 3338
 
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