Looking for a solution to a lightning issue

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VinceS

Senior Member
Nearly every time a thunder storm rolls through our neck of North Florida we blow all six of out SLC500's thermocouple cards. I think poorly of the design specs which didn't require shielded wire, and permitted the use of #21 J type thermocouple wire with exposed junction's three hundred feet away in a separate structure. Has anyone used a MOV terminal strip to isolate their thermocouples? Does such a device exist?


Oh, I already understand I need to pull a larger size shielded wire to keep my loop resistance under 100 Ohms. Alas, this is a tuff sell because is says loudly "Someone screwed up"... Thanks in advance for any advice...
 
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ohm

Senior Member
Location
Birmingham, AL
Vince, you have a problem. The T/C extension wire should have been shielded and grounded at only the receiver end. The sensor end shield should have been cut & taped. Exposed junctions are actually a good thing because at the distance you mentioned back to the input cards you probably have a significant difference in potential between the building steel. After I saw my T/C's were "measuring" with only one wire hooked up I never bought another grounded junction.

If you can replace the T/C input cards with 4-20 madc and run twisted shielded cable in conduit I believe your problems would go away.

You might try some sort of TVSS block to block the lightning hits but I think it's a long shot. If you can solve this without ruffling some tail feathers you should run for Secretary of State.
 
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VinceS

Senior Member
Still looking for TVSS terminal strips for j type T/C's

Still looking for TVSS terminal strips for j type T/C's

MOV's are the working part of a TVSS system. "Most TVSS's are made of Metal Oxide Varistors (MOV's) connected in parallel with the load they protect." Thats what I'm actually looking for... And yes trouble, much trouble.... But I can't blame the installing EC, unshielded wire was specified for cost issues at the local level.... Thus the politics of offending the person which specified the cheeper cost are in play...
 
beanland

beanland

Senior Member
Location
Vancouver, WA
Distances

Distances

Running thermocouples for those long distances is not a great idea. If it could be done again, I would recommend hockey-puck thermocouple to 4-20mA transmitters located at the sensors. Use isolating type transmitters. This provides decoupling between the wire of the thermocouple and the wire of the 4-20mA loop. The 4-20mA loop will be less susceptable (sp?) to noise.
 
ELA

ELA

Senior Member
Occupation
Electrical Test Engineer
You did say that you are blowing out the tc. conditioning circuits so " a simple noise upset" is not the immediate issue. You are probably going to to have to isolate the circuit.
Lightning is inducing a large common mode current in the long cable "antenna". Twisted pair would still conduct this common mode noise and would probably not solve the issue. Shielding is good if the shield is terminated properly but even that may not prevent it if the induced currents are large enough.
I have not heard of MOVs for tc lines since they would form another junction , but maybe circuit protectors that compensate for that exist?

I think your best bet would be wire inside of conduit or to isolate the tc's with an optic link.
 
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VinceS

Senior Member
Thanks ALL...

Thanks ALL...

I just thought I'd toss this out there to see what the "brain trust" thought...
 
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