Is Concrete Conductive OR Insulative? Internet says both

Discussion in 'General Electronics Chat' started by realracer, Sep 7, 2016.

  1. realracer

    Thread Starter New Member

    Sep 7, 2016
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    Working on polymeric outrigger pads in mobile cranes
    Concern for static buildup and need for ground
    Unclear of this concern as seems asphalt and wood are both insulators (assuming dry)
    Concrete found listed as insulator in some sites and a conductor in others

    Trying to understand
    1/ what is true status of concretes conductibility?
    2/ if already using wood and placing it on concrete or asphalt or even rock, doesn't that negate the whole concern since already operating in an isolated state?

    Any help appreciated

    Best
    C
     
  2. realracer

    Thread Starter New Member

    Sep 7, 2016
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    For additional grounding, this is one of the statements I found on the internet about this question but this site contradicts that to claim it is conduction. Likely both are accurate somehow, my interpretation is lacking grounding to know. upload_2016-9-7_10-27-52.png
     
  3. SLK001

    Well-Known Member

    Nov 29, 2011
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    It is a very poor conductor. In a lightning storm, it is best to stick to the concrete areas as opposed to the grass if you get caught outside when a storm rolls up.

    I had a friend that was working for an electrical contractor. He and another worker were moving an aluminum ladder to reposition it. It contacted a 27kV feeder line. My friend was standing on the grass, while his coworker was on a concrete sidewalk. My friend was fried and was killed, but the coworker was only knocked to the ground.

    The contractor caught hell for even having a conductive ladder on a job site (ladder should have been either wood or fiberglass).
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Dry concrete would be classified as an insulator.
    Damp or wet concrete would likely have some low level of conductance.

    Dry wood on any surface would be considered an insulator.
     
  5. #12

    Expert

    Nov 30, 2010
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    I just measured a foot of dry concrete surface at 4.35 gig-ohms (DC). 400 megs per inch is a fairly good insulator. My router will not send RF through a concrete block wall and I have had adventures trying to get rid of TV signal reflections from concrete apartment buildings. That seems to say concrete is not, "ground". If it was, it should just absorb the TV signals. At high frequencies, it looks like a reflector, not a conductor.

    Static electricity is DC, so you should use a DC model. Dry concrete has so much resistance that I would say it wouldn't be a good way to dissipate static electricity, but it will dissipate if you give it enough time. If you want to change that, add water and sprinkle on a hand full of common salt. Another way is to bury a metal pipe in concrete. A huge surface area in contact with the concrete will decrease the resistance accordingly. Concrete with rain on it will be a better conductor than dry surface.

    Adding an insulator between a machine and the planet enables the machine to accumulate static electricity. In other words, operating in an isolated state allows static electricity. Rain changes everything by coating all surfaces with a better conductor.
     
  6. panic mode

    Senior Member

    Oct 10, 2011
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    dry concrete is an insulator, wet would be poor conductor. if your crane is used to move material (blower or vacuum or some sort of conveyor) static charge would build VERY fast and give really nasty zaps to anyone near (think taser on steroids). on equipment that moves material grounding is extremely important. consider using separate groudning rod and large gauge wire. do NOT count on outriggers as conductive surfaces.
     
  7. realracer

    Thread Starter New Member

    Sep 7, 2016
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    you guys are fantastic with the support. All of these answers were valuable (and the story SUPER enlightening - sorry for the loss)

    As we move to design outriggers for heavy equipment like cranes in polymers the concern for static buildup in the equipment has arisen.

    On one hand seems like a reasonable concern, polymers are insulators and the application currently uses wood.
    On the other hand, those metal outriggers are commonly placed on insulators like wood, concrete, asphalt, rock...

    IF static build up was a concern wouldn't current practice of wood (or concrete, etc) as matting have revealed that?

    All inputs appreciated and valued.

    Thank you for the gift of your time in response.

    If you need help with plastics I can offer that in return

    Best always
     
  8. realracer

    Thread Starter New Member

    Sep 7, 2016
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    Panic mode...can you share more?
    your comment to not use outriggers as grounding appeared to have some specific experience behind it.

    Thank you always
    C
     
  9. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    There might be a lot to be learned while looking at grounding for swimming pools. In that case, the re-bar that's inside the concrete is done in a specific way and connected to ground. There are potentials that get created across the ground in a storm situation. A "computing" center had the ground potential change across 200 or so feet in a data center because of a storm, so they had to change to fiber to get across the room.

    Possibly a good thing about wood is that it does conduct when it rains, providing a better path to ground.

    There might be cases, where grounding your rig via a ground rod or to the structure might be appropriate.
     
  10. realracer

    Thread Starter New Member

    Sep 7, 2016
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    Your response appeared to have experience behind it...can you expand on the statement to NOT use outriggers as grounding?
    Am certain your direction is right, but I don't have your knowledge to language it to the others in this conversation with me.
    Anything you can do to deepen the understanding behind your advise is appreciated greatly.

    Thank you again and always.
     
  11. #12

    Expert

    Nov 30, 2010
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    Maybe you can add carbon to the polymer to reduce its electrical resistance?
    Yeah, gonna have to do the math about weakening the structure.:(
    If you want a good ground, you're just going to have to pound a rod into the planet and wire the machine to the ground rod.
     
  12. realracer

    Thread Starter New Member

    Sep 7, 2016
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    actually we do have ESD rated polymers, a whole platform of them but unfortunately they are out of financial acceptability for this application.
    This is an application with a bit of an economic ceiling for acceptability in the market.
     
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you think concrete is an insulator but the ground is conductive may want to Wikipedia "Ufer ground" where a grounding electrode is encased in concrete. This is especially effective in dry climates.
     
  14. panic mode

    Senior Member

    Oct 10, 2011
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    you cannot get low resistance path like that - at least not reliably... unless your crane has outriggers submerged in mud or sea; lookup salt water conductivity.

    there is no magic... ohms law still applies.... to get lower resistance you want material used as "conductor" to have few characteristics - path to be short, have large area and low specific resistance (high conductivity). when everything else fails or is out of our control, increase contact area.

    outriggers have comparatively small area. compare that with Ufer ground which has electrode embedded into foundation of entire building.
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    With cranes, you have benefits and concerns with conductivity and grounding.

    1) if you avoid grounding, then concern with contacting power lines is less. The Crane can be energized but no current will flow through the Crane after it is in contact with one power cable (assuming the outriggers are not grounded) and the entire thing is electrically "floating".

    The bad news is that charge can build up and sparks or discharge through an operator is possible as someone climbs aboard.

    2) if the thing is grounded, power lines become more problematic but static buildup from wind / sand erosion of the plastic or weather-related static and other natural charging phenomena.

    I think the best solution would be an ESD type of situation - (connection to ground with thousands to tens of thousands of ohms* of resistance). That way, the crane can discharge in seconds but avoid high voltage potential between Crane and ground.

    * exact values will depend on capacitance of the crane relative to ground (charge required to create a given potential).
     
  16. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    It is HIGHLY dependent on moisture content......so.....depending on the time of day and the weather.....good luck finding an answer! :)
     
  17. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Watch your wording: S=1/R, so if R = 100,000, S = 1/100,000 = 0.00001 S

    0.00001 is a low number, thus high resistivity is the same as low conductivity. Did conductivity measurements on semiconductors professionally.
     
  18. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I wrote a QST article a while back on "Why MHO is mo' better. MHO was a perfectly good term, because it's easy to remember "With Mo' MHO you have mo' current flow. Alas, in 1971 the International SI committee dictated that all electrical units had to be named after dead science guys, and since Siemens was a dead science guy and Mho wasn't, the siemens won out. I still use and teach MHOS though. :)
     
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  19. Tonyr1084

    Active Member

    Sep 24, 2015
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    Saw a video. Some kid playing on a roof. Waving a broom stick (wooden) around like a sword. Made contact with the power lines and got a zap that got his attention. He wasn't harmed, but wood DOES conduct. Humidity changes the game. Static is low under humid conditions but is high under dry conditions.

    I don't know where concrete falls on the tribo-electric scale. However, anything that CAN take a charge will. And that charge potential changes with mass.

    [edit] found the video:
     
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  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That broom was too light weight to be wood. The light ones are thin rolled sheet metal.
     
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