AC Test System

Discussion in 'The Projects Forum' started by krae, May 3, 2013.

  1. krae

    Thread Starter New Member

    May 3, 2013
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    Hi there, I am new to dealing with hands-on electrical work. I am hoping for some help in verifying I have this set up properly, mainly since I am paranoid about shocking myself!

    I am building a system, as shown in the picture. The whole thing is in a vacuum chamber with proper electrical feedthroughs. I have a heater with a radiation shield (these are in one unit) with an AC temperature controller. The hot lead is connected to the heater and the neutral lead goes to the shielding around the heater. The shielding is also connected to the chamber, which is grounded. Does this configuration look okay?

    One concern I have is the copper in contact with the heater face. I do not want any current running through these. They are in contact with a copper block with a water cooling loop. The water cooling loop is attached to the building water supply. Can I assume that this is grounded? Will I have current running through these copper bars? Should I physically ground them by running a wire to ground?

    Thank you for your help!
     
  2. #12

    Expert

    Nov 30, 2010
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    What country are you in?
     
  3. krae

    Thread Starter New Member

    May 3, 2013
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    I am in the US.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    What exactly is the heater face? I mean, if it's part of a resistive heater element then it is live and will conduct directly to the copper. The copper might even short part of the heating coil. If the face of the heater is ceramic, then it's insulated from the copper.
     
  5. #12

    Expert

    Nov 30, 2010
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    OK. The heater should have been insulated when it was manufactured. In the US, we use a third wire called, "bond" that is wired to the planet. That is for shielding and safety purposes. The water pipes are almost certainly connected to the planet. The way it should work is that power and neutral are wired to the heater and all the shields are wired to the planet. Please check the heater and see if it is compatible with this method.

    While you're at it, use an ohm meter to see if the heater is insulated so that the hot part is not connected to the power or neutral wire.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Depending upon the building. Plastic or copper pipes throughout?
     
    shortbus likes this.
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    And in these times you can't even count on what's just in the house. They finally ran city water down my road a couple of years ago. The hook up was up to the home owner. The plumber I hired said most people were using plastic pipe from the water main to the house. I paid extra for copper. Just wonder in the future how many electrician's will bond something to water pipes thinking it is an "earth" connection?
     
  8. #12

    Expert

    Nov 30, 2010
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    Here's something I suspect: The water is a continuous connection to the planet and it has impurities in it, therefore, the water is a partial conductor to Earth ground, regardless of the pipes...unless the water has been disconnected from the supply pipes.
     
  9. krae

    Thread Starter New Member

    May 3, 2013
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    Thanks for your help. I am not sure about the piping. This is in a new laboratory facility, and have all safety precautions possible here. Is it standard to have grounded water pipes in places like that?

    I tested parts of the system with an Ohm meter. The resistance that it shows jumps around a lot, so I'm assuming that I take the lowest value? I got a low resistance (<1 Ohm) for all of the following.

    heater face to heater shield
    copper bar to hot lead
    hot lead to neutral lead
    hot lead to chamber wall
    heater shield to hot lead
    neutral lead to chamber
    chill loop to chamber
    hot lead to green ground
    neutral lead to green ground

    Will I have current running through the copper bars or back to the neutral line? The heater face is metal. The guy with the heater company told me to set it up like I described earlier. He said that I would be fine, as long as the copper bar and water loop are grounded. However, I don't fully understand this.
     
  10. #12

    Expert

    Nov 30, 2010
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    Either you're doing the measurements wrong or this circuit is going to be like dropping a toaster in the bathtub.
     
  11. krae

    Thread Starter New Member

    May 3, 2013
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    Great, that's exactly what I'm afraid of. Why do you say that? I am completely new to this stuff, so please forgive me.

    What could I be doing wrong in the measurements? I do notice that the resistance value jumps around quite a bit for some the measurements from ~400 ohms all the way down to ~0.3 ohms.
     
  12. #12

    Expert

    Nov 30, 2010
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    Call somebody to help you, somebody that knows how to make measurements that don't jump around, somebody that will know for sure what the readings mean, somebody that is NOT going to make a beginners mistake. This could mean the difference between popping a circuit breaker or getting in the way and getting killed.
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    That may be true, although I personally wouldn't trust it. It isn't too unusual to have a water main break and the lines drained while a repair is being made. How good is the electrical ground through an empty plastic pipe? I would rather trust the U-ground connection on an electrical outlet. Code says they have to make their way back to a driven rod.
     
  14. #12

    Expert

    Nov 30, 2010
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    The question here is not, "Can I trust the water pipes to ground the experiment". It is, "If the heater is not insulated, will the contact with water pipes contribute to the fault current."

    Probably yes.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    That's what happens when the details of the measurement (surface area of probe, how hard it's thrust through a surface layer of corrosion, on and on) matter more than the actual resistance you're trying to determine. Artifacts, in other words.

    You never said whether your heater mechanism is insulated from the rest of your apparatus. It matters.
     
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