1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the reason for using electrically insulating joints in a water system?

  1. Sep 21, 2012 #1
    I'm living in France and have just replaced my "chauffe eau" - a 200 litre hot water tank with built in 2.4kW electrical heater and thermostat.

    The instructions say that it should be installed with an electrically insulating joint where the hot water outlet pipe is attached. I bought a pair of joints (they are only sold as pairs - 20 euros per pair) and their instructions said that one should be installed on the cold inlet and one on the hot outlet. They are sold as prolonging the life of the tank.

    I have dutifully installed these joints. They are made of metal but with a tough plastic washer preventing the two ends coming into contact.

    I assumed that the function of the "dielectric joints" as they are called is to prevent the steel tank being corroded by electrolysis resulting from being in contact with the copper water pipes.

    However, there is something I don't understand. The copper hot and cold water pipes are bonded to the ground connection of the house's electrical supply, in accordance with French electrical norms. Likewise, the steel case of the hot water tank is connected to the ground connection of the house's electrical supply.

    Here is what is puzzling me. I'd be grateful if anyone could point out what I am missing. I can't see what is the point of having the electrically isolating joints if the pipes they are meant to be isolating are connected, via the grounding cables, to the case of the tank anyway.

    What am I missing?

    Martin
     
  2. jcsd
  3. Sep 21, 2012 #2

    nsaspook

    User Avatar
    Science Advisor

    The grounding of the metal parts is for basic electrical safety not for the prevention of electrolysis. The "dielectric joints" are for the prevention of galvanic corrosion caused by two electrochemically dissimilar metals that would use the ground connections to provide a low resistance path for current from the dissimilar metals battery if it existed.

    http://www.midastech.co.uk/pdfs/TB002-Galvaniccorrosionguide.pdf [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. Sep 23, 2012 #3
    nsaspook - thank you for your reply.

    I had already well understood that the electrical bonding of the hot and cold pipes (and the casing of the hot water tank) to ground was for electrical safety. And the reference given is all good sense about electrolytic corrosion but it does not seem to resolve the point I have not understood.

    If both sides of the insulating joint are connected together by thick copper cable, they are going to be at the same voltage (give or take a fraction of a microvolt). I have not understood the reason for insulating the joint, if both sides are at the same voltage anyway.

    I must be missing something here but I cannot figure out what it is I am missing. I'll be grateful to be enlightened.
     
  5. Sep 23, 2012 #4
    The point is that the electrical safety bonding is not in contact with an ionic liquid.

    The pipework is.

    So even if the pipework is all of the same metal the possibility of a galvanic corrosion cell exists due to local potentials being set up via the liquid.

    This is how most corrosion cells work - a blob of liquid in contact with a bare metal surface forming a (small) corrosion cell within the contact region.
     
  6. Sep 23, 2012 #5

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    Ostensibly the water has very little electrical conductivity, and the idea of the electrical insulation between steel and copper is to substantially reduce the electrical circuit as compared to having steel and copper in contact.

    With both gounded, they are at the same potential, but as long as they do not contact, the circuit is open, except for what leaks through the water.
     
  7. Sep 23, 2012 #6

    nsaspook

    User Avatar
    Science Advisor

    Pure water has very little electrical conductivity but almost any impurity from hard water will be concentrated in the piping near the hottest part of piping increasing the chances for a galvanic corrosion cell.
    http://media.rd.com/dynamic/81/86/19/Oct06_JoinCopper-1_size2.jpg
     
  8. Sep 23, 2012 #7

    Thank you. I'm trying to understand.

    I believe that the current flows through the water, whether or not the steel and copper are in direct contact (in either case, they are connected together via the grounding wires).

    We have copper and steel in contact with water containing ions from the dissolved salts, CO2, and so on, which results in a short-circuited electric cell. An electric current flows from the copper to the steel via the electrical contact(or grounding connection) between them. It returns from steel to copper via the water.

    The plastic gasket increases the length of the return path through the water by 2mm or so. Is it this that makes all the difference?
     
  9. Sep 23, 2012 #8
    The interior of the old dielectric joint that I removed from the old water tank looked much like your picture. The picture seems to show an isolating joint - I wonder if it functioned as it was supposed to before being removed and sectioned to display the corrosion or whether it was faulty in some way?
     
  10. Sep 23, 2012 #9
    No it doesn't work like that at all.

    Galvanic corrosion is a local near surface effect.

    Two adjacent areas of metal surface, in contact with an ionic fluid, form anodic and cathodic regions.

    Current flows through the metal surface and returns through the ions in solution (or the other way round), driving the cell and corrosion.

    This process is unaffected by connections to grounding cbales or even the metal on the other side of the pipe wall. It is a surface effect a few microns deep - even the pipe wall thickness is large in comparison.
     
  11. Sep 23, 2012 #10
    Thank you. Does that explain why the insulated joiners are necessary and do their work even though the pipes on either side of the insulated joints are electrically connected? If it does, I am sorry to say I have not followed the reasoning.
     
  12. Sep 23, 2012 #11
    What I am suggesting is that the non-conductive washer breaks the local return path for the galvanic cell current.
     
  13. Sep 23, 2012 #12

    nsaspook

    User Avatar
    Science Advisor

    Yes, they have an electrical isolation washer and hence the need to ground both sides. I would say they are inadequate for most installations because things are just too close causing them to fail in a few years if you have hard water.
     
  14. Sep 24, 2012 #13
    Thank you. If I have understood what you are saying, two adjacent areas of metal surface form anodic and cathodic regions even though there is just one single metal present. Or have I misunderstood (again)?
     
  15. Sep 24, 2012 #14
    Thank you. I am still struggling to make sense of all this.

    It seems that even isolating joints don't prevent corrosion, because of the path through the water remaining quite short. So I am not sure of the benefit of them. And they cost 20 euros the pair.

    Maybe a 50cm length of drinking water/hot water rated polythene tube would be far better (and cheaper) than the dielectric joints. Because now, the current would have to flow through 50 cm of water (in a 14mm dia tube) and it really would have a high electrical resistance in its path.
     
  16. Sep 24, 2012 #15

    nsaspook

    User Avatar
    Science Advisor

  17. Sep 24, 2012 #16
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What is the reason for using electrically insulating joints in a water system?
Loading...