Dielectric Joint Compounds

[sbmar]

Gentlemen:

Hopefully this is the right place for this-------I have been struggling with coming up with a plausible explanation (in layman terms) as to defining a particular trait that dielectrics or "near" dielectrics" seem to possess when applied in-between, or on to electrically conductive materials (in this case, I am referring to electrical terminals or metal threaded connections)


QUESTION: Why do most any common petroleum ("Vaseline" for example, or synthetic based dielectric (or "near" dielectric) compounds (from common lithium grease, many pipe dopes, silicone based dielectric greases and even #2 diesel) seem to exhibit the quality of a perfect dielectric compound when tested with a typical DVM, or at lower AC or DC voltages (maybe up to 300V ?) , and yet when put in-between two electrical terminals / connections, or other mechanical connections ( treaded, bolted, etc) , the insulating properties of the compound disappear completely and in general use, seems to make for a better electrical connection??


Is this a phenomenon that that has been discussed in the past in a semi-technical fashion, but I cannot find a reference to? I believe there are many misconceptions about this subject as to how could a dielectric do anything but weaken the connection or insulate two surfaces. A very common example of this property is one you may have under the hood of your car is a WIF (water if fuel) sensor circuit that is part of the fuel filter system on your Dodge, BMW, etc., diesel engine. The probe is immersed in #2 (a "near" dielectric) , but as soon as water is present, the probes complete the circuit although #2 is still in the mix and still coating the probes.. The reason I mention this particular example is because we are current developing a "universal" WIF circuit instrument that can be easily adapted to aftermarket uses.

A few years ago I wrote a small piece for a marine forum on the subject, but I am still not happy with my thoughts and explanations on the subject– a small excerpt:


Dielectric compounds (greases and the like) are used in most connections to help maintain a good electrical contact which is established by the mechanical connection of two or more pieces, whether it be a fixed connection or a switched/sliding contact connection.

To put it another way, always use non-reactive, non-conducting, moisture displacing dielectric greases or lubricants when making electrical connections. These "moisture displacers" prevent moisture and oxygen present in the environment from contacting the metal surfaces of electrical contacts, thereby limiting corrosion and preventing the buildup of resistance within the connection. The basic principle used here is the "exclusion principle" and simply put, it is using a material (a dielectric grease or lubricant in this case) to occupy all of the spaces near and around an electrical connector that could otherwise be occupied by contaminants (possibly conductive) or moisture.

Is there anyone out there that would care to share some thoughts on this subject or steer me to some reference material that would cater to the less formally educated on the subject?


Tony



Tony Athens

http://www.sbmar.com

 

[VEBill]

I'd assumed that it was as simple as: such greasy materials simply keep the water out so the connector doesn't corrode.

One obvious reason to use an insulating material is so that it doesn't short-out adjacent connector pins.

 

[roydm]

Vaseline (petrolium jelly) on rubbing contacts like a crane drum controller cuts down the mechanical friction. I'm sure if you were to look under a microscope the actual contact point has no layer of grease.
For battery terminals vaseline excludes the air so that the terminal doesn't oxidize. I think this would apply to marine connections also, the grease excludes air or moisture from the joint.

 

[stevenal]

Anti-oxidant is the key property. Suggest you substitute anti-oxidant for dielectric in your description above. Some anti-oxidant compounds contain conductive particles (use in compression sleeves, not bolted connectors). Also, "lubricant" is not descriptive either, since that property is not involved. Check Burndy and Anderson connector websites.

 

[Compositepro]

The other key point is that these greases do not interfere with the metal to metal contact. They are displaced by the contact pressure. Gaps are filled so corrosion will not occur. There are dielectric greases used for electrical insulation and to improve heat transfer to heat sinks. These contain electically insulating particles to prevent metal to metal contact.