Dielectric Test

Hello Guest,

There is the "Dielectric Test", the "Dielectric Strength Test", and also "Dielectric Constant".

Both Tests involve using gradually increasing the voltage being applied between 2 electrodes, for a specified time, with an insulating medium between those electrodes.

The insulating medium is called "The Dielectric", and may be air/gas/glass/oil/plastic or any one of a number of materials which are electrically insulating (remember the perfect insulator does not exist, there is always leakage, however small), at normal voltages.

Careful use of Laboratory Test Equipment is needed, along with stringent safety requirements, and measurements of leakage current through the Dielectric are taken and constantly recorded as the applied voltage is increased.

The dielectric constant is the relative permittivity of a dielectric material

There is a constant for all Dielectrics, known as "The Dielectric Constant" = for that material, and makers of capacitors and circuit designers use these Tables such as here:

http://www.asiinstruments.com/technical/Dielectric%20Constants.htm

Please note that the Dielectric Constant of Air is 1 = Reference standard (The Standard had to start from somewhere, and air is normally readily available as a reference point).

A capacitor maker may save materials, costs, and size by careful use of insulating media with a high Dielectric Constant, as long as it can withstand the maximum breakdown voltage expected in a circuit.

Capacitors "Normal Rated Voltage" is marked on the capacitor case at less than 50 % of that breakdown voltage - A capacitor marked for duty at 50 Volts DC, would normally survive a peak DC impulse of 100 Volts.

I remember some 45 years ago, seeing my first "Tantalum" capacitors which had a Dielectric of TANTALUM OXIDE at 11.6, which meant they were extremely small sized for the capacitance stated on the outside.

Please appreciate the above is rather as shortened explanation.

Should you need further info, please reply here.....

Sparkstation

Please note that the Dielectric Constant of Air is 1 = Reference standard (The Standard had to start from somewhere, and air is normally readily available as a reference point).

Isn't the reference standard of "1" for a vacuum?

I think the dielectric constant of air varies with pressure and relative humidity. I've seen a value for dry air of 1.00059 at 1 atmosphere of pressure.

Hello joeconed,

Thank you for your kind advisory reply, and much appreciated

It all seems to depend on which source you take.

I do note the headline from:

http://my.execpc.com/~endlr/dielectric_const_.html

Table 14 shows the approximate dielectric constants of a variety of materials, including capacitor dielectrics, and coax cable insulations. These are rough averages of numbers from several handbooks and various manufacturers' literature, which often disagree significantly. Most are for 25C but some are at 20C.

When I started work long ago, the Reference Standard was Dry Air at 20 degrees Celsius, and still appears to be so, according to many reputable sources .

It does appear that during the intervening years, some Reference Standards quotes may have decided to alter the Reference material to a Dry Vacuum.

For most practical purposes, the 0.00059 difference would be of little importance.

As I said in my earlier Post, Air was originally used as the definitive Reference Point = 1, because it is "readily available" in most testing situations.

It is also good to remember that "Nature abhors a Vacuum", and a perfect vacuum is impossible to achieve, even in the depths of inter-galactic space.

If you are able to show in a Reply Post when, and by which Authority, the Reference Standard was officially changed, please advise here

Then I shall tuck that new Dielectric Constant Reference Point in my Memory Bank, thank you.....

Sparkstation

Here's the link to the info I provided

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/diel.html

I have to agree with you that it really depends on the source that one choses to use. I also agree that the difference for vacuum and air appears to be insignificant to me. However, some scientist may disagree. I'm sure a mathematician would since they tend to deal in numbers to the nth degree.

Can any body suggest the suitable Meter ( the manufacturer's name and address ) for measuring the leakage current during the dielectric test.

Murali.

You really shouldn't have to purchase an ammeter separately. An ammeter of suitable range is always integral with all dielectric test sets that I have seen. An exception (of a sort) is an insulation resistance tester which has an ammeter but it reads out in megohms which is really the parameter of concern. You can convert the megohm reading to current by simply dividing the megohm reading by the test set voltage.

Hello Guest,

There are manufacturers who make Testing sets for the purpose.

Should you need one: Dielectric Test Meter ( Type those words into Google Search Engine)

You do not need a "Special Meter", if you are able to interpret meter readings successfully.

Any microammeter/milliammeter will do the reading correctly,and you should take notes (through the protective screen) of the progressive readings as the Voltage is steadily increased..

To tabulate successive readings in graphical form is of great help.

Ensure you connect the meter in the GROUND leg of the circuit, not the high-voltage leg.

If you are using a recording type of meter, best to use a battery-powered one: If the recording meter requires a Mains Power supply for operating, then be aware of potential problems.

Ensure also you have a transparent protective screen, between you and the meter.

Ensure you are aware at all times that you are working with High Voltages, and take proper safety precautions to prevent accident or death.

That way you don't forget about the high voltage the meter case and leads may rise to, if the Earth wire connection open circuits at any time.

Experience will be the best teacher, as you can follow the graphical curve of your readings, and are able to interpret those readings taken during a Test, so you do not have to test the item to destruction.

But of course, if you are in a Test Laboratory situation, you would know all the above, anyway....

Kind Regards.....

i think this answer is more accurate for the meaning of dielectric test:

Definition: The application of a higher than rated voltage for a specified time, in order to determine the capability of insulating materials to withstand breakdown.

Definition Copyright ©1989 CRC Press LLC. All rights reserved.

thanks

Hi there. I just wanted to add a description of what is meant by breakdown. When the voltage is increased across the dielectric at some point electrons manage to pierce the dielectric and electricity starts actually flowing through it . This process is not reversible as the material is physically damaged. So don't try to test the breakdown voltage of something you need to use later. The voltage at which this happens is directly proportional to the thickness of the dielectric and the dielectric constant of the material used as dielectric. Also the temperature of the dielectric has an effect. Usually a voltage equal to half of the breakdown voltage is considered safe although this percentage may vary when you go to higher voltages. When dealing with alternating current the peak voltage must be considered and not the RMS value.

I think this much should be enough.

Sorry for joining this discussion with a related question. I have come across an application where `filled epoxy' is used in between two metals for electrical insulation. I wish to know, will there be any problem, if any porosity or air pockets are found in this sandwitched epoxy layer of 5-6 mm. Is air a better insulating medium than filled epoxy material. Filled epoxy is epoxy resin filled with fine sand / silica.

Hello Humble Ess,

Because many epoxy products actually absorb water, some more than others, if there are cavities in the complete product, they may become partially filled with moisture, more and more as time progresses.

If you are intending to make this product, then it would be best to:

1. Ensure the Sand/Silica is totally dry, and uniform in size
   
2. Ensure the mixing is done in a vacuum stirrer (Easy to make one yourself - just a mixing chamber connected to a vacuum pump)
3. If you can keep the mixture warm while stirring/setting, it does assist with removal of voids
   
4. Ensure the Epoxy was fully set while still inside a vacuum chamber

That should ensure there are no voids or trapped air pockets.

Years ago, I used exactly this method, to eliminate voids in 2 pot cast resin for High Voltage special Electrical Service materials.

If you need further assistance, advise here, thank you....

The application is to prevent / reduce electrolytic corrosion in salt water by insulating the two surfaces of different metals by fillling the 5/6 mm gap with `Epocast' or `Chockfast' epoxy material. Since these filled epoxy materials are very viscous, they may not completely fill the gap between metal surfaces. I wish to know if any air pockets or voids remain between the metals, will it affect the insulation value?

Hello again, Humble Ess,

"I wish to know if any air pockets or voids remain between the metals, will it affect the insulation value?"

The answer is YES, as stated earlier.

The normal situation is that over time, a series of thermal expansion/contraction cycles will occur during the service life of your manufactured article.

If your article is ever going to change temperature, during its service life, any air pocket will raise pressure as it expands.

Epoxy Resins, when cured, absorb water, some very slightly, but all seem to absorb water to some degree (water is the only known "Universal Solvent", even dissolving glass or Platinum).

The water being absorbed, may be via the air humidity, or actual liquid immersion - it happens just the same.

As pressure in a cavity in most Epoxy Resins increases, due to temperature rise, the enclosed gas migrates via tiny "imperfections" between the Epoxy Resin lattice bonds, until that cavity pressure stabilises at the lower level.

Because the Epoxy Resin has absorbed tiny amounts of water, in microscopic amounts at first, as the epoxy article cools down, some water is drawn into each cavity, by atmospheric or immersion pressure.

After many or few, (dependent on the water absorbency of the cured Epoxy Resin, and the number of voids, remember these may not be seen unless under a microscope), each void ends up with more and more water inside, until there is no more air in the void, only water.

It is a common phenomenon, not just to Epoxy Resins, but many other materials commonly used.

That is the reason for ensuring the Epoxy Resin is both stirred and fully set under a vacuum.

The vacuum pump extracts all, (well mostly all), of the air in the mixing/curing chamber.

Thus this minimises the voids in the cured material = Epoxy Resin in your situation.

A simple and cheap test of water absorption can be by Dielectric Testing, or weight, using electronic scales, because absorbed water weighs more than air or gas voids in the epoxy.

In any Testing situation, you always test the article before it is put into service - mark it with Serial Number or other, and then a comparison may be made at a later date.

You don't say what the actual installation is:

1. Shipping - normally protected by sacrificial anodes, which are easily replaceable.
2. Oil Rig - Same as above
   
3. Power Pylons - Hot dipped galvanized, lifetime 30 - 90 years- by then because of increased electrical load, a larger set of transmission lines is normally required, complete with new pylons and adjacent to the original route - or different route as needed
   
4. Other (yet to be advised)

You have also not advised what voltage difference is between the dissimilar metals.

That is easily checked by noting their respective positions in the Electrochemical Series Tables.

An Electrochemical Series Table commonly referred to for galvanized steel installations is here:

http://www.azom.com/details.asp?ArticleID=1285

To obtain the Electrochemical Difference in voltage between different metals etc, just the difference between them, remember H₂ (Hydrogen is 0.00 the Reference).

http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials

Example: Two elements in aqueous conducting solution: What electric potential exists between them?

Plate 1 = Made from Lithium - Li⁺(aq) + e⁻ → Li(s) −3.05

Plate 2 = Made from Silver - Ag²⁺(aq) + e⁻ → Ag⁺(aq) +1.98

The Electrochemical difference is thus 3.05 + 1.98 = 5.03 Volts per cell.

So your Electric cell, (commonly called a "battery" which is correctly describing more than one cell connected together), made with a Lithium Plate and a Silver Plate, can deliver 5.03 volts.

That is because Li is electronegative -3.05 volts, with reference to Hydrogen at 0.00, while Ag is +1.98 with reference to Hydrogen at 0.00.

So you see that dependent on your choice of "cell electrodes", lesser or increased voltage is electrochemically obtained.

If the electrode materials are very far apart, eg Lithium (Li) and Fluorine (F), (You could theoretically get 6.10 volts per cell - but because of extreme reactivity, your LiF cell would vanish in a quick reaction = explosion)

I am uncertain of your skills, so apologise if the above seems rather elementary to you.

To sum it all up, if you give further advice on the nature of the installation characteristics of your particular situation, it may be that further assistance is given you here.

Thank you..... .....

Hi Sparkstation,

Thanks for your info which is really informative.

I will now first do a mock-up to see if the epoxy can fill the 5/6 mm gap completely. If it does, will then carry out dielectric test. This will take some time, but I will surely let you know the results.

Thanks once to you and all others for their valuable suggestions.

Such fillings are carried out in Vacuum if Hi-Reliability is needed.
Set-up for filling is in a Chamber having a Vacuum-Pump to suck air from the chamber.

It is a "Destructive-Test", so only sample of material is tested.

it is the ability of the dielectric to withstand high voltages without breakdown. After breakdown the insulator transforms into the conductor and high current flows through it. This further causes high thermal stresses and may cause melting and loss of mechanical strength of the insulator. Apart from standards; (2*operating voltage)+1000V = test voltage.