The Engineering Behind Dielectric Testing

Dielectric Testing on bucket trucks

 

Dielectric testing refers to the process of assessing electrical insulation. When it comes to checking the insulation in mobile homes, you have to test the insulation at a slightly higher voltage compared to the normal voltage. This method of approach makes sure that insulation does not start at a marginal level. With an accurate voltage setting, dielectric testing using direct current (DC) offers the same results as utilizing alternating current (AC). However, DC testing provides better information on performance and safety enhancements.

The objective of dielectric testing

 

Dielectric testing is not a complex process. It does not cause any damage to the materials tested. This simple method corroborates the sufficiency of electrical insulation to withstand temporary power surges. Lightning strikes and short circuits are the major reasons for voltage spikes on power lines. These types of power surges last only for a small duration. That is to say, transient spikes last only under 20 microseconds.

Dielectric testing can also be used to verify whether particular insulation has sufficient performance headroom. This type of verification is needed to make sure that the insulation does not fail due to aging, vibration wear, moisture, and other types of reasons. Depending on the environmental conditions, you need to make the necessary changes to the voltage level used in the dielectric testing. In severe conditions, an increased amount of higher dielectric test voltage needs to be used.

Dielectric testing method

This type of testing is done by applying a high voltage between two conductors that are believed to be electrically insulated from each other. Under normal circumstances, the voltage is kept below 1000V. If there is complete isolation between two conductors, the application of a large voltage variation between two conductors would prevent the current flow between them. So, the insulation is capable of surviving or withstanding the application of a large voltage potential between the conductors. That is exactly why it is known as the dielectric withstand testing. .

Here are the two results that suggest an insulation failure:

1. Excessive flow of current

2. A sudden electric breakdown

Reduced insulation resistance is what causes excessive current flow during the dielectric test. If the insulating material that divides the two conductors has low insulation resistance; you can expect an increased flow of current.

An instant dielectric breakdown also suggests insulation failure. It may occur due to electrical discharge or arcing through the air or through insulation material or its surface.

Learn about the test voltage

You cannot expect proper stressing of the insulation material with reduced dielectric testing voltage. This situation permits insufficient insulation that is unacceptable to pass the test. On the contrary, a very high voltage could damage the insulation material that would have been otherwise sufficient for the application.

For 120 – 240 V alternating current mains wiring, you can use 1000 V and also double the operating voltage. That is to say, if you have 120-V wiring, you can test it with a voltage of 1240 V ac. You need to add 1000 V and two times of 120 V. So, the total is 1240.

What about the duration of dielectric test?

 

For creating the right amount of stress on insulation, one minute has been kept as the standard duration for applying test voltage. The duration of application can be lowered by increasing the test voltage. To accommodate large volume needs, test duration can even be lowered to 1 second by increasing the test voltage by 20%.

Alternating current and direct current

120 Vac is considered as the minimal voltage in the USA. The frequency of voltage is 60 Hz. The waveform of voltage is sinusoidal. 120 Volts is the RMS (Root-Mean-Square) value of the voltage. The RMS value of an AC voltage offers an equivalent to the DC voltage heating value. For example; applying an AC voltage of 120 RMS to a resistor and applying 120 V DC would deliver the same heat output.

An overview of test voltages

What is the ultimate purpose of dielectric testing? A dielectric test exerts stress on the insulation for a small period to find out that it doesn’t fail. A 60-Hz ac voltage testing is done basically because of the convenience. To get the dielectric testing done, you can use a transformer with a high-voltage secondary winding. It produces an increased voltage required to perform this type of test.

It is hard for a 60-Hz ac test voltage to simulate real events better than a dc test voltage. You may find increased voltage spikes on the 120-V ac mains. These surges are not alternating current, but momentary voltage transients with a particular duration that is gauged in milliseconds or microseconds.

When it comes to choosing between AC and DC voltage for testing, you have to assess the goal of the test. As the voltage gets higher, you need to apply an increased amount of stress to the insulation.

The voltage peak of an AC test voltage of 1000 V RMS is 1414 V. Hence, if you use a dc test voltage, you must elevate the test voltage to 1414 V DC to create a similar amount of stress on the insulation.

Several national and international testing and standards writing organizations approve the difference in test voltage for DC compared to test voltage for AC. Some of these organizations and agencies are:

 

The American National Standards Institute

Underwriters Laboratories

The Institute of Electrical and Electronics Engineers

Factory Mutual Corp.

International Electrotechnical Commission.

Dielectric Breakdown Evaluation

At the peak of the AC waveform, the electrical stress on the insulation stays at the highest levels. So, dielectric breakdown takes place at the peak of an AC test voltage. The pace at which electric breakdown occurs is incredibly fast. Since this type of abrupt breakdown takes place at the peak voltage of the AC waveform, you can find both AC and DC voltages precisely the same. Further, the durability of the AC waveform peak voltage is higher than the breakdown process.

Pros and cons of dielectric testing

In the past, it has been hard to generate DC test voltages. This situation has led to the need for more efficient, expensive, and advanced test equipment. This disadvantage has been resolved to a great extent by the performance enhancements and safety improvements achieved from utilizing DC test voltages.

Let us throw light on some extra background information to explain the benefits of dielectric testing better. When you apply a voltage difference between two conductors that are divided by an insulator, an electric charge is created. The electric charge produced stays proportional to the capacitance between the conductors and applied voltage. We can put this theory into a formula. Let us use Q to represent charge and V to denote voltage. C can be used for capacitance. Then, you can represent the mathematical relationship between charge, voltage, and capacitance as Q = C x V.

When it comes to practical applications, the capacitance can subsist because of discrete capacitors. However, the capacitance is formed unintentionally when two conductors with a difference in voltages are kept together closely. You can come across this type of capacitance in multi-conductor electrical wiring, transformers, and electric motors. The charge changes with the variations in the voltage. For example; if the voltage oscillates between positive and negative directions, the charge will repeat the same movements.

Another basic concept you need to be aware of is that whenever voltage changes take place, electric current flows through a capacitor. The logic is simple. When there is a voltage increase across a capacitor, the quantity of charge also goes up. Electric current is basically a measurement of the number of changes in charge that occur over a while. The letter I’ is used to represent current. Ampere is the unit to measure current. Coulombs is the unit used to measure the quantity of charge Q. What is 1 ampere of current? It refers to a charge flow of 1 coulomb per second.

When you blend the concept of current and capacitance, you can arrive at the following conclusions:

A changing charge is generated by a changing voltage. Changing charge refers to electric current flow. Thus, whenever voltage changes occur, the current starts to flow between two conductors. Due to the capacitance between the conductors, the current flow can take place even when they are divided by an insulator. As the capacitance between the two conductors increases, the current flow also increases.

When dielectric testing is done using AC voltage, there will be an electric current flow between the two points being examined. You cannot expect this current to represent a failing test result due to a reduced insulation resistance. So an AC dielectric tester should make up for this permissible current flow.

How to achieve this task? The easiest way to do this is to permit the tester to discover a considerable quantity of current without showing an additional current failure. If you use the same dielectric tester device to test multiple products, you have to set the current limit set point to accommodate the equipment with increased capacitance. That is to say, you should desensitize the dielectric testing device to help it ignore current levels.

This method of approach can generate two dangerous issues. A desensitized AC dielectric tester cannot identify the difference between 5 and 15 mA. What would happen if a testing circuit has a capacitance between two conductors that generate 5 mA to flow in standard conditions while performing the test? If you check a device with faulty insulation that permits 300% of the standard quantity of current flow (15 mA), the desensitized dielectric testing device would find it as an acceptable test result.

A desensitized AC dielectric tester may send down a dangerous level of current to the human body without shutting down. This situation can seriously injure or kill the operator. When you do dielectric tests with a dc test voltage, the electric current flow occurs only when the voltage shoots up from 0 V to the highest test voltage. In this situation, the voltage shoots up within 1 or 2 seconds and the current flow stays at minimal levels. On the other hand, an AC test voltage oscillates between positive peaks to negative peaks 60 times per second.

Remember that the sudden increase in a DC test voltage occurs over a timeframe of 2 seconds. So, it creates only 1/120 or less than 1% of the current flow of an AC test voltage. When the DC voltage goes up to the final test level, the current inevitably stops completely. The quantity of current flowing during a DC dielectric test is trivial irrespective of the capacitance amount that exists in the device under test.

When you make a comparison between AC and DC dielectric testing, you can associate several benefits with the latter. The major benefits can be summarized as follows:

– The maximum permissible test current can be adjusted to a very convenient low level such as 1 mA.

– The DC dielectric testing device activates a shutoff mode when it finds that more than 1 mA of current flowing during the test.

– With a highly sensitive test method, A DC dielectric tester helps the operator detect even marginal constructions that an AC tester would often fail to notice.

– Since DC tester offers the best protection for the operator with lower test-current levels.

A few nanoseconds are enough for a dielectric breakdown. An altering 60-Hz AC voltage becomes an un-altering DC voltage instantly. Both AC and DC dielectric testing is good enough to verify the insulation suitability if the peak voltages of these two withstand tests remain the same. To keep the peak voltages at the same levels, the DC voltage utilized in dielectric testing should be 1.414 times higher than the AC RMS voltage used.

If you compare these two dialectical testing methods, you can find the DC testing method more advantageous. Because of the total insulation failure, both these testing methods offer an equivalent level of breakdown identification. Nevertheless, the improved precision of DC leakage-current detection identifies even marginal insulation systems. You can also expect enhanced safety of the operator with DC dielectric testing.

 

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