Megger vs VLF tan delta testing

(I know I'm not answering the question directly)

Most "meggers" only go to volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is bad, but it won't tell you that it's really good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.

A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-/

If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB .pdf
(Okonite has a bunch of info about testing).

(I know I'm not answering the question directly)

Most "meggers" only go to volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is bad, but it won't tell you that it's really good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.

A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-/

If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB .pdf
(Okonite has a bunch of info about testing).

Ok, interesting stuff.
I would agree that you didn't quite answer my question.
We have roughly 40 feeder cables on site. These cables have been meggered the past 50 years and like I mentioned, when we start getting to around 150 Megaohms, we know trouble is coming soon.

We do our preventative maintenance every 3 years. So roughly test 13 feeder cables a year.

The cables have been neglected by guy before me. His motto was we will deal with it when the time comes. Well, we had two shorted cables last year. Luckily they had back up feeds, but eventually back up feeds run out.

We have 3 cables that we are replacing right now.

Looking forward to the next 5 years, per the megger testing, it look like we need to replace 5 more additional cables.

During our shutdown in July this year, I have the option of VLF testing on the 13 cables. I can obviously do this the next two years as well.

I guess the real question is this:

Should I spend an extra $15,000 to test these 13 cables per year over the next 3 years? I can sort of fit it in the budget but I would like to know if its REALLY going to give me info.....such as, yes this cable meggered bad, but its really OK per the VLF test.

$15,000 (per year) sounds high to me, but maybe not. Please keep in mind these cables will be de-energized and disconnected for megger test already.

If the megger reads bad, but VLF says cable is good, than the $15,000 investment seems wise. Replacing these underground cables is super expensive. VLF (Very Low Frequency) tan delta testing provides additional insights into the condition of cable insulation that simple "meggering" (insulation resistance testing) cannot offer. Here's a detailed comparison and explanation of why VLF tan delta testing is beneficial:

### Meggering (Insulation Resistance Testing)
1. **Principle**: Measures the insulation resistance of a cable by applying a DC voltage and recording the resistance value.
2. **Frequency**: DC voltage.
3. **What It Tells You**:
- Indicates the general condition of the insulation.
- Can identify gross failures and significant degradation.
- Provides a snapshot of insulation resistance at a particular moment.
4. **Limitations**:
- Limited in detecting water trees or other aging mechanisms not visible through simple resistance measurement.
- Doesn't provide a detailed analysis of insulation health over time.

### VLF Tan Delta Testing
1. **Principle**: Measures the dissipation factor (tan delta) of the insulation by applying a sinusoidal AC voltage at a very low frequency (typically 0.1 Hz).
2. **Frequency**: Very Low Frequency (0.1 Hz).
3. **What It Tells You**:
- **Dielectric Losses**: Tan delta is a measure of dielectric losses within the insulation, which can indicate the presence of moisture, contaminants, or aging.
- **Insulation Quality**: Provides a quantitative measure of the insulation's dielectric properties and its ability to withstand electrical stress over time.
- **Condition Over Time**: Detects trends in insulation condition, allowing for predictive maintenance rather than reactive maintenance.
4. **Benefits**:
- **Early Detection**: Can identify insulation deterioration at an early stage, even before significant resistance degradation is evident.
- **Comprehensive Analysis**: Offers a more comprehensive understanding of insulation health, including detection of water trees, which are common in older cables submerged in water.
- **Condition Monitoring**: Facilitates ongoing condition monitoring, helping to prioritize cable replacements and prevent unexpected failures.

### Why VLF Tan Delta Testing is Worth the Investment
- **Preventive Maintenance**: By identifying cables that are likely to fail in the near future, VLF tan delta testing enables proactive replacement and maintenance, potentially avoiding costly unplanned outages and damage.
- **Long-term Planning**: Provides data that can be used for long-term asset management, helping to extend the life of cables and optimize maintenance schedules.
- **Cost-effectiveness**: While the initial cost of VLF tan delta testing is higher, it can lead to significant cost savings by preventing failures and optimizing maintenance.

### Conclusion
While "meggering" gives you a basic understanding of the insulation resistance, VLF tan delta testing provides a deeper, more comprehensive analysis of the insulation condition. It helps detect issues that might not be evident through resistance testing alone, especially in an older site with cables susceptible to water damage and aging. Investing in VLF tan delta testing can ultimately save costs by preventing unexpected failures and optimizing maintenance schedules.

Most important, You save money after you invest the VLF test and because of the extending your cable life-time

Electrical safety (Hipot) tester functions and application

Insulation resistance testing is likely to be required in motor winding, transformer winding and other applications involving cabling or insulated wire. Insulation resistance testing typically involves confirming that the resistance exceeds a defined high resistance value.

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In many instances, insulation resistance needs to be measured between several conductors. Examples include cable/connector assemblies, multiconductor cables and relays. To make this measurement, all the conductors except one are shorted together and the test voltage is applied from the remaining conductor across the bundled ones. Each wire is then, in turn, tested in this fashion (Figure 2).

Desirable insulation resistance test features 

Wide range of selectable test voltages

Accurate/repeatable high-resistance measurement

Programmable high-voltage switching accessory

Multichannel programmable testing

Pass on steady and Increasing voltage

Ground continuity

Ground continuity testing is performed to confirm that the conductive chassis of a device is safely connected to the earth ground pin on the power plug. This ensures protection against shock hazards even if the equipment suffers an internal short to the chassis. Ground continuity is performed by measuring by applying a low current (e.g., 50 mA) and calculating the resistance from the ground pin on the power plug to selected locations on the exposed surfaces of the DUT. Desirable ground continuity features include:

Accurate, repeatable low resistance meter

Plug adaptor accessory to speed testing

Ground bond

Where ground continuity measures the resistance of the safety ground connection, the ground bond test assures the integrity of the connection. Using the same test setup, a high current is passed through the circuit. If the ground bond is solid, the current passes without a change in resistance. If weak, the resistive heating of the current would induce a failure of the bond.

Desirable ground bond test features

Accurate high current source

Programmable test currents and test times

Plug adaptor accessory to speed testing

4-wire milliohm meter&#;providing a Kelvin connection for highly accurate low resistance measurements.

Hipot test station setup and operation

Because there is no substitute for operator competence, the importance of having trained personnel as the first step to a safe testing environment can&#;t be overstated.  Operators must understand the workings and importance of safety interlocks and why the interlocks should never be disabled. They must also understand the hazards of wearing metallic jewelry around electrical equipment and show how to interrupt power quickly in emergency situations.

On a regular basis, typically at the start of every shift, the tester itself should be checked by connecting the tester to both PASS and FAIL samples. These samples should be designed to confirm the proper operation of the tester based on the type(s) of tests to be conducted (hipot, insulation resistance, ground resistance, and ground bond). 

Location of the hipot test station

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The test area should be isolated from the factory assembly area. It should be located away from foot traffic to assure the safety of passersby and, of course, the safety of the station operator. The area should be conspicuously marked with internationally approved signage, such as "DANGER&#;HIGH VOLTAGE." During testing, the hipot tester itself should have indicator lights to denote when high voltage is present. 

The work area and bench surface should consist of nonmetallic materials; which means that metal work surfaces should be avoided, and no metal objects should not be placed between operator and DUT. All other metal objects should be grounded or be out of the test area altogether. An ESD mat is not a recommended platform for your test station, as it may cause erroneous readings for leakage and is unnecessary in this application. 

In addition, the test equipment should provide for immediate and safe removal of the output voltage using internal discharge circuity at the conclusion of the test or if the test is interrupted. Never remove power for the hipot tester. If there is a power interruption, use extreme care in any contact with the DUT. The safest approach is to leave the DUT connected to the hipot tester until power is restored and the tester can conduct its discharge function.

Operator safety considerations

The test station should have sufficient space for the tester and the DUT without the operator having to reach over the DUT to access the tester. There are many safety features that can be added to the test station to prevent the operator from encountering high voltage, such as guards or enclosures. When placed around a DUT they should be nonconducting and be equipped with safety interlocks that interrupt all high voltages when open.

Palm switches can be utilized to prevent the operator from encountering high voltage during testing. The basic operation of a palm switch requires the operator to use both hands to initiate a test with, potentially, a foot switch to activate the test. If one or both hands are removed from the while testing, the test is immediately stopped. No high voltage can be applied to the output terminals and DUT until both switches are pressed simultaneously. The palm switches are connected to the digital I/O on the hipot tester. When the switches are in the down position the start is enabled. Once one switch goes up the safety interlock is enabled, terminating the output voltage of the hipot test. This method is safe, quick, and effective.

Figure 3 illustrates two alternative approaches to the setup of a benchtop hipot test. In Figure 3a, the DUT is placed on the test bench and a combination of palm switches and a foot switch ensure that the operator cannot make contact with the DUT while the test is underway. The operator is wearing safety glasses.  As a practical matter, the use of palm switches is typically restricted to short-duration tests done on a repetitive basis with a series of DUTs. If this test setup is used for longer tests, operators will find a way to defeat the palm switches.

In figure 3b, the DUT is placed under a protective cover with interlock to isolate the operator during the test. The use of an enclosure is a more reliable means to assuring operator safety, particularly when testing requires longer time periods.

AC DC Hipot Test

Such a test applies a voltage to the DUT that is much higher than normal operating voltage; typically V AC plus twice the normal operating voltage. For a household appliance designed to operate at 120 or 240V AC, the test voltage is usually about to V AC.

A DC hipot test can usually be substituted for an AC hipot test. The best voltage for a DC hipot is normally higher than the AC test voltage by a factor of 1.414. A product that would be tested at V AC would be tested at V DC.

For double-insulated products, the required test voltages may be much higher, such as VAC or even VAC for a 120 VAC power tool. The voltage is applied between the operating circuits and the chassis or ground &#; the parts of a product that a consumer might touch or come in contact with.

Refer to Figure 5 for typical AC hipot test setup. The setup for a DC hipot test would be identical.

The purpose of the test is to make sure consumers do not receive an electrical shock when they use the product. This typically is caused by a breakdown of the electrical insulation. The test also detects possible defects in design and workmanship that cause components and conductors to be too closely spaced. The danger is that air gaps between conductors or circuit components may become clogged with dust, dirt, and other contaminants over time in typical user environments. If the design spacing is inadequate, a shock hazard can occur after a period of use. By subjecting the product to a very high voltage, the hipot test overstresses the product to the point that arcing may occur if the spacing is too close. If the product passes the hipot test, it is very unlikely to cause an electrical shock in normal use.

Withstanding a very high voltage means that a large margin of protection exists for the consumer. Regulatory agencies usually require a stringent hipot test as a product &#;type test&#; before releasing the product for sale to the public and another less demanding test to be used on the production line. As a rule, testing laboratories consider the hipot test to be the most important safeguard for the consumer. They may accept &#;design&#; or &#;type&#; tests for other types of tests, but always require hipot tests for 100% of the units in a production line.

HIPOT Testers & Test Equipment

Discover the pinnacle of electrical safety testing with our comprehensive selection of HIPOT Testers, engineered to ensure the integrity of insulation in electrical equipment. Our catalog features a versatile array of HIPOT solutions, including AC HIPOTs for routine compliance testing, DC Dielectric Test Sets for diagnostic applications, and VLF (Very Low Frequency) HIPOTs, which are specifically designed for testing the reliability of cable insulation.

HIPOT Tester Industrial Uses

Ensuring Electrical Safety: HIPOT testers are crucial in verifying electrical insulation in products and ensuring they are safe for use. They are widely used in the manufacturing of electrical appliances, cables, and components to prevent electric shock hazards.

Quality Control in Manufacturing: In the industrial manufacturing sector, HIPOT testers play a vital role in quality control. They are used to ensure that products meet safety standards and regulations, which is essential for maintaining brand integrity and consumer trust.

Preventive Maintenance and Troubleshooting: Beyond manufacturing, these testers are used in the maintenance of electrical systems. They help in identifying insulation breakdowns, preventing potential failures and accidents in industrial settings.

Compliance with Industry Standards: HIPOT testers are essential for industries to comply with various national and international electrical safety standards. They are used in testing a wide range of products, from small household appliances to large industrial machinery.

How HIPOT Testers Work

Basic Principle: The fundamental principle of a HIPOT tester is to apply a high voltage to a device&#;s insulation and measure the current that leaks through. This test helps in determining the adequacy of the insulation in protecting users from electrical shocks.

Test Process: In a typical HIPOT test, the device under test is connected to the tester. A high voltage, higher than the device&#;s operational voltage, is then applied. This voltage is maintained for a specific time to check the insulation&#;s ability to hold this stress without breakdown.

Measurement and Interpretation: The HIPOT tester measures the amount of leakage current flowing through the insulation. The results are then interpreted based on predefined standards &#; a low leakage current indicates good insulation, while a high leakage current suggests potential insulation failure.

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