Rentals Available

Do you have short term requirements for our testing equipment? If purchasing equipment isn't practical for your business at this time, please consider the following Diagnostic Test Equipment Rental providers.

Frequently Asked Questions

The following are answers to common questions about our products and diagnostic services. As always if your question is not addressed here, you may contact us online or call us at +1 (678) 445-2555 and we will be happy to assist you.

What is VLF?

VLF stands for Very Low Frequency and is nominally 0.1Hz, although frequencies as low as 0.01 Hz are still considered VLF.

VLF, like power frequency is still AC and the output waveform is also a sinewave, although there are some cheaper, less sophisticated VLF units on the market that have a quasi-sinewave or square wave output.

The practical benefits of VLF over power frequency for high voltage testing is the significantly reduced size, weight and cost of the test instruments, particularly when testing high capacitive loads like shielded power cables, large generators/motors etc.

Is a VLF AC test a destructive test?

It depends on ones point of view.

Elevated voltages (above normal) on a weak cable can cause cable failure at one of those weak points. AC high voltage testing at elevated voltages (at 50/60Hz or VLF testing) can cause weak elements in a cable to fail. It has nothing to do with the frequency of the applied voltage waveform and everything to do with the pressure - that is the voltage, applied.  

As long as one follows the correct standards for withstand testing and the cable fails this test, then depending on your interpretation of word "destructive", you have exposed and detected a severe weakness in the cable, that would very likely have failed in normal service. The big difference is that a failure under test has very limited current and therefore power injected into the fault, while a unplanned failure under service, can cause catastrophic and collateral damage to other devices in the area of the failure and customer interruptions etc.

Note that based on extensive research performed by a large university, they found that about 3% of cables may fail a withstand test if one applies the correct withstand voltages, so don't think that every cable you perform a withstand test will fail. Only those that have very severe defects will run to failure.

Also, learn more about VLF and Hipot testing here.

Where can I find the VLF Test Voltages for a VLF Withstand Test on MV Cables?

*Use of this table is voluntary and not binding.

Please refer to the latest version of IEEE 400.2 for the most up to date standard information.

How long a cable can I test?

The length of cable you can test depends on the cable being tested and the capabilities of the test instrument being used.

Regarding the cable, what is of primary importance is the capacitance of the cable. This value can be obtained from the cable's data sheet or by asking the cable manufacturer. This is normally given as per foot or per meter. Simply multiply this value by the distance of the cable in question and you have your capacitance.

Next look at the capabilities of the test instrument and the voltage and frequency that you will want to apply to the cable.


The HVA34 is one of the most popular VLF units on the market.

The HVA34 can test 0.5uF (0.5E-6 F) at full voltage (24kV RMS) & max. freq. (0.1Hz)
The cable that is to be tested has a capacitance of 100pF (100E-12 F) per foot.

What is the max. length of cable that can be tested using the HVA34 at full voltage and frequency?

So. . . 0.5E-6 / 100E-12 = 5000 feet of cable!

Remember that if a lower frequency is selected, such as 0.05Hz, this will be 10,000 feet of cable (half frequency, double the length).

Also note that if the voltage is decreased, the length of cable can also be increased. If for example 16kV is applied, then a cable of about 11,000 feet can be tested.

Regarding short cables, our HVA series equipment can test cables as short as a few feet/meters as long as the cable(s) is properly terminated and the shield is grounded.

Why do I need to use a 240V supply when using the HVA90 and HVA120?

The HVA90 and the HVA120 have very powerful output capabilities that may require a 240V input when testing long cables at elevated voltages.

For cables longer than 2000m/6500ft and when testing above 60kVrms, we recommend to use a 220/240V AC(50/60Hz) power supply rather than 110/120V AC (50/60Hz).

Power Capabilities (HVA90/HVA120):
110/120VAC 1.2kVA (limited) 50/60Hz
220/240VAC 3kVA 50/60Hz

Can the HVA find the location of a fault?

The HVA Series equipment cannot find the location of a fault.

It can be used to estimate the length of the cable if the capacitance is known or help condition a fault if fault location needs to be performed.

HVA series has the ability "burn on arc" for up to 5 minutes to carbonize the fault to make it easier to locate.

For information on fault location test services and our fault location test equipment, click here.

How can I check if my high voltage test lead is good for a HVA series test instrument?

Test leads generally get a lot of abuse during their lifetime and are often subject to damage and possible failure.

While having a broken test lead can lead to some frustrating troubleshooting to find the guilty culprit, there are some basic tests that can be done to either help eliminate or to blame the test lead for a possible faulty test result.

Using a conventional multimeter in Ohm mode, check the resistance of the main conductor and shield.

Note: The test lead should be removed from the test equipment and obviously not energized for these tests to be carried out.

HVA28TD, HVA34TD, HVA45TD: The main conductor should have less than a few Ohms from end to end to show continuity.
Questions regarding the shield continuity should be addressed by calling HV Diagnostics.

HVA30/34: The main conductor should have less than a few Ohms from one end to the other end - that is full continuity.
The Shield should show continuity (less than a few ohms).

HVA60: Depending on the version of test lead, the main conductor should have approximately 10kOhms, 2.6kOhms or 1.2kOhms from end to end.
The Shield should show continuity (less than a few ohms).

HVA90: The main conductor should read approximately 1.2kOhm from end to end.
The Shield should show continuity (less than a few ohms).

HVA120: The main conductor should read approximately 1.2kOhm from end to end.
The Shield should show continuity (less than a few ohms).

Can I change the test data in my reports?

You CANNOT change the test data measured by any HV Diagnostics test equipment in the reports.

This is important from a security perspective to ensure the integrity of the test data is maintained.

The only items that can be changed are non-measured user defined data such as cable length, manufacturer, names, work order, etc.

Why am I having issues communicating with my HVA VLF/DC test equipment?

Normally the COM port is automatically selected by the Windows operating system to work and communicate with our instruments.

If you are having communication issues, first make sure you are using the correct RS232 Serial cable that we supplied.

There are many versions of serial cables on the market that do not work with our equipment. Even if you have a cable that fits mechanically, it will not necessarily work. Also, we recommend using the USB to serial adapter interface that we supplied with this serial cable.

In the HVA Control Center Software, under the “Settings” tab, ensure that the “Select Com Port” is set to “AUTO”.
Make sure you have the latest software.

For HVA Series VLF/DC units (NOT VLF/TD), check the menu item “Instrument Settings” to ensure the USB Flash is set to “NO” to activate serial communication.

(USB Flash should be set to YES when downloading reports to USB memory sticks)

Finally, if the above does not work, in the Device Manager of the Windows operating system, check the COM port allocated to that device. Once you know the COM port number assigned to that device, you need to make sure the HVA Software uses that same com port number to link correctly to the instrument.

Please contact us if you need the latest HVA Control Center software and we will provide a free download link.

Which software works with my HV Diagnostics test equipment?

Click here to download our testing software.

*email address required

This software is used for reporting VLF Withstand test data and test sequence editing and creation.

HVA Control Center (Latest Version 3.14):
HVA28TD(FW 1.XX.X), HVA30, HVA30-7, HVA34, HVA60, HVA90, HVA120


This software is used for measuring and reporting Tan Delta diagnostic data when interfacing with the following test equipment:

TD Control Center (Latest Version 3.23):
TD30, TD60, TD90
TD60-MC, TD90-MC, TD120-MC


This software is specifically for configuring, measuring and reporting VLF Withstand results and Tan Delta diagnostic data:

HVD Control Center (Latest Version 3.70):


This software is specifically for configuring, measuring and reporting Partial Discharge diagnostics with the following PD test instruments:

HVD Suite:
PD30-E, PD60-2 , PD90-2 , PD120-2
PD60-2/TD, PD90-2/TD, PD120-2/TD

This software is specifically used for configuring, measuring and reporting dielectric breakdown with the following oil testers:

BA Control Center (Latest Version 3.02):
BA75, BA100

How often does my test equipment need calibration?

All HV Diagnostics test equipment require annual calibration excluding the below exceptions.

*Exceptions: TDR 1205CXA and EZ-Cable ID

What is the difference between a Test Sequence and a Test Report?

A test report contains the unique test data from your cable test.

You can use our reporting software to open these data files and subsequently save them as a PDF report or print them.

A test sequence will have the voltage and time profile – number of steps, voltage level and duration.

For example, for a withstand test you would setup one step at the required voltage and time.
Similarly, for a Tan Delta test you can set 3 or 4 steps and the HVA unit will automatically adjust the voltage and time accordingly. This way you can set up your test, and let it run automatically without any user intervention.

What is the password for my Bluetooth enabled equipment?

The password for Bluetooth connection to our equipment is “welcome”.  

Bluetooth enabled equipment includes the BA series, Tan Delta series, and the Integrated VLF/Tan Delta instruments.

*Note: Sometimes with Windows 10, manually connecting to Bluetooth enabled equipment may take up to 5 or 10 attempts before Windows finally prompts the user to enter the password.

How do I connect to the TD Control Center software?

TD Control Center Version 3.21 and newer have an AUTO function that makes it easy to connect.

On the bottom left side of the software is a drop-down menu. “AUTO” should be selected then you can click to “Connect to TD System”.

Once you have clicked this, a pop up will display the units within range. Select your unit and click “Connect”.

Your unit and the software will then be connected.

How long / short can the cable be in to perform a Tan Delta test?

The length limitation is dependent on two parameters: the cable's capacitance (that is length dependent) and the load capability of the test instrument being used for the test.

How short?
For a Tan Delta test the capacitance should be a minimum of 5,000pF to have enough current to run a test. To find the minimum length, divide 5,000pF by the capacitance per foot of your cable (contact manufacturer). On average, the capacitance per foot is 100pF/ft. The resulting number will give you the minimum needed to test.

Example: 5,000pF / (100pF/ft.) = 50ft. of cable.

How long?
Each unit has its own specifications so check your unit manual for its capabilities. For example, our HVA28TD can test up to approximately 5,000ft. (0.5uF) at maximum voltage and maximum frequency. Once the load exceeds that capacitance(0.5uF), the frequency will automatically drop to accommodate the load. You must pay attention to this frequency drop with Tan Delta diagnostics because you need to maintain 0.1Hz to evaluate a cable with Tan Delta.  

Note: 1 mile = 5,280 ft.

Why is there a difference sometimes in resistance value between a DC mega ohmmeter test and a VLF AC resistance value?

DC Resistance measurements (Mega Ohmmeter) measure the current drawn under an applied voltage and divide this current value into the applied voltage to get the resultant so called insulation resistance (ohms law).

The HVA line of VLF/DC test instruments, unlike many other VLF units on the market, also measure the resistance as well as the capacitance of the load.

Note the resistance value displayed is not the impedance of the load but it is the actual real component of the overall current. The real component IR of the total current IT is then divided into the applied AC RMS voltage to give the resultant insulation resistance.

The capacitance of loads like cables play a very small role when measuring steady state resistive leakage currents under DC, but play a major role under AC (including VLF) conditions.

Therefore the real resistance value of the load under AC applied voltages may not be the same as the resistance of the load under applied DC voltages. Most cables are naturally under AC applied voltages when in normal operation.

Can I perform a TD test if there are splices in the cable?

Yes, a Tan Delta test can be performed on a cable with splices without any need to compensate for the splice.

Does HV Diagnostics offer training?

Yes! HV Diagnostics, Inc. offers NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics using our world-class high voltage electrical test equipment.

Our training is a combination of both theory and practical. We have extensive application knowledge that extends well beyond our instruments. Students will learn how to correctly operate HV Diagnostics test equipment and properly setup our instruments to various electrical termination interfaces. We offer both in-person and online courses for VLF and Tan Delta training.

Due to its specialized nature, we only provide in-person training for Partial Discharge as part of a three-day intensive course that includes VLF / TD.

For more about on our Training Services and a peek into our state-of-the-art Training Center, click here! NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics.

*Note to New Equipment Users:
To provide the required level of professional support with HV Diagnostics test equipment, we strongly recommend new users complete our training course(s). Incorrect use of equipment and/or testing application can lead to negative user experience, unsatisfactory results, incorrect data, and probable damage to the equipment.