Get answers to frequently asked questions about DV power test equipment.

Micro Ohmmeters

Do your Micro Ohmmeters create magnetic transients that can inadvertently cause relays on a breaker under test to operate?

Previous generation of Micro Ohmmeters, which were designed 5 years ago or earlier, generate DC current which contains high level of AC component, 100 Hz or 120 Hz. Those devices had full wave rectification, but without additional filtration. Because of that, magnetic transients might induce current on a secondary of a CT and cause breaker control or a relay to operate.
Modern Micro Ohmmeters (like our RMO series) generate a filtered (true) DC current that contains low level of AC component, lower than 1%. These devices do not create any transients. DV Power devices RMO series and other modern Micro Ohmmeters generate DC current with automatically regulated test ramps. During the test our devices ramp with increasing current before measuring and decreasing current after the measurement. By sloping current up and down, magnetic transients in a breaker’s CT are virtually eliminated.

Which current cables do you suggest for use with RMO300 device?

We propose cables 2×10 m 35 mm2 instead of current cables 2×10 m 50 mm2. Cables of 35 mm2 weigh less than cables of 50mm2 and they are sufficient for 300 A. Our RMO300 is more powerful than other Micro Ohmmeters and there is no need for thick cables in order to generate 300 A.

What is the meaning of PFC? What are you referring with DC/DC and supply?

PFC (Power Factor Correction) is an input electronic unit. It’s function is to make our devices load power network with cos φ ≈ 1. That makes that our device draws active power but not reactive power from the power network. This unit is fitted only in modern devices, such as ours. DC/DC is unit that transforms high input voltage to output voltage (5 V, 600 A). This enables galvanic isolation between input and output of the device. Supply is a short form of Supplier Unit. That is our small inter-unit that supplies our electronic components.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

Do you produce a Micro-ohmmeter with test currents up to 60 A?

Yes, we produce Micro Ohmmeter RMO60E. Maximum test current is 60A and the test voltage is up to 12V DC. This RMO60E device is designed for protective earth resistance tests in accordance with the IEC 61010-1 safety standard Annex F. The RMO60E device is not intended for winding resistance measurement of transformers nor test objects with high inductance.

Which test current should be used for contact resistance measurement of circuit breakers?

According to IEC 56 (now IEC62271-100) this resistance value should be measured using a test current between 50A and the breakers nominal current. According to ANSI C 37.09 the lowest test current is 100 A. Other national and international standards specify similar regulations.

Is RMO300 adequate model to measure the contact resistance of lead-acid batteries in UPS?

Our experience is that the measurement of resistance values below 10 μΩ to as high as 100 μΩ should be performed with the test current higher than 50A. Using test current below 50 A would not produce the right analyses. The test current of Micro Ohmmeter should be at least 100 A or 200 A to measure the contact resistances. We have experience with measurement of contact resistances of lead-acid batteries in a battery factory. There we have measured the contact resistance of battery with the test current of 300 A. It proved that the RMO300 (RMO300A and RMO300G)is adequate model for this task.

Coil Tester & Power Supply units

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between Power Boxes POB30D, POB30AD and Coil Tester & Breaker Supply units POB30D and POB30AD?

Actually, there is no any difference. Since POB30D, POB40DL, POB30AD and POB40ADL are not only power supply units, but also minimum trip voltage testers, their name is changed from Power Boxes to Coil Tester & Breaker Supply units.

The POB30AD current pulse width (DC) is 140ms. However, on occasion customers from China want the pulse width of 700ms. Is it possible? Some old circuit breakers close pulse is about 700ms. Can the pulse width be adjusted?

DC pulse width (140ms) was chosen because our German customers specified it. The reason behind this is the possibility of a coil could be blown (damaged) in the MT Volt menu.

In the MT Volt menu by pushing TRIG button, a pre-selected DC voltage lasting 140ms is brought to a desired coil. If the coil was not actuated, the voltage automatically increases for the selected step, and after 900ms pause the voltage is generated again, lasting 140ms.

If the DC pulse width is increased from 140ms to 700ms in the MT Volt menu, the coil could be overheated and because of that burn out.

For that reason, we can offer two possibilities for your market with our SAT30A device:

  1. We can adjust DC pulse width of 700ms in the Volt DC menu, but we have to keep the pulse width of 140ms in the MT Volt menu.
  2. We can adjust DC pulse width of 700ms in both, the Volt DC menu and the MT Volt menu, but we have to increase the pause from 900ms to 2,8s in the MT Volt menu.
Can the POB30AD or POB30D can be equipped with the built-in thermal printer.

Our POB30D and POB30AD are only Power Boxes, and unfortunately they are not timers. Because of that, we have not built in a thermal printer. We are developing a Power Box with a timer, and then our POB30D and POB30AD will be equipped with built-in thermal printers.

Winding Ohmmeters

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. The dual reading characteristic will speed up the measurement when it is used to test windings on delta-delta connected windings on three-phase transformers
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

How long does it take to measure windings of, for example, a 63MVA transformer?

We have successfully tested our instruments on 300MVA transformers and got stable results in 20-50 seconds. We believe that the stable results on the 63 MVA transformers can be obtained within 30 seconds.

Do RMO-T devices have RTD input for winding temperature monitoring?

Our devices can generate continuous test current and they can continuously measure resistance change. That way it is possible to observe changing of winding temperature.

Do RMO-T devices have a temperature compensation feature?

When used together with our DV-Win software, our RMO-T devices have a temperature compensation feature. The user needs to select the temperature of windings, the reference temperature value, and the material of the windings.

Why is it necessary to ground the winding under test of a transformer?

For safety reasons you must always ground one end of the measured winding of transformer to the transformer case. Dangerously high voltage, which can be fatal, may occur at terminals if you don’t earth one side or if you open the measuring circuit during the process of current discharging.

Can we transfer all, or a selection of memorized data from the RMO-T series devices, or only the last test result?

If the DV-Win PC is used in a supplementary manner, after the tests are finished, it is only possible to transfer the results of last 100 tests. The alternative is to connect the PC during the test. Then, during the test, results can be transferred in Excel spreadsheet form, from 1 sec, to up-to 300 sec intervals. The maximum test duration time is unlimited.

Do you produce these devices with rechargeable batteries?

DV Power does not produce Winding Ohmmeters with rechargeable battery. Winding Ohmmeters with rechargeable batteries have test voltage value of only 12 V. Our Winding Ohmmeters have test voltage value of up to 60V. This significantly increases the speed of testing and enables the maximum current to be reached faster.

What would happen if during a measurement of power transformer winding resistance a current lead got disconnected from the Power Transformer?

We strongly recommend that the RMO-T equipotential connector is always connected to protective ground (PE) before measurement. If any current lead gets disconnected from the transformer during a measurement, the energy stored in the transformer magnetic circuit will be automatically discharged through voltage sense cables and a built-in discharging circuit in our RMO-T device. However, it is strongly recommended to check all connections in order to avoid such accidental disconnecting.
Our RMO-T devices use mechanical protection against accidental disconnection of current cables. If you plug-in current cables into sockets of RMO-T and turn them (lock them) in clockwise direction, current cables cannot be accidentally disconnected from the instrument.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are normally designed to reach saturation when the current is 1,2 times the peak value of the no-load current. The no-load current is normally in the range of 0,2 to 5 percent of the nominal winding current. When measuring DC resistance the test current should be at least 1,2 times the no-load current of the transformer. This is to ensure that the transformer core is saturated to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Three-phase Transformer Turns Ratio Tester

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

Why do I often get the message “Excitation current too high” while measuring a current transformer?

The TRT series devices, just like any other competitive device on the market, are designed just for the verification of current transformers. The accuracy class of CT is tested according to relevant standards, for example IEC 60044-1 or similar. Usually, the impedance (inductance) of CT is very low, and the power required is higher than the power available from the TRT, a result indicating the “Excitation current too high” will be displayed. In that case try selecting a lower test voltage.

I don’t understand what is tested in the Accuracy Check menu. Is it a calibration test of the instrument or just another way to test a transformer?

The Accuracy Check menu is designed for accuracy control/calibration of the TRT device. It is not another way to test the transformer.

What happens if a wrong transformer vector group is entered? Should it be evidenced by results of the test? Which values would indicate that mistake?

If the wrong vector group is selected, you will most probably get the wrong results. Turns ratio results as well as phase angle results will be notably wrong. This is because testing procedures are different for different vector groups. However, there are certain vector groups where testing procedures are the same (for example Dd0 and Yy0). In that case, if you select Yy0 while testing Dd0 or vice versa, the results will be good.

Three-phase Transformer Demagnetizer

How do I know that demagnetization was successfully completed?

We do not believe this type of verification is necessary. However, if you would like to check, the best way would be to compare the results of excitation current measurement before and after the demagnetization. This test can be performed with our TRT instruments.

When is it necessary to use DEM60C?

When suspecting remanent magnetism, or when test results such as FRA or magnetization/excitation current show possible remanent magnetism, you should use the DEM60C to perform fully automatic demagnetization.

Why is it necessary to perform demagnetization?

The remanent magnetism causes various problems such as erroneous diagnostic electrical measurements on a transformer, or an inrush current at start-up of power transformer, or incorrect operation of protective relays due to magnetized CT cores. Even if no remanent magnetism is present, demagnetization does not cause any harm, while the process is quick.

If I have one of the RMO-TD instruments with demagnetization feature, do I need DEM60C as well?

No, the instruments from the RMO-TD series can perform single-phase demagnetization efficiently.

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