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High-Voltage AC know-how

The high voltage test DC

Do I feel safe?

Am I doing everything right?

You will know for sure in a few minutes.


Safety tests are mandatory and are part of every final inspection of your electrical product.
Learn the most important facts about the high voltage test DC.
We explain the WHY?, WHERE?, HOW? and also the possible danger!
And if you would like to learn more, you can download even more detailed information at the end of this page free of charge!

 

RIso or HV-DC ?

The basic principle of the high voltage test is very similar to the insulation resistance test. This is because both test methods are concerned with the quality of the insulation.
This can be done by measuring the insulation resistance or by testing the electric strength with high voltage and simultaneously measuring the flowing leakage current. So what is the main difference between the two test methods? It is the test voltage and the test current. Both are usually significantly higher in the high voltage test. In addition, the demands on the quality of the test voltage are not as high.

In principle, the high voltage test with DC is quite similar to the insulation resistance test. Provided that the voltage quality is high with regard to low ripple, it can also be used to measure the insulation resistance in MΩ or GΩ. In this case, however, SCHLEICH also consistently refers to the test as insulation resistance test.

For the HV-DC, the basis of the evaluation is the flowing current. This corresponds to the same evaluation as for the HV-AC.

 

WHY?

Safe insulation is the central protective measure to ensure electrical safety. It ensures that the user does not touch live conductors and that no short circuit can occur between the conductors or to the housing of the equipment. Because if it did occur, a life-threatening current could flow through the user if he or she touched the housing. Obviously, the protective earth conductor should ensure that this does not happen. But in the worst case it could also be defective. And it would also only be an evasion of the effect, not of the cause.

 

In order to guarantee all this, the insulation must work perfectly! And this must be proven and documented by you in a high voltage test before the electrical product is delivered.

This test is not mandatory for all electrical products. However, it may well be required for the certification of the electrical product in the type test. If it is required during manufacture, it is a routine test. This means that every piece, i.e. every single electrical product you put on the market, necessarily requires a high voltage test with DC.

 

WHERE?

Basically, there must be good insulation between current-carrying conductors or between these and housing parts. Typically, this is done by insulating the electrical conductors against dangerous contact, i.e. covering them with an insulating material. But this protective sheath must be removed at the latest when the electrical conductor is connected to other electrical components. At these points, the insulation is guaranteed over a safe distance. It is then a matter of safety distances through clearance and creepage distances.

In addition, current-carrying conductors can also be insulated from each other, e.g. by means of casting compounds, insulating foils or solids.
When is which type of insulation used?
This always has to do with the design of the electrical product, the type of specification such as high temperature or mechanical load etc.

Now it is certainly understandable that insulations in a luminaire, an electric iron, an electric motor or a high-voltage insulator in a power station have very different requirements and designs.
From this diversity, quite complex electrotechnical insulation structures result from case to case.

 

HOW?

Since the insulation has “something to do with the voltage”, the test is carried out with a defined test voltage level. This can be ramped up or applied directly to the device under test at its full magnitude.

The aim is to measure the current through the insulation or to detect a breakdown. This is because it is the evaluation criterion for the insulation. It must not be greater than a specified maximum current.
The upper limit of the current can vary greatly from product to product. In the standards there is little or nothing to be found about this limit. And there is a good reason for this – because the amount of current depends heavily on the capacitive component in the insulation.
The HV-DC does not change polarity 50 or 60 times per second like the HV-AC, but the capacitance of the device under test must first be charged.

This test is not mandatory for all electrical products. However, it may well be required for the certification of the electrical product in the type test. If it is required during manufacture, it is a routine test. This means that every piece, i.e. every single electrical product you put on the market, necessarily requires a high voltage test with DC.

 

 

 

Often the high voltage resistance is measured one after the other between all conductors involved. These can be combined groups of conductors or individual conductors and of course the housing or housing parts. It quickly becomes clear that the test can and must be carried out at a wide variety of locations, depending on the complexity of the electrical product.
This could be done by scanning the test points with a test probe – an approach that can quickly turn out to be lengthy and costly.
For 25 years, complex tests have therefore always been carried out automatically at any test points via the SCHLEICH-typical matrix, which is fully programmable:

SCHLEICH switching matrixes switch flexibly in 2- and 4-wire technology. Especially 4-wire technology is of great importance in automated systems and plants. It guarantees a safe contact control of the test voltage and thus process stability.

 

Test paramters typical norm values SCHLEICH | from standard to customized
minimum required test voltage 1,000 – 3,000 V DC 50 bis 100,000 V DC
max. permissible test current 1 – 10, 50, 100 mA 0,1 – 5.000 mA
minimum test duration 1 s from 0.1 s to 1 month
start-up ramp off; 1 s – 1 min off; from 0.5 s to 1 month
down ramp off; 1 s – 1 min off; from 0.5 s to 1 month
voltage profiles off; in 5 steps off; in any number of steps with any profile steps

With this range of requirements, it is of course ideal to use a test device that covers as many of the world’s standards as possible.
That is SCHLEICH’s strength.

 

The TEST CURRENT?

An insulation always consists of an insulation resistance and a capacitor? Why a capacitor? Was that even built in? …

 


The test always takes place between two electrical conductors/poles. In an abstract way, these two poles form two metal plates facing each other. Between them is the insulation. And this construction corresponds to that of a capacitor. As a result, the whole insulation structure therefore also behaves strongly capacitive. This effect is particularly evident in electric motors, generators and also transformers.

These are physical factors that have nothing to do with good or bad insulating properties of the device under test. They and of course the standard specify the maximum permissible current..
The decisive factor is now the power of the high-voltage source. SIt must be able to supply the necessary charging current to charge the capacity in the time you want. The required level is therefore determined by your time requirements for the test. After charging, the test current drops to almost zero. The advantage of HV-DC: The test voltage source does not have to be as powerful as with HV-AC.

Low-End – The high voltage test DC is carried out in the simplest and cheapest case with a rectified alternating voltage. However, even after charging, a current still flows in the capacitance, which depends on the residual ripple of the test voltage.

High-End – In most cases, SCHLEICH supplies HV-DC test devices with highly stable DC voltage for high standards. And this with a residual ripple of partially less than 1 V! In this case, the measurement of the insulation resistance at high test voltages is again very well possible.

 

The DANGER?

If an alternating current of less than 12 mA flows through the human body, it is classified as non-hazardous.
If a DC high-voltage source can only supply a maximum of 12 mA, it is therefore considered to be safety-limited and non-hazardous. Special protective measures are not necessary in this case.

But in addition, the charge in the device under test must also be taken into account. Here a charge of max. 250 mJ is permissible without safety measures. In the case of electric motors and generators, this is quickly exceeded. Appropriate protective measures are then absolutely necessary.

This includes:

  • electrically isolated high voltage
  • Enclosing of the test station
  • Safety test pistols
  • Two-hand start with corresponding safety relay
  • Test cage or test cabin – double-circuit monitored with approved safety relays and, if necessary, also a safe guard locking
  • Compliance with performance level PLe, SIL3, Kat4 …

 

Performance Level PLe, SIL3, Kat4 …

The demands on safety engineering are correspondingly high for a test current greater than 12 mA. The relevant international standards must be complied with worldwide.

SCHLEICH test devices meet these requirements!

It should be noted that there is a wide variety of test devices on the international market which look like a supposed bargain, but do not comply with the legally required safety regulations of EN50191/VDE0104!

 

All set? Want more details?

Our mission – know-how, know-how, know-how… Those who understand the test methods with technical and normative certainty will get the most out of their test device.
– Dipl. Ing. Martin Lahrmann

Yes – tell me more. I want maximum security for our customers, our company and myself.

Send me more detailed information from the SCHLEICH test method handbook.

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