für ANNA angelegt 2004-06-25 für ANNA aktualisiert 2004-06-26

Description of a measuring method

Uvdr

Varistor Voltage



The Procedure

Test of varistors and protection diodes with constant test current.
Measurement of D.C. resistance with high voltage.


Measuring Ranges

Voltage according to layout of the device up to 2kV or 3kV.
Switch between two measuring ranges:
Range NV: 800V resolution 0.2V
Range HV: 2kV/3kV resolution 1V

Current according to layout of the device up to 2 mA or 10 mA.

Measuring Parameters

Tms: measuring time - duration from start to reading measuring data.
Umax: voltage limitation if the test object is absent or does not react.
Ims: test current

Properties

The test current is stabilized electronically and constant over the measuring range. To avoid damages with defective or missing varistor, a voltage limitation can be set.

Tips and Tricks

Generally the test goes very fast (Tmess = 0.3s) and heats up the Varistor very little. With longer measuring time the Varistor is heated up more and the varistor voltage can rise according to type and manufacturer strongly. From hand measured values with a constant current source essentially higher varistor voltages can be found.
This effect is very annoying in the test. Nevertheless, for the user it is very desirable. Because with a raised leakage current the Varistor heats up, the varistor voltage rises and the leakage current sinks again.

One can also check parallel connections of varistors and resistor or indicator lamps:
1. measure Lamp or resistance with Umax as a parameter adjusted clearly below the varistor voltage to be expected and evaluate the measured current or the resistance (only with PC control).
2. measure Varistor
determine current of the lamp at varistor voltage, e.g., with 100k and 430 V = 4.3 mA, raise the measuring current accordingly, e.g., 4.3 mA + 1 mA = 5.3 mA and evaluate the measured voltage.

Parallel connections of Varistoren and gas discharge arresters or spark gaps are very difficult to check. Usually the varistor is so dimensioned that its varistor voltage lies under the sparkover voltage of the spark gap and it's possible to measure the varistor voltage.
The spark gap shows effect only when a dynamic current pulse is applied.

A varistor connected in series with a gas discharge arrester is popular in the protection of low-voltage nets to get a safe separation to the protective earth. Mostly two varistors are switched between the current-leading poles. In this varistors one can measure the sum of their varistor voltages.
For the test of the gas discharge arrester in this case there is a very special method - see Uz-stat



Overview about the measuring methods.



End of Doc