"Testing Suitcase" - The Development

Authors: Walter Schossig, Germany, and Thomas Schossig, OMICRON electronics GmbH, Austria

"Testing Suitcase" - The Development

Electrical engineers with discernment recognized quite early that detailed testing is essential for reliable operation and during commissioning. Testing of substation equipment during operation avoids damages on the assets.

To move measurement devices and test equipment, boxes made of wood have been developed quite early. 

Figure 1 shows an isolation testing device according to Brugger developed in 1903 by Hartmann & Braun (H&B).

The connections marked with + and – are connected to DC supply 100…120 V (Battery -B, Inductor J or dynamo machine D.) Once the equipment under test is disconnected from the grid operating the button T makes it possible to read resistance on scale. Utilizing the inductor requires the button pressed permanently- this can be done by turn to right slightly.  Multiplying the scale values by 10 or 100 according to the position of the switch U.

 Measuring the resistance was possible according to Wheatstone‘s bridge method. Figure 3 shows the device produced by Norma, Vienna, Austria in 1910.  The measurement range can be set via the connectors between 0.1 and 1000 Ohms. Measuring the DC current was possible via the zero Galvano meter with high sensitivity. AC current could be measured via noise detection on head phones. See Table 1.

To test voltage relays Todd developed a scheme with a transportable test transformer in 1922. (Figure 2)

The apparatus was connected to the device under test. It was checked, that the contacts close at the right voltage.

If desirable a table showed the relation between the lever settings and the closing or opening volts. Or a curve may be made showing the length of time required to close the contacts.

Another setup developed to test voltage relays is shown in Figure 5.

Before installing and before making periodic tests, the relay should be tested for "Grounds". While this is really a test for love metal-to-frame defects it will readily detect defective spots in the insultation (Figure 4).

Leads with heavy spring clips on the ends have been found very convenient for making quick connections and caught several hundred Amperes (Figure 7).

Voltmeters came with two or more ranges, i.e., they indicate full scale on 150 V, or by changing to another terminal, they indicate full scale on 300 V; or on 300 and 600. A good combination is a 75- and 150-V voltmeter, with an external multiplier, making full scale 300 V and 600 or 750 V.

To understand how challenging setting up and testing relays was at this time explains a look into the devices under test. The tester needed to understand the principle and the operation of the device to prepare the tests properly.  Figure 8 and Figure 9 show an overload relay.

So, the setting up was quite complicated and errors could occur and have been dangerous.

Practical help was delivered by transcripts and tables as shown in Figure 6 and Figure 10.

To locate faults in feeders and wirings a "magneto and bell test" as visualized in Figure 12 was used. Open circuits, breaks, different potentials and many more problems could be found with bells.

A portable testing set consisting of several dry cells mounted in a portable case with a bell and small lamp shows Figure 11.

The National Electric Light Association published its "Relay Handbook" in 1926. Almost 1,000 pages of technical content explain relays, circuit breakers, measurement transformers, testing and fault calculation- Acceptance and installation test have been recommended for any relay. Also, intermediate inspections should be performed. Figure 15 shows the device to test the electrical conditions of the relay. This was not to test the relay operation at any particular current value.

Also, maintenance tests as covered in the last issue have been explained. To perform them it was again recommended to connected the test leads via bull dog clips (Figure 13).

To test power-directional relays devices as shown in Figure 14 have been used. Single phase power directional relays have been usually so connected that at unity power factor the current leads the voltage by 30 degrees. The testing equipment consists of a 360-degree single phase power indicator and a 50 VA 110 to 8-volt transformer.

In addition to verifying its 30-degree relation the indications of these instruments furnish a check on the operation of the directional element of the relay. The tests have been made without disturbing the relay connections and while the feeder carrying load.

The company S&H produced laboratory equipment and transportable measurement devices for measuring dielectric losses, frequency measurement and rotation indication. (See Figure 16.)

Also R. Schmidt, from AEG, developed such a device in 1923. Figure 17.

Figure 18 and the figure on page 70 shows a secondary test set developed by Voigt & Haeffner in the 1920s.

Cable accessories are shown in Figure 19. See Table 2.

Figure 20 shows the scheme.

Overcurrent and overvoltage relays have been quite common at this time already. The increasing size of the grids and new requirements regarding reliability demanded supervision and test. This was the idea to have a portable test set, capable to be connected to any low voltage available at this time.

In terms of weight and size the apparatus should not be too big. Also, the price should not be too high. Solving this requirement resulted in the patented method and device as shown.

Taking into account that calibration and testing was not a long taking action higher stress and burden was acceptable.

To calculate the calibration transformer and adapt it to current needs the demand regarding Volts and Amps for existing relays have been considered.

Engineer de Buhr developed a scheme called "serial parallel scheme". Summarizing all this it was possible to develop a device (25 kg) to produce currents between 0.4 and 1200 A. The device could be connected to any nominal voltage up to 500 V. The variable resistor was realized as slide resistance divided into 4 zones.

Every part of the resistance belongs to a certain coil.  The coil as well as the resistance part is controlled via changeover switch. A current indicator indicates via red label, if the highest load is reached.

The current measurement devices have been hanging on springs. The measurement range was between 5 and 20 A. To reduce the load for the test set an additional high current cable was available (Figure 19).

If devices between 20 A and 1200 A shall be calibrated scheme I (Figure 20) is used. In the range from 0 A till 20 A the current can be taken from the grid directly- scheme II is used for.

The mobile relays testing equipment produced by S&H combines all apparatus within single housing. (Figure 21). The necessary operations are done by operating switches.

The test set consists of the basic unit which could be connected to 110 V or 220 V (50 Hz). It delivered currents from 0 to 100 A, voltages between 0 and 200 V could be produced. The connection to the grid was done via fuses and 2-pole switches. For this connection as well as connecting time recording equipment, clock and other accessories the terminal connections have been available.

In addition, a controllable transformer is there with fine and raw tuning possibility (continuously).

This tapped transformer comes with 20 taps and is realized as autotransformer (Figure 22).

The auxiliary equipment for the portable variant (Figure 24) consists of multiple switchable high current transformer with one primary and 8 secondary windings. Those could be operated in series, in parallel or mixed.  This allows different combinations (250 A/ 10V; 1000 A/ 4 V).

To measure the time registration units (Figure 23) or clocks have been used.

The head of the Elektrowerke A.G. in Berlin, M. Neustätter, wrote in 1929:

"For me it looks like that the test sets available on the market are designed too tight because the current transformers used as well as the controlling inductors allow only short time of operation.

It is possible to combine relay testing equipment for the own purpose and achieve longer time of operation possible. Of course, this causes bigger dimensions and it is not so easy to move them. Nevertheless, there is a huge advantage- the relay tester can do his job more calmly as with the handy equipment." (Figure 25).

When doing isolation measurement, the value measured slightly depends from the testing voltage. To avoid this the German VDE demanded to perform such tests with voltage over 1000 V. It was not only about measuring the value to ground, also between different lines of different voltage levels this should be measured.

The source of current was grid’s voltage normally, a battery or crank inductor. Such inductors delivering DC voltage became quite common because they have been easy to use and always ready. The „Fixohmmeter" by H & B (1924), Figure 26 and Figure 27 required for the full scale 0.001 A only. The accuracy of measurement depends from speed and evenness rotating the crank.

The rotation speed is approx. 180 rotations/ minute. To keep the rotation constant - which demanded some experience anyway- a centrifugal governor assisted the engineer. The huge scale supported the operator as well as the shoulder strap.

walter.schossig@pacw.org        www.walter-schossig.de

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