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By:
Brian Burke, Application Engineer, KoCoS America LLC
Guy Wasfy, Managing Director, KoCoS America LLC
Jürgen Dreier, Product Manager, KoCoS Messtechnik AG

Bonding connections and ground connections are an essential component for the safe functioning of power and communications engineering systems and for ensuring personal and equipment safety.
Unwanted voltage potential differences are to be avoided by means of ground connections and grounding procedures. These voltage potential differences can occur between metallic components and ground, which can endanger human safety and/or technical equipment.
Metallic components must be connected to ground potential to prevent dangerous voltages. Voltage drops are reduced by grounding all non-voltage-carrying parts and by extensive ground potential equalization (grounding system). It is important to make sure these grounding connections have a low resistance. Resistance measurements must be made at both potential and ground connections to ensure that a sufficiently good low resistance connection is achieved and maintained.

Below is an example of application in power distribution where grounding and equipotential bonding and their good connection are important.  However, there are many other applications where grounding and equipotential bonding must be taken into account (Rail vehicles, aviation industry and aircraft maintenance, etc.). Typical applications for equipotential bonding and grounding are found in power distribution substations in low and medium voltage networks and especially in high voltage substations. The grounding of all non-voltage-carrying parts and extensive ground potential equalization reduce voltage drops that can occur due to capacitive or even inductive voltage coupling.

An example of these non-voltage-carrying parts is the mechanical disconnect switches used to remove a switchgear from service. Due to the need for human interaction with these physical switches, it is critical their grounding connection is not compromised. These straps are metallic braids secured via bolts to the switch and ground connection point. These straps can become poor conductors due to corrosion or physical damage. A compromised strap could lead to improper grounding of the switch causing a hazardous touch potential when the switch is to be used. To ensure the straps are functioning accordingly, the connection across them can be tested with a micro-ohm measurement. A failing strap will have a high resistance reading where as a properly conducting strap will have a low resistance reading. High resistance of the connection or, in the worst case, a failing strap could require the bolted connection points being cleaned and reconnected or the strap being replaced entirely.

Aside from bolted ground straps, the grounding connections can be bonded through exothermic welds. These welds result in superior mechanical and electrical bonding. Especially for conjoining dissimilar metals, such as joining a copper grounding rod to a galvanized metallic structure in a substation. These welds, when done properly, create a solid and reliable connection (Figure 2) Due to improper heating or unwanted moisture, the welds can have pitting or gaps (Figure 3) and be considered poor quality both mechanically and electrically. Performing a micro-ohm resistance measurement across these connections can give insight into the weld quality. The more solid the weld connection is, the lower the resistance reading will be. A poor quality weld could result in a less reliable grounding connection.

The resistance measuring instruments from KoCoS Messtechnik AG are ideal for measuring equipotential bonding and earth connections. The PROMET SE precision resistance measuring instrument is used to determine low-resistance in the μΩ and mΩ range. With adjustable test currents of up to 200 A, in conjunction with a four-wire measurement method, the systems provide measurement results for the highest accuracy requirements. The use of state-of-the-art power electronics and the robust design guarantee maximum reliability for mobile use.

The PROMET SE is ideal because it is battery operated and does not require a mains connection. Some of the connections described may be at height so that the tester is standing on a ladder or similar. Without the battery operation, the meter would also require a portable generator. Finally, the PROMET SE is very lightweight and easy to transport.  It weighs less than 5 pounds and can be conveniently transported on-site to test the many ground connections.

If you have questions or additions to the resistance measurement or to our measuring devices? Then contact us via the comment function here on the blog or by mail to info(at)kocos.com.

When distributing electrical energy, it must be remembered that poor current connections cause losses that must be compensated for by additional power from the power generator.

The power loss at the contact point depends on the current and the resistance: P = I²·R

When transmitting high currents, the aim must therefore be to achieve the lowest possible contact resistance at the connection points. The contact resistance is influenced by several variables and increases in the course of the operating time due to aging. By testing at the installation site, a faulty connection can be detected and eliminated.

The quantity for assessing an electrical connection is the resistance. The resistance of an electrical connection is in the micro ohm range. These small resistance values require special measurement technology, such as resistance measurement in four-wire technique (Kelvin method).

In order to assess the quality of a connection, the PROMET SE and PROMET R300/R600 resistance measuring instrument is able to determine the quality of a connection. Due to two voltage measurement inputs, a simple and quick determination of the quality, e.g. of screw connections on bus bars, is possible. The determination is made via the quality factor. This is defined by the ratio of the resistance of the connection over the overlap length to the resistance of the bus bar of the same length.

The quality factor K is the ratio of the resistance RCON of the connection over the overlap length lCON to the resistance RREF of the bus bar of the same length lREF.

     K = RCON/RREF

     RCON: Resistance of the connection
     RREF: Resistance of the bus bar

Therefore, when making an electrical connection, care must be taken to limit aging and provide a low-maintenance and reliable connection.

By determining the resistance or quality of a connection, the correct connection can be verified during installation and maintenance and a reduction in electrical losses, an extension of service life and an increase in plant safety can be achieved.

Do you have questions or additions to the resistance measurement or to our measuring devices? Then contact us via the comment function here on the blog or by mail to info(at)kocos.com.

Automation in the field of low-resistance measurement is increasingly required in the factory or laboratory. Be it in the automotive/electromobility sector, in the investigation of soldered or welded joints of high-current connections or in a wide variety of other applications.

For special requirements, e.g. for use in test stands, the easy-to-use PROMET PI programming interface is available for control and measurement with the PROMET R300/R600 resistance measuring devices. This can be used in COM/ActiveX-supporting as well as in .NET environments.
By programming the measurement sequence once, it is possible to integrate the PROMET R300 or R600 resistance measuring instruments into the test equipment via the programming interface and to perform measurements automatically.

A driver is installed with the programming interface, via which the connected devices are addressed. Communication between the software/PC and the external PROMET R300/R600 is made possible by the installed ActiveX component. This allows communication via USB or Ethernet interfaces.
As an example, an Excel sheet is used to control the PROMET R300/R600 and to evaluate the measurement results in this description, via which the programmed VBA macros (Visual Basic for Applications) are executed. Programs can be modified and adapted according to the requirements.

The PROMET R300/R600 precision resistance meters are an ideal tool for characterizing components for high current and low resistance due to their four-wire measurement and ability to accurately measure both current and voltage. 
As demonstrated in the article, a resistance measurement system controlled via external software can be easily integrated into an automated application. Using the PROMET R300/R600 resistance measuring instruments to perform such measurements simplifies the test setup, reduces programming time, and enables efficient test sequences.

Further information on the use of the PROMET PI programming interface can be found in the application report PROMET R300/R600 - The intelligent way to measure resistance!

Do you have any questions about resistance measurement or our measuring devices? Then contact us via the comment function here on the blog or by mail to info(at)kocos.com.

Postscript
The EPOS 360 three-phase signal generator can also be integrated into your own test applications in a similar form via the EPOS PI programming interface!

Resistance measurement with PROMET - Thanks to Ohm!

After Alessandro Volta created a source that supplied electric current in 1801 with the so-called Volta Column, it was possible to explore the effects of electric current. Many researchers made numerous discoveries and observations, but the mysterious effects of electric current could not be revealed.

It was only through the discoveries and research of Georg Simon Ohm that the facts could be explored. Without his research and without the resulting fundamentals of Ohm's law, the outstanding developments in electrical engineering would not have been possible.

Georg Simon Ohm, born March 16, 1789 in Erlangen, died July 6, 1854 in Munich, was a German physicist.

The decisive measuring instrument for the discovery of Ohm's laws was the torsion balance galvanometer constructed by Ohm. The torsion balance galvanometer consists of a thermocouple (A) in which the ends are kept at different but uniform temperatures (B). A magnetic needle (C) on an adjustable suspension (D) and a device with which the various test conductors (E), i.e. the variable resistance, can be contacted.

If a test conductor is connected so that a current flows, the magnetic needle is deflected. The position is read off a scale. The deflection or the read scale values form a proportional measure for the magnetic effect of the electric current, thus the current intensity.

Ohm was able to deduce the law from these measurements:
I = Uq / (Ri + Rv)
Current = Source Voltage / (Internal Resistance + Variable Resistance)

Ohm published his results in 1826 and initially received little recognition. It was not until 1841 that Ohm received the Copley Medal of the Royal Society of London, which corresponds to today's Nobel Prize, as an award for his work. In 1893, the World Electrical Congress in Chicago gave the designation "Ohm" (sign Omega: Ω) to the unit of electrical resistance.

With Ohm's torsion balance galvanometer, only the first step in the development of resistance measuring instruments is described in this article. The history of resistance measurement shows the changes from the age of early experimenters to today's computer age, i.e. from measuring bridges to the first electronic devices to today's digital measuring systems. Developers always used the latest ideas and systems to make the products more useful and user-friendly. Technological change drove the development of measuring instruments and realized technological advances.

 

KoCoS is committed to this development and offers a diverse range of resistance measuring instruments with the PROMET series. PROMET precision resistance measuring devices are used to determine low-resistance in the μΩ and mΩ range. With adjustable test currents of up to 600 A in conjunction with a four-wire measuring method, the systems provide measurement results for the highest accuracy requirements. Typical applications are, for example, the determination of the contact resistance of switching devices and the resistance determination on inductive loads such as transformers. The use of state-of-the-art power electronics and the robust design guarantee maximum reliability for mobile use, but also for stationary use in the laboratory and factory.

Do you have questions or additions to the resistance measurement or to our measuring devices? Then contact us via the comment function here on the blog or by mail to info(at)kocos.com.

Saving of working time through simultaneous resistance determination at three measuring points

For switchgear in the medium-voltage and high-voltage level, the switchgear standard IEC 62271-1 requires a static resistance measurement of the main circuit in order to exclude an unacceptable heating of the current path.

Traditionally, the measurements are carried out one after the other and individually at each phase. The main circuit is supplied with 100 A direct current and the voltage drop is measured. If the measured value, i.e. the voltage drop is within the specified limits, the test is passed and the results can be recorded/stored. This measuring procedure is time-consuming as the three phases are tested one after the other.

To increase productivity and improve reliability, the measuring method for resistance and voltage drop measurement can be optimized with the PROMET R300 or R600.

The PROMET R300/R600 resistance measurement systems are equipped with three voltage measurement inputs, allowing parallel measurement at three measuring points, for example to measure the resistance of three main contacts statically.

 

In order to perform a simultaneous measurement of three main contacts, the test objects must be connected in series and provided with a test current connection. Since a four-wire measurement is carried out, it is important to ensure that the voltage connections are between the high current connections and that they are connected exactly at the points where the resistance is to be determined.

Connection example for a measurement on three test objects connected in series, e.g. three interrupter units.

In stand-alone mode, the three static resistance results with the measurement details (actual test current and voltage drop, measurement ranges etc.) can be stored in the measurement device.

 

The data stored in the device can be read out and managed with an easy-to-use PROMET software. The clearly displayed measurement results can also be output in a PDF test report or exported as CSV data.

The described simultaneous measuring method for the acquisition of three resistances thus saves working, changeover and measuring time!

As a further automation option, PROMET R300/R600 are equipped with interfaces for connection to the ACTAS 2.60 switchgear testing software. Resistance measurement can be conveniently integrated via the ACTAS test software. Even without an additional ACTAS test system, automated test sequences and a comprehensive analysis of the test results can be carried out without any problems.

 

The method is not only applicable in switchgear testing, but also in applications such as e.g. in the field of e-mobility, where several resistors have to be detected at the same time.

If you have further questions, please leave a comment or contact us directly.