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Low Resistance Testing Using GGT and RMO-EH Series

DV Power produces a comprehensive range of low-resistance ohmmeters and accessories which cover most measurement applications. This application note gives an overview of low resistance measurement in terms of:

  • Integrity/continuity testing of earthing systems
  • Lightning protection systems in substations
  • Wind turbine lightning protection systems/foundation grounding
  • Continuity (bonding) of protective conductors in many different applications
  • Railway joints, lines, vehicles, and rail track joints
  • On-board aircraft electrical systems – bonding
  • All connections and joint types (welded, crimped, bolted, soldered, or tied)
  • Oil and gas pipelines bonding

Such applications usually require the use of long test leads (dozens of meters) and contact resistance measurements less than 0.1 Ω. Therefore, it is beneficial to follow the Four-Wire Kelvin procedure to accurately measure low resistance values while eliminating the resistance of the current test leads. The GGT’s and RMO-EH’s combined current and voltage sense leads are specially designed to fulfill Kelvin’s 4-point principle.

Figure 1. GGT and RMO-EH 4-wire low resistance measurement principle

Two wires (C1 and C2) carry the current, known as the source or current leads, and pass the current through the object under test. The other two wires (V1 and V2) known as the sense or potential leads are used to measure the voltage drop across the test object.

Our test devices for such applications are designed to overcome different possible errors that could exist because of the contact and lead resistances, and external influences like temperature, induced currents, noise, etc. Attention should be paid when selecting the test current because it should not exceed the rated current of the equipment under test (e.g., protective bonding tests of LV equipment).

Ground Grid Integrity Testing

The ground grid integrity and continuity test is described in IEEE 80 standard, and it is one of the most efficient test methods and techniques for measuring the electrical characteristics of grounding systems. It is also known as a high-current test method, whose purpose is to check the continuity of buried ground conductors and connectors, ground rods, and bonding connections by injecting a high-test current (up to 300 A). The integrity of earthing systems is critical to its effectiveness as the key reliability and safety mechanism of the installed equipment and electrical installations.

Figure 2. Ground grid integrity test using a high current DC source (GGT series)

GGT is a powerful device that provides currents up to 300 A and gives highly accurate measured resistance values (usually less than 10 mΩ). High output voltage enables testing with long cables and measurement of a wide range of resistance values. To simplify the test procedure additional GGT-M module has been developed. It provides remote control of the test device, monitoring of measured results, and increases the safety of personnel. Since the substation contains dozens of grounding points, features that save time and simplify work are crucial.

Verification of the ground grid integrity

Figure 3. Ground grid integrity test using the RMO-EH series

For electrical substations and other facilities in which the availability of power supply is limited or when the lightweight, handheld, and easily portable test set is required, our battery-operated RMO-EH device is ideal to verify the condition of the earthing systems. It provides a maximum of 10 A test current, and test leads even up to 200 m (on reels). The weight of the RMO-EH is less than 1 kg.

GGT and RMO-EH series provide accurate continuity measurement within in-service grounding systems, overcoming background noise, stray currents, etc., and enabling testing multiple times faster than with conventional methods.

Wind Turbine Lightning Protection System Testing

Due to the increase in height of the wind turbines and their exposed location, the risk of direct lightning strikes and corresponding damage becomes considerable. The most vulnerable part of the wind turbine is the blade. To protect the wind turbines, different lightning protection systems are installed whose structure must have a very low resistance path to the ground. It begins with the receptors placed on the blades and continues with internal conductors that lead the path into the ground.

To ensure that the lightning protection system will work when needed, the resistance path from the blade receptors to the ground should be measured at regular intervals.

The RMO-EH is the perfect solution for the low resistance measurement of wind turbine lightning protection conductors. It is lightweight, battery operated, supported with long test leads, and easily can be lifted with the crane to desired test point. A test current of 1 A or higher is recommended for this test.

Figure 4. Wind turbine lightning protection system resistance measurement

Wind Turbine Foundation Grounding Testing

Before the concrete casting and cabling works of the wind farms, the continuity of the grounding conductors should be checked. Usually, those are the 50mm2 copper-based conductors, interconnected between the inner terminal lugs and outside grounding electrodes. The resistance values of the connections are mostly less than 5 mΩ. For such an application, we recommend our GGT series.

Figure 5. Valued customer KSBR company from Finland is testing wind turbines foundation grounding with
GGT Series

Railway Lines and Track Joints Testing

The electric train power system consists of a power supply wired to the train’s electric motor through the contact wire. To allow the current to flow, the rail track is used as a return path. The trains must always have the access to the power supply. Moreover, the track will be earthed, and cross-bonded to prevent the voltage from rising to a dangerous level. To ensure that the electric train power system will be operational constantly and to reduce the risk of unexpected failure, the following routine tests should be conducted periodically:

  • Catenary continuity resistance measurement
  • Railway track joint efficiency and earth bonding resistance measurement
  • Railway vehicle’s resistance measurement

The catenary continuity resistance measurement could be performed for each section in a way that one end of the section will be grounded using the auxiliary ground rod and on the other side a micro-ohmmeter (like GGT or RMO-EH) will be connected. In this way, the electric circuit will be established and simply contact resistance calculated by using the ratio of the measured voltage drop and injected test current. The resistance of the running rail will also be included. Bad conditions of the contact conductors’ joints may lead to overheating, and therefore should be detected, and fixed in time.

Figure 6. Continuity test of railway contact network – principal scheme

According to the IEC 62128-2, the longitudinal resistance of the running rails should be low. The rail joints and crossings should be welded or connected by rail joint bonds of a low resistance value so that the longitudinal resistance of the rails is not increased by more than 5%. Experiences show that the 1m long rail sample has a resistance of several tens of μΩ.

The track-to-track cross-bonding should be provided at suitable intervals to ensure the continuity of the traction return current path and the proper distribution of the traction return currents so that the touch voltages will not exceed the permissible levels, under both operating and faulty conditions.

Rail Vehicle Body Resistance Measurement

In accordance with the IEC 61991 standard, the resistance between the vehicle body and the rails must not exceed 50 mΩ for locomotives and passenger trains and 150 mΩ for wagons. This is to prevent dangerous voltages and avoid serious injuries.

Figure 7. Rail vehicle body resistance measurement

Railway Ground Grid Bonding Test

The railway’s ground grid bonds should be inspected during the construction stage to avoid possible earth resistance rising to an unacceptable level. The continuity test with GGT or RMO-EH micro-ohmmeters will indicate whether some of the many joints have slight weaknesses. In this case, bad connections which might not be visually seen will be detected. A low resistance path to the earth helps to protect signaling and other low voltage equipment of being damaged from lightning strikes.

On-board Aircraft Electrical Systems Bonding Test

Aircraft manufacturers have strict procedures for measuring the bonding resistance, during the production and maintenance stages. Electrical bonding prevents static electricity build-up that can interfere with radio and navigational equipment and prevents dangerous static discharges in aircraft fuel tanks and hoses. It also provides lightning protection by allowing the current to pass through the airframe with minimum arcing.

According to the standard EN 3371:2019, there are different bonding tests that should be conducted and will be mentioned below. The test devices used to measure this bonding resistance must be accurate, reliable, and able to measure with long leads.

Primary bonding tests

It suggests a test of the conducting parts of the aircraft structure, bonding parts, and conductors, through which considerable energy might be discharged (lightning discharge currents). The structures are generally used as a return current path for the sum of the currents of the onboard electrical installation. Under a current of 300 A (main structure test), it is usual to measure resistance from 0,2 mΩ to 2 mΩ between two points of a 30 m distance.

Primary bonding measurements should be carried out on the assembled structure before the installation of the different systems. The test current and duration of the tests must be defined by responsible personnel. The acceptable resistance values between the reference point and different points under test, for different parts of the structure, are mentioned in EN 3371:2019 standard.

Secondary bonding tests

Return paths of main currents to aircraft structure should be tested with a current of 150 A applied for a minimum of 1 min before recording the resistance values. These values must not exceed 1 mΩ or 0,05 mΩ, depending on the measured category.

Return paths of non-main currents to the aircraft structure should be tested with the test currents in a range of 1 A to 10 A – not higher than the nominal current of the test object. The measured resistance values of the conductors connected to light alloy parts should not exceed 0,05 mΩ, or 1 mΩ when connected to a stainless-steel part.

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January 12, 2023

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