Fault loop impedance testing on non-standard electrical networks

In the UK, electrical systems exist that operate outside the traditional and commonplace 230/400V network that we normally see in domestic and commercial installations.

For engineers looking to make their safety tests and inspections on these systems, this can cause a problem when using standard test equipment. One such electrification system exists within Network Rail, which operates part of its network at 650V. As standard test equipment can not operate and test live at these voltages, extra work and downtime is needed to be scheduled in order to make the safety tests required. This adds time and costs to the maintenance plans.

The solution to test on a live network was found from the test equipment manufacturer, Sonel. Based in Poland, Sonel provides measurement solutions globally. Working in a large number of countries, the company has been able to build a profile of products capable of working on an extended range of test voltages. Its high voltage high current fault loop impedance meters are suitable for testing on many alternative system voltages, including 110/190V, 115/200V, 127/220V, 220/380V, 230/400V, 240/415V, 290/500V and 400/690V.

This variety of voltages can be seen in Poland, where 690V can be found in industrial applications; in Germany for some of the larger automotive manufactures; and also in larger renewable projects being commissioned across Europe. The Sonel meter’s overall operating voltage range is from 100V up to 750V, making it ideal for the application found at Network Rail and for many others where a traditional loop impedance meter may not be capable of making the required measurement due to various factors. The factors where fault loop measurements are sometimes difficult to make can include unusual or non-standard voltage levels and testing at transformers where the higher reactance requires a higher test current in order to make an accurate test measurement.

The principle application within Network Rail is for the testing of its 650V based power distribution systems. This type of distribution system can be found in areas of the network such as the signalling power feeders. Existing fault monitoring equipment is already installed at these locations, however, the need to test live for the fault current (Ik) and loop impedance (Zs) measurements was also required.

Electrical systems commonly seen in the UK are either TT, TN or IT systems. For the 650V distribution system found at Network Rail sites, an IT system is employed. The power is distributed by two phase conductors, – 325V and +325V (650V Ph-Ph), to supply power on a number of the key systems for safe rail network operations. Where the Sonel device is expected to be used most is in removing the requirement for conducting conductor and circuit protective conductors (CPC) continuity testing for the IT systems, mainly on the individually earthed systems used. The meters from Sonel will make it possible for engineers to use a tester that can measure the phase loop impedance whilst the system is live and therefore not requiring planned maintenance shutdowns.

Along with the 650V electrical network, Network Rail also has standard 230/400V systems. The meter can make these measurements through a simple range selection change, meaning the same instrument can be used for multiple test applications.

The process of approval for the product took just over two years. Extensive testing by Network Rail engineers on the network was conducted to ensure the metering was safe and reliable to use before proceeding to issue product approval for use on the network.

Sonel provided support during the process to rewrite, test and issue an updated version of the instruments’ firmware. This was achieved within two weeks of request, and enabled the meter to make measurement calculations in accordance with Network Rail’s requirements and to ensure the level of accuracy and consistency they required for the test results.

The Sonel MZC-306 has gained approval by Network Rail for use in its signalling power feeders to test the fault current and loop impedance measurements and is fully PADS approved under the Network Rail Certificate of Acceptance PA05/07453.

In addition, the wide variety of electrical systems that the meter can test not only makes it ideal for the 650V railway electrification network, but also for use in a wide variety of LV electrical installations that may be encountered in modern electrical systems, such as solar PV renewables, commercial properties, public facilities, workshops and factories.

“Extensive testing by Network Rail engineers on the network was conducted to ensure the metering was safe and reliable to use”