MRU-200 – Earth Resistance Meter with Surge Functionality

The MRU-200 is a new earth resistance meter, designed and manufactured by Sonel, a Polish company. It is a highly functional instrument, suitable for users who have to make repeated measurements under various and often difficult field conditions.

The meter provides an exceptional range of functions, including comprehensive methods for taking static earth resistance measurements (3 and 4-wire measurements; two types of clamp measurements), as well as a unique method of surge resistance measurement. A significant advantage of this functionality is the possibility of making auxiliary surveys that support system diagnostics (measurement of root-mean-square currents using two types of clamps; protective and equipotential conductor continuity checks). Furthermore, each function maintains a complete set of supporting data (interference signal voltage and frequency values; back-up electrode resistance values, etc.). Given the number of functions on hand, the user can set up virtually any method of measurement under any conditions and anywhere in the world.

Undoubtedly, the opportunity of making earth resistance measurements using a surge function is a novel idea, and a unique option offered by the MRU-200. This method is designed for dynamic parameter diagnostics of lightning management systems.

An essential difference that must be taken into account during the analysis of earthing system parameters, with reference to protection from electrocution and from lightning, is that for electrocution the currents present in the system have relatively low frequencies (50 Hz, 60 Hz), whereas lightning protection involves very rapid pulse strikes that considerably increase the inductive component of the earth electrode. In this case, only the part of the earth electrode that directly contacts the lightning strike location will be used effectively for the dissipation of the lightning current. This is an essential difference: an earth electrode featuring a low static resistance and warranting good basic protection may fail to ensure acceptable lightning protection parameters. This situation is common in the case of distributed earthing systems, whose dynamic resistance may be several times higher than their static resistance.

The surge measurement method supported by the MRU-200 fully complies with the definition laid down in IEC 61024-1 (Building site lightning protection. General principles). The procedure allows the determining of a routine value, i.e. surge resistance (Rd), which is equal to the quotient of the voltage and current peak values. In compliance with the standard, surge resistance is a routine value since the voltage and current peaks usually occur at different times. Hence, surge resistance is deemed a good indication of the effectiveness of the earthing system under conditions of tightened or special protection.

The procedure involves the production of an electrical effect corresponding to a lightning pulse. Two numbers define the pulse parameters: the leading edge duration (t1) and the time to half-peak (t2) (see Fig. 1). The MRU-200 allows one to select two pulse forms: 10/350μs and 4/10μs. According to the IEC 61312-1:2001 standard (Protection from electromagnetic lightning pulses. General principles) the 10/350μs pulse form is typical of the first strike of lightning. The same pulse is defined as the model pulse in the EN 62305-1:2006 standard (Lightning protection - Part 1 - General requirements).

The surge method may be also used for measurement of earth resistance of high-voltage transmission pylons. This system has been developed to allow the determination of the earth resistance of the entire pylon, including hoop iron systems and the pylon footing resistance. Moreover, it can be used for live high-voltage lines where traditional 3-wire methods require the total shutdown of the transmission line. Conventional 3-wire measurements ignore the pile footing resistance, leading to an overestimation of the actual values.

The MRU-200 can measure surges in 4-wire systems. Contrary to appearances, this is an essential advantage since this ensures independence from the impedance of the conductors: for the test pulses used by this method, the impedance may exceed 0.5 Ω in a conductor 2.2 m long. With such short test pulses, measurement using a method other than a 4-wire system means that the result will include a considerable error due to the impedance of the conductor that connects the meter ports with the object being measured.

The use of a special shielded cable for surge measurements is an important innovation. This 50 m long cable is included in the standard package and ensures pulse minimal interference being transmitted by the electromagnetic field for the duration of the measurement.

The MRU-200 has great resistance to user-induced errors, with protection from voltage sources that do not exceed 250 Vrms. Hence, accidental connection to the mains will not damage the unit.

This is a particularly outstanding meter due to its intuitive operation. Thanks to the large graphical display (240x160 pixels, 4 shadows of grey), all messages and commands can be given in the written form instead of symbols; this means the user can understand them without the need to consult the operations and maintenance manual. One of the keys has been dedicated for use as a help function, with on-line diagrams of the wiring circuits that are used for specific measuring functions.

Compared with other instruments on the market, the MRU-200 earth resistance meter is outstanding, being equipped with many functions that allow the optimum measurement method to be easily selected for virtually any conditions, including the checking of the effectiveness of specific protection systems. The instrument features a novel design, is simple to use, and comes with a considerable kit of accessories. It is certainly a remarkable instrument, and we recommend it to anyone who needs the extensive measuring potential combined with high reliability.