Sonel adjustable electrical resistance measures

   Adjustable electrical resistance measures (resistance boxes) have been used for long time in metrology for adjustment and calibration of electrical resistance meters. In practice they represent in-series set of individual resistors, which may be switched on to the circuit or switched off from it thus adjusting total resistance. In spite of apparent engineering minimalism of this equipment important technologies and original engineering solutions are hidden in a case of resistance box.

   The first prototypes of modern resistant boxes were manufactured as early as in the XIX century. Resistance coils made of alloys were used as the basis of such electrical resistance measures. At first argentan or platinoid were used as the most popular alloys but due to instability of their parameters in time they were substituted by konstantan and manganin gradually. At the same time switching circuit for resistance boxes was also developed. It suggested initially direct connection of inner coil leads using additional facilities, for example, switch plugs or plugs.

   Over the years this circuit was replaced by lever switching of required resistance rates. It allowed implementing rather convenient interface with a user, who could adjust resistance set by turning adjustors of individual decades to pre-labelled positions of switches. Among exotic modifications of adjustable electric resistance measures it is worth mentioning resistance simulators, which diverge from classic circuit of in-series resistors connection and allow simulation of high resistance. But this method of resistance production is unacceptable for a number of application cases.

   Though different modifications of adjustable electrical resistance measures were used, including usage of resistors and electronic switching control in their structure, resistance boxes of lever type with manganin resistance coils became the most popular by the end of the XX century.

Drawbacks of popular resistance boxes

Meanwhile actual level of instrument engineering development set a rather high bar for metrological servicing of modern resistance meters. Generally used digital testers of electric safety parameters use in measurement process short-time large values of current passing through measuring circuits, and that is why their usage is not intended for calibration of popular resistance measures with low values of permissible dissipation. Nowadays modern megaohmeters are rather sort of teraohmmeters designed for measurement of resistance with limit of tens of teraohms and measuring voltage up to 10kV. In this situation use of the vast amount of popular resistance boxes is not possible for calibration of such testers both due to incompatibility of the upper limits of resistance ranges and limitation of permissible dissipation. It should not go unmentioned that design of resistance boxes involving usage of mechanical switches for decades to set target resistance values   does not allow any automation of calibration process. Certainly, all the above functional constraints and design defects serve as the reference point in technical assignment for manufacturing of device of the next generation with modern electronic control and usage of recent developments from leading manufacturers of resistors.


      SRM-0R1-4k1 is a an example of classic adjustable electric resistance measures with lever-type switches, and at the same time it belongs to a separate class of specialized reference standards due to a number of its features of construction. Resistance measurement in fault path by digital devices is made due to short-time power supply circuit closure to the reference resistor built-in into a measuring instrument. It stipulates passage of large current values in measuring circuit, which can reach up to 40 A of the main frequency. The accompanying attribute of this process is heating of conductors and drift of metrological performance in result. Besides the above facts for the purpose of metrological emulation of such network it is necessary to consider another important factor – when working with alternating current we should simulate no just resistance but its active component. Usage of widely used resistance boxes is unacceptable in this case and it will lead either to damage of their components or deviation from preset resistance values. In order to support this process in correct way it is necessary to use specialized equipment such as SRM-0R1-4k1.

   The base of this decade resistor is represented by manganin coils with bifilar winding, which are coated with special varnish and aged in thermal chamber. Material and technology used for manufacture of resistance coils ensure stability of reproducible resistance and minimization of environmental impact. Among others it allows ensuring stability of the decade resistor performance in result of coil material heating upon passage of high-amperage currents. Auxiliary active cooling circuit prevents potential effect of these processes. Bifilar winding of manganin wire is responsible for elimination of inductive component in reproducible resistance; it allows damping of reactivity occurring at work with alternating current.

   Being a part of resistance reproducing circuit, ad hoc decade switches influence minimally to zero resistance of the decade resistor and are able to operate with high-amperage currents. Operating range of this reference unit is within limits of 0.1 – 4,100 Ohm with class of accuracy of 0.05 thus completely meeting needs for calibration of electrical safety testers in regard to active resistance of the fault path. And finally, though SRM-0R1-4k1 is designed for certain tasks, it can be used for classic measurements without involvement of power supply circuit.


   Going to general cases from special-purpose engineering solutions it is worth mentioning the whole line of close to classic decade resistors of MC series. “Close to classic” means that they still have a unique feature, which will be mentioned later. In general the whole МС series is an example of decade resistors of lever switch type based on film resistors used for wide scope of metrological applications. Based on quantity of decades the series consists of six modifications, which cover range of resistance from 0.1 Ohm up to 100 MOhm partially or completely. Each modification may be manufactured in three versions according to requirements to the class of accuracy, up to class of accuracy of 0.05.

   When comparing decade resistors of МС series and adjustable electrical resistance measures widely used in the last century then the main advantage of МС series consists in increased permitted dissipation per a degree of one decade. For six junior decades of the decade resistor this parameter is equal to 5W and it is decreased up to 0.5W in megaohm area. This feature allows using decade resistors for specialized metrological tasks in regard to adjustment of some measurement modes of devices intended to control electric safety parameters. Definitions specific for this sphere, such as contact voltage, resistance of grounding device, resistance of protective conductors, imply physical processes with passage of measuring signals of high values both for alternating and direct current.  Usage of MC series as the reference unit for the above values allows complete metrological identity at parameters calibration of wide range of residual current devices, grounding devices and large number of ohmmeters.


We have been discussing modern adjustable electrical resistance measures with manual control and now it's high time to mention reference standard equipment ensuring perspective user interface and also featuring unique characteristics. Our company produces several modifications of resistance calibrators of SRP series. They are designed on the basis of high-voltage film resistors with good values of TCR (thermal coefficient of resistivity) and VCR (voltage-variable coefficient of resistance) coefficients. It allows ensuring stability of characteristics while calibrating various resistance meters with wide range of testing voltage rates.

   Resistors and banks of switches of the calibrator are placed in a separate inner shielded and sealed module, which is controlled using keyboard or special-purpose software. A value set by a user is processed by the central processing unit that implements switching of required resistor cells. The circuit is formed by the calibrator from in-series resistors and connected to input connectors at the front panel. Special attention in the calibrator design is paid to issues of the user and equipment safety as far as potential voltage in measuring circuit may reach 10kV. The safety is ensured due to use of sensor keyboard, wide use of plastic fluor in the measuring circuit, inner separation of the calibrator body into process execution module and command unit, remote control and galvanic isolation of USB interface.

   The line of resistance calibrators of SRP series is represented by modifications with their main characteristics shown in Table 1.

Table 1 – Main Characteristics of Product Line of Calibrators of SRP series.




Range Relative error, % Connection to PC
SRP-50k0-10G0 2.5 50 k .. 10 G 0.1 – 1 +
SRP-50k0-100G0 50 k .. 100 G 0.05 – 0.5
SRP-50k0-5T0 5 50 k .. 5 T 1.5
SRP-10G0-10T0 10 10 G .. 10 T 1 – 1.5 -
SRP-100G0-20T0 100 G .. 20 T 1.5 - 3

   As seen from the above specified data the product line of calibrators varies depending on metrological tasks to be solved by calibration laboratories. Let’s view certain representatives of this series in more details.

SRP-50k0-10G0 and SRP-50k0-100G0

   Calibration of different meters with function of electrical resistance measuring, digital and analogue megaohmeters of initial level (including those with enhanced accuracy) is widely used by calibration laboratories, which test instruments for electrical quantity measuring. Tested devices of this class are characterized by fairly good accuracy characteristics and relatively low measuring voltage. Calibrators SRP-50k0-10G0 and SRP-50k0-100G0 are designed for convenient solution of this metrological task. At that the first calibrator has minimum cost while the second one ensures the best metrological performance in КС series. Basic configuration of both calibrators includes USB interface and free software for extension of their functional capabilities. In particular, tuning mode in control program allows calibration of analogue resistance meters due to smooth adjustment of reproducible resistance.

Interaction with calibrator

   Calibrator interface is represented by graphical LCD display, sensor keyboard and software.

   Besides direct input of resistance values, manual control mode provides option for start-up of built-in calibration procedures. Usage of these procedures minimizes calibration time due to semi-automatic sequential search of preset check points. Operator’s intervention is required only for transition between these points. Usage of these calibration programs reduces costs for complete calibration cycle significantly. By default calibrators are supplied with pre-loaded procedures for calibration of the most widely used SONEL insulation resistance meters.

   Software of the calibrator allows extension of ability for its control. The main program tab emulates complete manual control allowing usage of keyboard and mouse for entry of required commands.

   There is Tuning tab for calibration of pointer-and-scale instruments. It allows recording and storing of resistance values set at the calibrator when ohmmeter indicator is approached smoothly to the calibrated scale marks at both sides. The produced data array may be exported to text file for further basic error calculation of the calibrated meter.

   In Programming tab the operator may create required calibration procedures for all types of resistance meters calibrated in the laboratory. The operator needs to enter name of the calibrated device only and to fill in check points for selected measuring resistnace values in compliance with   calibration report.

The procedures created by the operator may be stored at the computer for further usage. The calibrator memory may store up to 10 of the most in-demand procedures. They may be started up both at active connection to the computer and in free running mode.

   Moreover control program for SRP calibrator provides option to change main settings of the calibrator such as sensitivity of sensor keyboard response, display brightness, time adjustment for sleep mode activation and to update built-in software of the calibrator when new versions appear.


            SRP series ensures compatibility of input connectors with majority of modern meters, including SONEL measuring instruments and majority of equipment produced by other manufacturers.

   Standard SRP package includes all required connecting wires for assembly of reliable measuring circuit. When it is necessary to calibrate obsolete resistance meters or modern devices with unexpected process solutions in regard to input circuits auxiliary accessories may be used such as metal bushings of the respective diameters and alligator clips. Using this switching method it should be kept in mind that in this case additional error is introduced into the measuring circuit, especially to the upper limit of resistance.

   Besides, another design of terminals is used for new generation of SONEL ad-hoc high-voltage meters of insulation parameters. In order to ensure their compatibility with KC calibrators special adaptors are included into optional set for this series.


   Summarizing brief excursus into Ohm’s law implementation we can say that electrical resistance is the most interesting segment of physical quantities with large amount of nuances and the whole array of theoretical foundation. Engineering solutions provided by us in this area have been serving as sentinels of metrological service providing uniformity of measurements. Permanent engineering perfection and 36 months of warranty are unchangeable attributes of our activity in the sphere of production of electrical parameters measuring instrumentation and equipment for their metrological servicing.


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