DC Networks’ and Power Sources’ Line and Loop Resistances

Power distribution

DC networks are an ever-expanding field in the high and ultra-high voltage locations. The lowered losses at transport over large distances and ability to bridge between incompatible AC systems make it an attractive option.

Substations and industrial locations in middle voltage range also depend on DC islands: DC control circuits, supplying DC sources, DC charging stations for the battery banks, power lines and grounded loops. The installation’s protective measures include circuit protection based on internal resistance of the source and automatic trip-out protection like fuses. Fuses are a practical solution that will regularly operate in case of faults, errors or shorts where resistances in the circuit are low enough.


Inspection of DC circuits is mainly dependant on impedance measurements in line or loop circuits. Periodical inspections of DC circuits can discover bad contacts on bus bars, condition of batteries, conditions of power switches, contactors or fuse holders. The resistance result at the power source needs to be compared with the result at the outer edge of the circuit where voltage drops shall not exceed evaluated border limit. Internal resistance of the batteries shows their quality and condition. It can be measured in either parallel or series connection, either whole system or partially.

Industrial solution must support live testing: no disconnection of the DC batteries or DC power sources during testing. MI 3144 compares loaded and unloaded battery’s voltage and calculates its internal resistance. Measured and calculated values are compared to the reference in documentation. The batteries from 3 V DC up to 260 V DC can be evaluated.

The measuring principle enables also live testing of loop resistance of DC networks and DC power Sources for wide range of applications. Quality of the battery banks and separate cells can be evaluated and measured in percentage of the voltage drop during loading. Cells must be be compared one to another to eliminate possible power failures.

MI 3144 is a multifunctional instrument that can be used either together with MI 3155 for the most extensive industrial and medium voltage applications, or by itself. It can be controlled using the Android application aMESM. DC testing can be done without MI 3155. 

Four-wire type of connection with Kelvin crocodiles for high precision and sta¬ble results elimi¬nates connection problems even in an aggressive environment.

Internal resistance of the Power Source with Automatic trip-out ability evaluation with high accuracy on DC circuits.

Measured result of live loop resistance on DC control circuits at the field end to be compared with battery source for evaluation of the conductors, relays and contacts quality.



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