Find your nearest distributor
 Get in touch
 Log in

Low Resistance Ohmmeter


  • BT51
  • Overview  
  • Application  
  • Downloads  
  • High test current
  • Four terminal measurement
  • Two measuring ranges (2000 mΩ. and 20,00 mΩ.)and maximum resolution of 0,01 mΩ.
  • Protected against inadvertent connection to the mains supply

The BT51 Low Resistance Ohmmeter makes measurements by passing a current through the conductor under test and also monitoring the voltage across it. The test current is limited by a simple current limiting circuit and is measured by monitoring the voltage across a resistor. The test current is maintained at a nominal 2 A, and as the measurement is ratiometric, the reading is unaffected by any current variations.


Instruments that measure low resistance accurately and give the result directly are invaluable in many applications. The BT51 is a stable, accurate, reliable, low resistance ohmmeter equally suited to precision laboratory applications and to applications in the field.

Example Uses

Commissioning and maintenance of substation equipment, where measurements can be made on such things as busbar joints, switch and circuit breaker contact resistance, fuse resistance, cold lap welded joints in aluminium earthing strip and earth bonding

Maintenance of overhead transmission lines, where “hot” joints can be tested before and after their remaking or recompression

Bond testing aircraft frames, including the bonding of electronic dischargers and fuel tanks

Testing earth bonds in mines

Rail bond testing, where a rail is used as part of a communication system or for power transmission

Testing the integrity of lightning conductors

Electronic equipment, where measurements can be made on such things as resistors, track resistance of printed circuit boards (quality control of plating thickness), resistance of plated-through holes on printed circuit boards, contact resistance of relays, resistance of shunts, thick film circuits, etc.

Domestic and industrial wiring installations, where ring main continuity and circuit protective conductor continuity can be measured and the integrity of earth bonding checked in compliance with the l6th Edition IEE Wiring Regulations


The BT51 uses the four-terminal method of measurement. The main advantage of this method is that the resistance of the test leads is not included in the measurement. This is an important factor when the value of the measured resistance is very low. In the diagram, resistance RX is measured only between points A and B; other resistances in the current loop either side of A and B are ignored.

Good connections to the item whose resistance is being measured are very important. Test leads for the instrument may take the form of duplex hand spikes, which enable connections to such things as busbars and aircraft frames to be made easily. Crocodile-type clip leads are used where a more rigid connection is necessary (e.g., when varying contact resistance tends to introduce errors). Sometimes the current connections are made with crocodile clips and the potential connections with spikes. This may be the case, for example, where multiple measurements have to be made.