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.

The instrument has a 3 1/2 digit LED display to facilitate use in low light levels. Two LED’s on the front panel indicate battery condition and whether test current is flowing when a measurement is being made. A neon lamp indicates the presence of a dangerous voltage if the test spikes are accidentally placed across a live circuit. This warning takes place whether or not the instrument has been switched on. The ohmmeter is protected up to 240 V a.c. by a relay circuit that keeps the instrument in a safe mode isolated from the mains supply. Again, this safeguard is effective whether or not the instrument has been switched on.

Operation of the instrument is simple, there being only one range switch to set. Power is supplied by internal rechargeable cells and the charger unit is incorporated into the case. Duplex Connect hand spike test leads are supplied and other types of leads are available. The instrument is built into a robust, portable case that is weatherproof and shockproof and has a hinged, detachable lid.

Applications

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 aluminum 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

Principle of Operation

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.