Protection relays are also sensitive to the a.c. ripple, which can exist within the output current of the test equipment. These small ripples can look like a potential fault, for example, a.c. signal and also trip the Circuit Breaker under test. Is this a reason to keep these relays in place?Smooth current output enables protection to remain in place during testing thus maximizing safety for the user.Wheatstone and Kelvin bridgesA Wheatstone bridge can be used to measure resistance by comparing an unknown resistor against precision resistors of known value. A Kelvin double bridge is a variant of the Wheatstone bridge and can be used for measuring very low resistances. Wheatstone bridgeA pioneering method for measuring resistance was devised in 1833 by S. H. Christie and made public by Sir Charles Wheatstone. The simplest arrangement is a square pattern of four resistors with a galvanometer connected across one diagonal and a battery across the other (see Fig 38). Two of the resistors are of known appropriate values and comprise the ratio arm (A + B). A third has a known value which can be varied in small increments over a wide range, and is thus designated the rheostat arm (R). The fourth is the resistance being measured, the unknown arm (X). Fig 38: Wheatstone bridge circuitviiThe bridge is considered balanced when the rheostat arm has been adjusted (tweaked) so that current is divided in such a way that there is no voltage drop across the galvanometer and it ceases to deflect (is nulled). The resistance being measured can then be calculated from a vii Electrical Meterman’s Handbook; Third Edition; 1965; page 479knowledge of the values of the ratio resistors and the adjusted value of the rheostat arm. The basic formula is: X = B/A x RWhere:B and A are the ratio resistors R is the rheostatThe Wheatstone Bridge can be constructed to a variety of ranges and is generally used for all but the highest and lowest measurements. It's suited to a range of about 1 to 100,000 A.Kelvin bridgeThe Kelvin Bridge (also known as the Thomson Bridge) is used for precision measurements below the typical range of the Wheatstone Bridge. Sir William Thomson (Lord Kelvin) devised the concept circa 1854. The classic arrangement has six resistors in a rectangle, bisected by a galvanometer (see Fig 39). A comparatively large current is passed through the unknown resistance and a known resistance of a low value. The galvanometer compares the voltage drops across these two resistances with the double ratio circuit comprised of the other four resistors.Fig 39: Kelvin bridge circuitviiiFor very low measurements, the Kelvin Bridge has the advantage of nullifying extraneous resistances from leads and contacts by employing the system of double ratio arms. The resistances of the connecting leads are in series with the high resistance ratio arms and not with the reference or tested resistors. The two pairs of ratio resistors (A/B, a/b) are paralleled with each other and connected across with the galvanometer. One pair (a/b) is in series with the unknown (X) and the reference standard (R). The latter is an adjustable low resistance, usually viii Electrical Meterman’s Handbook; Third Edition; 1965; page 480a Manganin bar with a sliding contact. When potential is balanced across the two parallel circuits, the unknown is equivalent to the parallel ratio multiplied by the adjusted reference value.X = A/B x RA connecting link (Y), sometimes called the yoke, shunts the ratio pair (a/b) that are otherwise in series with the unknown and standard, but has minimal effect on the accuracy of the measurement so long as the two pairs of parallel ratio resistors are kept exactly equal (A to a, B to b). Lead and contact resistances are included in the value of the ratio pairs, and any effects can be nullified by keeping the resistance of the yoke extremely low. Keeping the yoke resistance low also accommodates the large test currents often used in Kelvin Bridges without causing unwanted heating effects.DLRO microohm and milliohm applications listAviation ■Assembly of components ■Interconnection of equipment ■Repair and maintenanceRail, including tram and underground ■Rolling stock and infrastructure ■Track high current joints ■Signalling systemsMarine ■Power wiring systems ■Protection systems ■Ship-to-shore bonding ■ Cable ■Connection points ■Cathode protection system testingOil and gas pipelines ■Bonding between welded joints ■Grounding systemsAutomotive and electric vehicles ■Battery connections ■Weld quality ■Quality of crimped connections ■Assembly robot welding cablesCable manufacturers ■Quality control ■Cable lengthComponent manufacturers ■Quality control ■Resistors, inductors, chokesAll types of mechanically assembled joints which need low resistance values ■Bolted ■Welded ■Compressed ■Crimped ■Soldered ■Conductive adhesive ■Joints subject to ■ Stress ■Vibration ■ Heat ■Cold ■ Corrosion ■FatigueCable manufacturers ■Motors and generators ■Coil and turn-to-turn shorts ■Bar-to-bar tests ■Coil balance — cold to full load current comparisonwww.megger.com 3332 A guide to low resistance testing www.megger.com
