May, 2016
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Page 79 When Lightning Strikes — Keeping Aircraft Flying Continued from previous page
developed to provide accurate valida- tion of the resistance of grounding loops and parallel resistance paths. These methods involve using current clamps to indirectly induce a current to flow in the loop, and to monitor the current flowing and the voltage developed in forcing that current to flow, and hence deduce the resist- ance of that complete loop. Whereas bond testing uses DC current, loop testing techniques use induced AC current of a known frequency. The use of AC current introduces further complication in that the values measured are actually impedance, and are phase-affected. The critical portion of the measurement is the in- phase resistive value, which must be calculated using designed phase cor- rection algorithms. Any accurate and reliable test system must be able to isolate and report the true resistance value of a loop, using these tech- niques.
Testing a Ground Loop Characterized loops vary from
simple to complex paths through large numbers of conjoined components and surfaces. Each joint between these components and surfaces contributes to the overall loop resistance. It is therefore critical that one be able to isolate and measure the accurate resistance value of each contributing factor in the overall loop measure- ment, and to accomplish this while leaving the assembly intact. Accord - ingly, the measurement and charac- terization of these joints is the third element to be considered in compre- hensive ground circuit testing. While the loop test measures
the resistance value of a complete loop, joint testing involves measur- ing the resistance of each joint with- in that loop. The characterization of each joint within a loop enables the user to detect individual joints that may be close to resistance tolerance, or the joint that is causing a loop to be reported as high-resistance. The joint resistance values
within a loop are measured by carry- ing a loop measurement technique one step further. While the current of a known frequency and magnitude is injected into the loop, a measure- ment is made of the voltage drop that results across a given joint within that loop. Using calculations extend- ed from the previously described loop measurement methods, the individ- ual joint resistance value is deter- mined using a current and voltage phase correction algorithm, a tech- nique necessary to ensure the accu- rate reporting of resistance. The described methodology
allows the test to report the accurate value of the joint resistance without concern for alternate current paths that cause a common bond test to report false values. This allows users to accurately characterize each joint that in a loop, without manually dis- assembling the loop to otherwise facil- itate isolated bond measurements.
A complete Test Strategy A complete test strategy
employs each of these three men- tioned methods (bond, loop and joint) to ensure the robustness of the ground circuits in aircraft, trains, vehicles and structures. MK Test Systems’ BLRT automatic bond and loop resistance test system addresses
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Lowest Cost of Ownership
all three. The BLRT system includes an
integrated computer and test man- agement software, which allows download of complete test require- ments and sequences from manufac-
turing management systems. It auto- matically creates the test flow and stores measured results, guarantee- ing the accuracy of recorded meas- urements, as well as full traceability. In the OEM space the BLRT is
used to ensure that the complete grounding circuit is implemented and within resistance limits as the product is manufactured. It maxi- mizes test efficiencies through dra- matic reduction in test times by elim- inating the need for a paper record, and by guiding the operator through the correct test processes with inte- grated visual aids and prompts. The real-time record that is created can provide instant feedback to automat- ed statistical process controls that are used in best-practice manufac- turing environments. In the MRO space these tests
Correct way to test an electrical grounding loop using a pair of clamps.
can be carried out to gauge the degradation of ground circuits and
Continued on page 87
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