Low frequency loop impedance, Physics

Low frequency loop impedance:

Low frequency loop impedance testing is a useful method complementary to DC bonding testing.  A visual inspection of cable bundle shields, complemented by a low frequency loop impedance test, gives good confidence in the integrity of the shielding provisions. 

Low frequency loop impedance testing is a method developed to check that adequate bonding exists between over braid (conduit) shields and structure.  To achieve the shielding performance required, it is often necessary that both ends of a cable bundle shield be bonded to aircraft structure.  In such cases, it is hard to check bonding integrity by the standard DC bonding test method.  If the bond between shield and structure at one end is degraded while the other one is still good, there is little chance to find this defect by performing DC bonding measurements.  The remaining bond still ensures a low resistance to ground but the current loop through the shield is interrupted, causing degradation of shielding performance.  The fault can easily be detected by performing a low frequency loop impedance test. 

The test set-up requires simple test equipment, refer to Figure.  A current of about 1 kHz is fed into the conduit under test while measuring the voltage necessary to drive that current.  Other versions of the loop impedance test arrangement use different frequencies (200 Hz is typical), and provide the resistive and reactive parts of the loop impedance. 

1919_LOW FREQUENCY LOOP IMPEDANCE.png

The test equipment consists of a generator operating at 1 kHz feeding an injection probe and a current monitoring probe, connected to an AC millivoltmeter.  A voltmeter connected to the generator enables the voltage necessary to drive the current to be measured.  1 kHz is a high enough frequency to drive the injection and the monitoring probes and is also enough to avoid specific RF effects, like non-uniform current distribution along the loop under test. 

If, in practice, the current is set to 1A, the voltage figure, when expressed in millivolts, gives the loop impedance in milliohms directly.  The loop impedance is normally in the range 1-100 milliohms.  In this range, accurate results can easily be achieved. 

If too high loop impedance is found, the joint determining the problem has to be identified.  This can be performed by measuring the voltage drop across each joint.  The joint with the high voltage drop across it is the defective one, refer to Figure. 

1442_LOW FREQUENCY LOOP IMPEDANCE1.png

As there is no need for a wide band swept RF generator, the test equipment can be quite simple and easy to handle.  Hand held battery powered test equipment, especially designed for production monitoring and routine maintenance, is available on the market.  

 

Posted Date: 9/13/2012 8:17:40 AM | Location : United States







Related Discussions:- Low frequency loop impedance, Assignment Help, Ask Question on Low frequency loop impedance, Get Answer, Expert's Help, Low frequency loop impedance Discussions

Write discussion on Low frequency loop impedance
Your posts are moderated
Related Questions
Is CPH a quantum energy unchangeable? Answer- Yes, the quantum energy (photon) is made of lots CPH, different between photons depend to the number of CPH that they are made of.

The central part (photosphere) of the sun is very hot and  all possible wavelengths of the visible light. Though, the outer part (chromospheres) having of vapours of dissimilar com

a) Sketch an IV characteristic of a photodiode, showing how the curve shifts as a result of the illumination intensity incident on the device being varied. b) Indicate on the c


what about capacitor with dielectric & air electric & electrolytic capacitor

difference b/w concave grating and rownald mounting

why sky is blue

in some case we take y=-g but in some case we take y=g... why???

The Time Base Control 3 Shows the effect of changing the time base control on the display when there is an input voltage varying in time when the frequency of the voltage is hi

The driver of an SUV slammed on the breaks when he saw a tree 52 m away blocking the road. The SUV slowed down at a constant rate of 4.0 m/s2 for 3.6 s before reaching the tree. Wh