Sunday, 13 March 2016

Dynamic Contact Resistance Measurement, DCRM


First question which strikes in our mind is that why do we need to do DCRM even though we already had Contact Resistance Measurement,CRM? Seems good question and this question will take you to the depth.
So, first of all we need to know about the contacts of Circuit Breaker. Basically, a CB has two types of Contact.
  • Main Contact– Main contact of CB are designed to carry current when the CB is closed but it is not designed to take arcing stress during Closing / Opening operation of CB. Therefore, obviously Main Contact will make after the making of Arcing contact during Closing operation and it will separate before the separation of Arcing contact during Opening operation.
  • Arcing Contact– Arcing contact is provided in CB to take all the arcing stress.
BreakerFrom the picture, it is clear that how arcing contact and Main Contact work. When we measure the CRM, we do it after closing the CB so the measured resistance is basically the Net Resistance of Main Contact and Arcing Contact. If the Resistance of Main Contact is R and that of arcing contact is r the both of them are parallel when Breaker is closed. So measured resistance during CRM = R*r/(R+r)
Next, If I ask you how will you determine whether the Arcing Contact assembly is properly working or malfunctioning then will you answer by doing CRM? Obviously your answer will be a big NOOO………..
How will we confirm then? It’s by conducting DCRM.
Why dynamic? As because the arcing contact travels while Opening / Closing operation to fully take closed position / open position. Therefore the word Dynamic came in picture. Therefore as you can guess, DCRM measures the contact Resistance during dynamic condition. Basically it is the measurement of Arcing contact resistance when contact travels.
How do we conduct this test and what are the results?
This test is conducted by DCRM kits available in market of many makes like SCOPE, DOBLE etc. What we do, we extend the 220 V DC supply to the kit and form the kit 220 V DC is extended to the nearest point of Trip Coil-1 & TC-2 and Closing Coil. By this manner we take the control to kit for closing / opening of Breaker. For taking the Breaker Open / Close feedback to the kit, we connect the kit to the NO contact of Auxiliary Switch of CB. Now, we need to get the travel of contacts, for that we connect kit to the closing / opening lever via a Transducer. By this manner, we are now able to control the CB, Status of CB and contact travel in kit.
DCRM kit now injects 100 A / 25 A current through the contacts and plot a curve of Current through the contact during dynamic condition, Travel of contact and Dynamic Resistance with respect to Time. Straight forward, a fingerprint of CB we get.
How does the fingerprint looks and how to interpret?
DCRM
This a typical fingerprint of healthy CB. How do we analyse this signature of CB?
See the graph, there are three quantities namely Resistance (Dynamic), Current and Travel of Contact. Lets start when the CB is open, obviously Resistance will be high as shown by horizontal red line. But as soon as we give close command to the CB, first Main Contact makes and resistance decreases to point P1. After that Arcing Contact start closing, resistance varies from P1 to P2 and finally when it is fully closed the resistance is constant P2 to P3. At P3, CB is fully close. Now we will see what happens to the Current.
Initially current is zero as CB is open. As soon as we give close command to the CB,at P1 current increases and at P2 it will be highest as CB is fully close here. Current will remain at its maximum up to P3.
At P3 we give opening command to the CB by the Kit, so Main Contact will open at P3 and Resistance will increase slightly to P4, and then Arcing Contact start opening and at P4 it fully opens and hence Resistance take the maximum value. Likewise current also, decreases to its original value i.e zero.
Now we focus on Travel of contact. Leaving this portion for you to think but in case you need help write in comment box.
Now, see the dynamic period is from P2 to P3 as in this period Arcing contact is travelling to get at fully close position. Therefore, during the dynamic period the Resistance curve is almost flat which signifies constant resistance during dynamic period which in turn meand Arching contact and main contact assembly are properly working and contacts are in good condition. Now take a look at this Signature of CB.
Bad CB
See the dynamic resistance(Green curve) during Close / Open period is not flat rather it is fluctuating which means contact condition is not good or assembly is malfunctioning.
That is all which I can convey. If you have more information share it by writing in comment box, I will be very happy. Thank you!!!

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