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why dc voltage is applied in insulation resistance testing/

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DC is easy to regulate and control. There's no advantage to using AC or any difference in the test results, so DC is normally used.

Edit: Another factor is that testing with an AC will give some small current due to circuit stray capacitance, which would give errors in calculating the insulation resistor unless you also measured the phase of the signal.
 
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The test is to make sure that the insulation can withstand a constant stress for a specified lenght of time without bresking down.
The main reason for using DC to test insulation is that DC presents a constant stress on the insulation material. AC is, of course, a constantly varying stress, which allows the insulation to recover to some extent when the voltage falls into the trough of the waveform. A DC stress is constant, and will not allow the stress on the insulation to recover until the stress is removed or it breaks down.

Dave M
 
can any one tell why there is drum test of power cable before laying...? power cable like 132 kv

A 132kV power cable will be a VERY expensive item.
It will be a fairly difficult job to lay the cable.

The company who made the cable, and the company who delivered the cable to the work site will be interested to know that it is in good condition BEFORE it is layed into the ground. Then they know that they have delivered a product in good condition and can get paid for it.

The company laying the cable in the ground want to know that the cable is good or bad before it is layed.
If the cable has been layed and it is faulty, the installation company will get the blame for damaging the cable. So they want to be sure the cable is good before they lay it.
If the cable is faulty before laying, the installation company will know not to start the installation. The cable must be repaired first.

JimB
 
i m preparing for interview for dubai muncipality approval ..so kindly help me out for answer..i will highly obliged...related cable laying
what is link box ? its uses and it advantage...
 
You should enter these questions into a search engine. msn.com or yahoo.com or bing
Most of us have not used >100kv.
 
i m preparing for interview for dubai muncipality approval
So, you are wanting us to provide answers to possible questions you may get at an interview for a job.

A job, which in my opinion you do not have the technical knowledge or practical experience to carry out.

JimB
 
I am an engineer. My definition is; one that can learn.
Anyone can fake their way through a interview. most people do.....
A engineer type person, will be able to do the job by 8:00 a.m., even if it takes a all-nighter reading books.
A person with out a job is one that did not read the books and did not ask the questions.
 
**broken link removed** and free little download on the subject of insulation testing. There is a small section on both DC and AC testing. I suggest you read and understand it.

Ron
 
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