Testing and Test Results on a Multifunction Test Set —
I received the following question from a field technician in Cairo, Egypt. They have the same types of cables and the same types of service that we have here in the US, hence the same problems. One of their field technicians had questions on testing and test results on his multifunction test set. I felt this discussion might be helpful to some of you struggling with similar issues.
Greetings, Sir —
I would like your help when using the EXFO 635G multifunction test set. I’m really grateful for your concern and taking some of your time. I have a lot of questions — here are some of them.
• What is the difference between a resistive balance test and an isolation test?
• When a RFL test is needed — is it for a hit in the cable, or for bad isolation on the cable pair?
• Can you detect a bad splice through a TDR test?
I’ll explain the Resistive Balance Test and an Isolation Test. However, before you run either test, you need to do a Longitudinal Balance Test.
LONGITUDINAL BALANCE TEST
Begin by disconnecting the end user’s equipment and the central office equipment or the remote equipment from the cable pair, and perform a Longitudinal Balance Test on the vacant cable pair. Any Longitudinal Balance Test that measures greater than 60dB is a Pass.
Any test that is less than 60dB is a Fail. In that case you must do the following:
Next Steps If You Fail the Longitudinal Balance Test
Test the vacant pair from the far end, perhaps from the central office or the remote.
Test the pair tip and ring to ground for crossed-battery from other cable pairs.
If crossed-battery is identified, then use your RFL function to locate where the crossed-battery is coming from, and fix the pair and then retest the longitudinal balance of the pair. If the longitudinal balance is greater than 60dB, then restore service.
If the longitudinal balance test still fails, then there could be more than 1 fault on the cable pair.
Test the pair for a tip to ring short, and then test the pair for a tip or ring ground. If a short or a ground is identified: again, use the RFL function to locate the fault and repair it.
Retest the longitudinal balance. If the longitudinal balance passes, then restore service. If the longitudinal balance test still fails, then the pair has a series resistance fault or the pair is split with another cable pair.
Now we can perform a Resistive Balance Test and then an Isolation Test.
RESISTIVE BALANCE TEST
Go to one end of the cable pair, perhaps the central office or the remote. Short and ground the cable pair. Be sure that the grounded short is 0 ohms.
Go to the other end and connect your test set. Perform a Longitudinal Balance Test. (See the test information provided earlier.) If the test fails, the pair has a series resistive fault such as a loose or failed splice connector.
Use the Resistive Balance Test to identify the series resistance fault, and then use the TDR to locate it.
Use your test set VOM to measure tip to ring ohms, and then measure tip and ring to ground ohms.
• Whatever resistance the tip to ring measurement shows, the tip and ring to ground should measure 1/2 of the tip to ring measurement.
• Therefore, if tip to ring measures 100 ohms, tip and ring to ground should measure 50 ohms.
• If tip to ring measured 100 ohms and tip to ground measured 55 ohms and ring to ground measured 45 ohms, then the tip has 10 ohms extra in series.
More than likely the resistive balance failed because there is a bad splice connection in one of the splices.
To locate the splice, use the TDR function on your test set.
The TDR baseline trace would be up or above the baseline like an open.
• If the splice is too far away from where you are testing from the TDR, then it will not show it.
• If the TDR does not show it, go half way back toward the grounded short and run the resistive balance test again.
If the Resistive Balance Test fails the series, then resistive fault is still toward the grounded short, so repeat the process and go half way again and remeasure the resistive balance.
A Fail means the series fault is still toward the grounded short. A Pass means that the fault is away from the grounded short and is toward the field of where you are testing.
• Look in the direction of the fault with the TDR.
• The splice may show up on the TDR trace because you are closer to the fault.
• If the splice does not show up on the TDR trace, then go half way in the direction of the fault.
• After the fault is located and repaired: restore service.
The Isolation Test is automatic (a button on your test set), and should always be the final test — and it’s critical that the tech doesn’t skip this test.
However, many techs do skip this test because they aren’t sure what it is, what it does, and why they should perform this final, automatic test.
They may think that because they’ve already done a VOM test when doing the Resistive Balance Test and all looks good, they are done. However, the VOM can be a false positive, so to speak. If you don’t do the Isolation Test, you will likely get the dreaded call: a repeat report from the same customer.
So, you go and move them to a new pair — and all looks good, right? But then you get another repeat call. I call this the Black Martini Rage Customer.
This could have been avoided had you done the Isolation Test. The reason this test is critical and is the final test is that the average VOM applies only about 15 volts DC on the pair — and under the low battery that is applied, the pair can show a Good test when in fact there is galvanic corrosion which can mask the leakage making the pair test Good.
The automatic Isolation Test hits the pair with up to 500 VDC. This positively identifies any leakage between cable pairs, or any leakage between cable pairs and the cable shield.
• If there is corrosion, then you fix it. If it’s in cable pair, then resplice it, cut out the corrosion, and you should be good to go.
• However, if the corrosion is caused by water in the cable itself, then you have to replace the cable. Today, much of the cable is filled cable vs. air core cable. The filled cable is more resistant to water damage but it is not impervious. Additionally, there is still plenty of air cable installed.
Therefore, you must ALWAYS DO AN ISOLATION TEST as the final test before putting the pair back in service. It’s easy:
• Press the button that’s labeled isolation test.
• Within the Isolation Test, a large amount of battery (up to 500VDC) hits the pair. This positively identifies any leakage between cable pairs, or any leakage between cable pairs and the cable shield.
• If there is corrosion, then you go back, fix it, and then repeat this test.
Often, I find that some technicians are uncomfortable asking what they consider are basic questions because they think they should know the answers. However, many companies provide little training, and the equipment providers do their best but the training is primarily hands-on. In-person training for many is reserved for larger customers. So don’t be uncomfortable asking any questions; I enjoy helping everyone from beginner to advanced techs. And advanced techs should also be curious about basic questions and procedures. Just because they’ve been doing something 15 or so years doesn’t mean they were taught the best process. I invite you to call me and I will walk you through it! 831.818.3930. I also like getting mail, so send me an email (email@example.com) about any of your tech concerns or questions.