Interpreting DLRO Results

[Warrengarber]

I have a question about interpreting the DLRO results. According to the NETA MTS 2019, 7.6.1.2 Circuit Breakers, Air, Low-Voltage Power, D. Test Values – Electrical, #3 "Microhm or dc millivolt drop values should not exceed the high levels of the normal range as indicated in the manufacturer’s published data. If manufacturer’s data is not available, investigate values that deviate from adjacent poles or similar breakers by more than 50
percent of the lowest value." Both the LV Air Switch and Molded Case Circuit breaker use this wording. When they refer to adjacent poles. I would read that as comparing A phase to B phase and B phase to C phase but not A phase to C phase.

Example: 151(A), 128(B), 98(C). 50% of lowest reading (98) would be 98/2=49. So phase A (151) is within 49 ohms of adjacent pole B (128), and phase B (128) is within 49 ohms of adjacent pole C (98). Phase A (151) is NOT within 50% (49 ohms) of phase C (98). A and C phases are not adjacent to one another in my mind.

Would these reading be acceptable? Are we supposed to also compare A phase to C phase? There have been questions similar to this on every test I have taken and I want to make sure that this simple question is not one that I am missing. Thanks for any input.

[SecondGen]

All phases are compared to each other, sometimes there can be a neutral pole. The way I like to calculate in the field is take the lowest value and multiply by 1.5. If lowest reading is 90, no other value can be above 135.

[cbrazzil]

It just wants you to take any phase that has the lowest reading and divide it by 50% and compare it to the other phases.

[brewhahamaster]

I have a question about interpreting the DLRO results. According to the NETA MTS 2019, 7.6.1.2 Circuit Breakers, Air, Low-Voltage Power, D. Test Values – Electrical, #3 "Microhm or dc millivolt drop values should not exceed the high levels of the normal range as indicated in the manufacturer’s published data. If manufacturer’s data is not available, investigate values that deviate from adjacent poles or similar breakers by more than 50
percent of the lowest value." Both the LV Air Switch and Molded Case Circuit breaker use this wording. When they refer to adjacent poles. I would read that as comparing A phase to B phase and B phase to C phase but not A phase to C phase.

Example: 151(A), 128(B), 98(C). 50% of lowest reading (98) would be 98/2=49. So phase A (151) is within 49 ohms of adjacent pole B (128), and phase B (128) is within 49 ohms of adjacent pole C (98). Phase A (151) is NOT within 50% (49 ohms) of phase C (98). A and C phases are not adjacent to one another in my mind.

Would these reading be acceptable? Are we supposed to also compare A phase to C phase? There have been questions similar to this on every test I have taken and I want to make sure that this simple question is not one that I am missing. Thanks for any input.

I know that they have asked these type of questions in the past. You are correct in your thought process and accurate in your computations. NETA will usually ask this question, with a couple of the answers falling into acceptable parameters. You then have to decide which set of answers are the MOST acceptable, in which case, would be the LOWEST values still falling under the 50% deviation parameter spelled out by the MTS/ATS.

[Sparky59]

I have a question about interpreting the DLRO results. According to the NETA MTS 2019, 7.6.1.2 Circuit Breakers, Air, Low-Voltage Power, D. Test Values – Electrical, #3 "Microhm or dc millivolt drop values should not exceed the high levels of the normal range as indicated in the manufacturer’s published data. If manufacturer’s data is not available, investigate values that deviate from adjacent poles or similar breakers by more than 50
percent of the lowest value." Both the LV Air Switch and Molded Case Circuit breaker use this wording. When they refer to adjacent poles. I would read that as comparing A phase to B phase and B phase to C phase but not A phase to C phase.

Example: 151(A), 128(B), 98(C). 50% of lowest reading (98) would be 98/2=49. So phase A (151) is within 49 ohms of adjacent pole B (128), and phase B (128) is within 49 ohms of adjacent pole C (98). Phase A (151) is NOT within 50% (49 ohms) of phase C (98). A and C phases are not adjacent to one another in my mind.

Would these reading be acceptable? Are we supposed to also compare A phase to C phase? There have been questions similar to this on every test I have taken and I want to make sure that this simple question is not one that I am missing. Thanks for any input.


Yes. It includes a comparison between the three (ABC) poles. Also, if you have several identical assets, you can examine the readings of like devices and evaluate based on the group of readings.

[AaronCBI]

I know there are already good answers, just want to throw in that in my experience all three lines are compared to each other.
I take the lowest reading and multiply it by 1.5 and use that as my highest allowable reading.

[Nick1234]

I know there are already good answers, just want to throw in that in my experience all three lines are compared to each other.
I take the lowest reading and multiply it by 1.5 and use that as my highest allowable reading.

This is the correct answer.
Things to note are some times there is a neutral that also has to be withing 1.5 of the lowest value. The only time where this may not be true is when using a low resistance ohm meter on bus in a switch gear ground bus is usually higher then the other phases due to it being usually a smaller bus and routed through different ares in the gear. This can be true about neutral as well where it might be much less then the other phases due to having less bus length.