Contact Resistance testing of BUS, difference between test currents 10A VS 100A

[Static93475]

Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks

[Kalbi_Rob]

Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks

Here is a link and excerpt from Megger's Guide to Low Resistance Testing
You will see they highlight that if performing manual test, you want to test at a current low enough not to heat up the metal, thus not changing the resistance properties of the metal:

https://www.testequipmentdepot.com/m...ce-testing.pdf

CURRENT SELECTION
Depending on the instrument chosen, the current selection can be either manual or automatic. The operator should select the highest current suitable for the test to provide the best signal to noise ratio for the measurement. On instruments that offer current levels in excess of 10 amps, care is required to minimize any heating of the sample that would itself cause the resistance of the sample to change.

Instruments designed for circuit breaker testing have much higher current characteristics and the operator must be careful when setting the test current level. Instruments designed specifically for transformer testing have a special high-voltage power level at the beginning of the test to saturate the winding. These units then switch to a lower constant current mode to measure the winding on the transformer.

I would highly recommend reading this material to fully understand what you are doing when you are performing DLRO testing.

[Static93475]

Here is a link and excerpt from Megger's Guide to Low Resistance Testing
You will see they highlight that if performing manual test, you want to test at a current low enough not to heat up the metal, thus not changing the resistance properties of the metal:

https://www.testequipmentdepot.com/m...ce-testing.pdf

CURRENT SELECTION
Depending on the instrument chosen, the current selection can be either manual or automatic. The operator should select the highest current suitable for the test to provide the best signal to noise ratio for the measurement. On instruments that offer current levels in excess of 10 amps, care is required to minimize any heating of the sample that would itself cause the resistance of the sample to change.

Instruments designed for circuit breaker testing have much higher current characteristics and the operator must be careful when setting the test current level. Instruments designed specifically for transformer testing have a special high-voltage power level at the beginning of the test to saturate the winding. These units then switch to a lower constant current mode to measure the winding on the transformer.

I would highly recommend reading this material to fully understand what you are doing when you are performing DLRO testing.

Thanks for the reply Kalbi, the BUS ducts were long runs (roughly around 500ft) we did the tests at both current levels and the results were almost identical 10A & 100A, NETA states to investigate any deviation from similar connections by more than 50% of the lowest measured value. I was wondering because someone ordered the wrong piece of equipment and we only had a 10A DLRO and we usually use a 100A.

[nolanholt]

Just wondering on some theory info about long bus runs on contact resistance testing prior to putting equipment into service. Testing it at 10A VS 100A, does testing it at a higher current make it a more accurate current reading. Thanks

From what I've picked up from tribal knowledge, I've been taught that a 100A ductor is generally used more for medium voltage gear or low voltage switches/motor starters/breakers that are rated for 100A+ if contact resistance is at the edge of (or past) the 50% acceptable value. The higher current seems to have more of an impact with "cleaning" the contacts using higher current to "blast away" any gunk or grease. I've been able to change a few pieces of equipment from a fail to a pass by doing this. I would say, however, that I generally use a 10A ductor for most applications, and have never used a 100A ductor for runs of bus.

[SecondGen]

From what I've picked up from tribal knowledge, I've been taught that a 100A ductor is generally used more for medium voltage gear or low voltage switches/motor starters/breakers that are rated for 100A+ if contact resistance is at the edge of (or past) the 50% acceptable value. The higher current seems to have more of an impact with "cleaning" the contacts using higher current to "blast away" any gunk or grease. I've been able to change a few pieces of equipment from a fail to a pass by doing this. I would say, however, that I generally use a 10A ductor for most applications, and have never used a 100A ductor for runs of bus.

This has been the case in my experience as well. Our company only has a couple of 100A ductors and they are typically used for MV or HV equipment. Most techs carry a 10A on their truck at all times.

100A ductors are very robust and the test connections are usually bulky clamps, which can make them difficult to work with. In this particular application, a 100A ducter may be beneficial for a long bus duct run but I've had success testing LV bus duct risers in large commercial buildings with 10A.

With all of that said, I've heard instances of breaker manufacturers not doing warranty work for 10A contact resistance tests, they want to see 100A. It's really all depends on the specific application but in general go with 10A and bump up the current only if readings are questionable.

As others have stated, be careful not to alter the device under test with excessive currents. Also be mindful of any components in the test circuit that can't handle such levels, such as when testing PT fuses.

[test11]

This has been the case in my experience as well. Our company only has a couple of 100A ductors and they are typically used for MV or HV equipment. Most techs carry a 10A on their truck at all times.

100A ductors are very robust and the test connections are usually bulky clamps, which can make them difficult to work with. In this particular application, a 100A ducter may be beneficial for a long bus duct run but I've had success testing LV bus duct risers in large commercial buildings with 10A.

With all of that said, I've heard instances of breaker manufacturers not doing warranty work for 10A contact resistance tests, they want to see 100A. It's really all depends on the specific application but in general go with 10A and bump up the current only if readings are questionable.

As others have stated, be careful not to alter the device under test with excessive currents. Also be mindful of any components in the test circuit that can't handle such levels, such as when testing PT fuses.

This is especially true with SF6. I've also found that you need adequate burn in time for consistent and accurate results. I've never found a direct answer as to why, but I believe it's due to initial electron absorption from some SF6. I've seen 20-40 percent drops in the minute value vs. the spot value.

[TrapFlyVenus]

This has been the case in my experience as well. Our company only has a couple of 100A ductors and they are typically used for MV or HV equipment. Most techs carry a 10A on their truck at all times.

100A ductors are very robust and the test connections are usually bulky clamps, which can make them difficult to work with. In this particular application, a 100A ducter may be beneficial for a long bus duct run but I've had success testing LV bus duct risers in large commercial buildings with 10A.

With all of that said, I've heard instances of breaker manufacturers not doing warranty work for 10A contact resistance tests, they want to see 100A. It's really all depends on the specific application but in general go with 10A and bump up the current only if readings are questionable.

As others have stated, be careful not to alter the device under test with excessive currents. Also be mindful of any components in the test circuit that can't handle such levels, such as when testing PT fuses.

WHAT IS YOUR METHOD OF TESTING THEM LONG RUNS? LONG LEADS OR A JUMPER

[SecondGen]

WHAT IS YOUR METHOD OF TESTING THEM LONG RUNS? LONG LEADS OR A JUMPER

For 10A testing, I typically like to use long leads. I have two 100' extension cord leads made up that can get me point to point testing up to 200' if need be. Jumpers are the way to go for bus risers that span multiple floors in a building.

[Kalbi_Rob]

WHAT IS YOUR METHOD OF TESTING THEM LONG RUNS? LONG LEADS OR A JUMPER

I personally have one of 2 methods I use.

First, I cut two 25' extension cords in half and flipped them so that I can insert additional extension cords to make the leads as long as I want. On the cut extension cord, one side gets 100A clamps (1 on hot and 1 on neutral which leaves ground as a backup) and the other gets banana plugs or fork spade connectors, which can connect to the DLRO V and C connection points. Since DLROs use Kelvin bridge, they have the ability to remove lead resistance from the final measurement and the leads can be as long as you need.

Second works if I can't run my leads to the end (multi-level bus duct, etc.), and it is atleast 3 wire system. To measure "A" phase, attach a lead from the DLRO current to "B" phase, and DLRO voltage to "C" phase. The other set of DLRO outputs (V and C) goes to the end of "A" phase. At the other end of the specimen (bus), take jumpers from "B" phase and "C" phase to "A" phase. Thus you use the other two phases as your DLRO leads and get accurate measurement with connection to either end of "A" phase. Repeat steps for other phases.
This measurement removes the resistance of the jumper and other phase resistance measurement.