testing voltage drop across a breaker

[mjmike]

What does testing for voltage drop across a breaker tell you and when do you perform this test?

I understand you put one lead of the DMM on the line and the other on the load, and read the mV. Thinking of this from a theory standpoint, it is like reading the voltage drop across a resistor. So, I am guessing this is to check for internal resistance of a breaker?

I was told anything higher than 100mV means there is a circuit breaker problem as a rule of thumb. However, wouldn't this reading change depending on the amp draw of the load and also on the type of breaker such as thermal magnetic ect?

Could this test also produce the same results by checking the voltage to ground at both the line and load side then comparing the difference?

Is it valid to check any other electrical components this way such as contactors, relays, etc?

Any info would be helpful.
Thanks

 

[kwired]

You are exactly right with most of what you said you are essentially checking across a resistance, the less resistance the less the drop will be. There will be some drop across all breakers or they would never be able to do what they were intended to do as there has to be some resistance in the thermal element of the thermal trip device or it would never heat up to make it trip. The amount of drop will also be in proportion to the amount of current flowing through that thermal element. What you are likely trying to find would be increased resistance because of a failure of something, either resistance in a connection or at the contact points.

 

[ptonsparky]

Yes checking across relays and contactors works. Any contact. Line side and load side checks to ground will not work because of the different scales. 100s vs .01

 

[templdl]

Millivolt tests across bread contacts are amusing to me. Although it may make sense but ohms law must be considered. If millivolts is measured that is a voltage drop which equates to contact resistance that there will be heating.
To to do a millvolt drop test the breaker must be loaded to some degree and the current passing through the contacts would generate heat.
Since the contacts actually rub together when they close the millivolt would change each time that you close the contacts. Comparing millivolt drops take from one pole to the others may be a diagnostic tool but each time the breaker is opened and closed it will change.
As a former breaker application engineer I find some of the field testing of breakers to be intriguing especially when measuring of contact resistance is attempted.

 

[zog]

Thats what microhmeters are for.

 

[templdl]

Thats what microhmeters are for.

And then what? What do the results represent? Resistance=heat.
Do you have so manufacturers values to compare to?
Every time a breaker is opened and closed that number will change.

 

[ptonsparky]

Millivolt tests across bread contacts are amusing to me. Although it may make sense but ohms law must be considered. If millivolts is measured that is a voltage drop which equates to contact resistance that there will be heating.
To to do a millvolt drop test the breaker must be loaded to some degree and the current passing through the contacts would generate heat.
Since the contacts actually rub together when they close the millivolt would change each time that you close the contacts. Comparing millivolt drops take from one pole to the others may be a diagnostic tool but each time the breaker is opened and closed it will change.
As a former breaker application engineer I find some of the field testing of breakers to be intriguing especially when measuring of contact resistance is attempted.

Amusing as it may seem, if I find 450mv drop from top of disc to load side of motor starter and 90 on the other two legs, I have reduced the potential problem area by 66%. Granted a good thermal camera and or microhmeter may be better but not all of us have those on our truck. Definitely not the latter.

That said, make sure you use all the necessary precautions to take those measurements safely. Thermal imaging cameras are getting better and less expensive all the time.

 

[GoldDigger]

Measuring the drop across a closed energized breaker is somewhat risky for your equipment. If the breaker should open, you will get full line voltage across your meter.
An autoranging digital meter might be fine, but an analog meter could be damaged.

In addition to the contact resistance varying each time based on wiping action, etc., there may be a difference in resistance between closing unloaded and closing under load.


Tapatalk...

 

[zog]

And then what? What do the results represent? Resistance=heat.
Do you have so manufacturers values to compare to?

Yes I do. Even if someone does not have the OEM's tollerances, contact resistance tests are required tests per ANSI/NETA standards as well as NEMA AB4

 

[Ragin Cajun]

Depending on the gear, size breaker, voltage, etc. measuring voltace "drop" across a breaker, IMHO, is not something I would want to do. For numerous reasons!

Thermal imaging is much safer and will detect any difference between phases.

RC

 

[templdl]

Amusing as it may seem, if I find 450mv drop from top of disc to load side of motor starter and 90 on the other two legs, I have reduced the potential problem area by 66%. Granted a good thermal camera and or microhmeter may be better but not all of us have those on our truck. Definitely not the latter.

That said, make sure you use all the necessary precautions to take those measurements safely. Thermal imaging cameras are getting better and less expensive all the time.

Thermal imaging will expose heating issues. I have an IR accessory for my Fluke 79 that is worth its weight in gold when I have a concern about heating. I was one of two breaker application engineers and I am aware what are legitimate tests which I go back to NEMA AB4 the manufacturers acceptable method for field testing molded case circuit breakers.
NEMA AB4 5.1: " To assess the electrical intergrety on connections and contacts in a circuit breaker. This can only be done by conducting a millivolt drop test across the line and load terminals of each pole with the contacts closed.
The millivolt drop of the circuit breaker can vary significantly due to inherent variability in the extremely low resistance of the electrical contacts and connections. SUCH VARIATIONS DO NOT NECESSARILY PREDICT UNACCEPTABLE PERFORMANCE AND SHALL NOT BE USED AS THE SOLE CRITERIA FOT DETERMINATION OF ACCEPTABILITY."

 

[zog]

Not the sole criteria by any means.

Also importnat to point out that you do not do a MV drop test with the breaker in service. AB4 and NETA/ANSI specifically say to disconnect it from the system and use either a 24VDC or less power supply or a microhmeter with at least a 10A output.

 

[templdl]

Not the sole criteria by any means.

Also importnat to point out that you do not do a MV drop test with the breaker in service. AB4 and NETA/ANSI specifically say to disconnect it from the system and use either a 24VDC or less power supply or a microhmeter with at least a 10A output.

Are you referring to a microvolt meter? Ohm meters don't like being used on circuits that are energized, such as the 24vdc and 10a output.
Without current flowing through the breaker the will be no voltage drop. Who is doing a MV drop test across a breaker that is not loaded going to work? What you're attempting to do is attempt to measure contact resistance indirectly by measuring a voltage drop. But this can not be done unless the breaker is loaded. As such with the breaker in service is the an issue with measuring the voltage drop between the line and load terminals? If you new the current and the voltage drop you could certainly determine the contact resistance is the results would mean anything anyway.
When it comes down to it the results may not have any relivancy.

 

[zog]

Are you referring to a microvolt meter? Ohm meters don't like being used on circuits that are energized, such as the 24vdc and 10a output.
Without current flowing through the breaker the will be no voltage drop. Who is doing a MV drop test across a breaker that is not loaded going to work? What you're attempting to do is attempt to measure contact resistance indirectly by measuring a voltage drop. But this can not be done unless the breaker is loaded. As such with the breaker in service is the an issue with measuring the voltage drop between the line and load terminals? If you new the current and the voltage drop you could certainly determine the contact resistance is the results would mean anything anyway.
When it comes down to it the results may not have any relivancy.

No, microhmeter, and I specifically stated the breaker is not energized. I get the feeling you don't know what a microhmeter is? They are more or less a Kelvin bridge, and supply the current source (A clean, stable, controlled one) needed to get the voltage drop.

 

[templdl]

No, microhmeter, and I specifically stated the breaker is not energized. I get the feeling you don't know what a microhmeter is? They are more or less a Kelvin bridge, and supply the current source (A clean, stable, controlled one) needed to get the voltage drop.

Regardless, what do the results even mean? Since I was one of 2 beaker applications engineers for a major breaker manufacturer and no where did we have a procedure for measuring breaker contact resistance as an acceptable test for breaker "health." As such where are these contact resistance test procedures coming from? It isc not a legitimately recognized test by the manufacturer that I worked for.
Resistance=heating. Also, contact resistance will in fact change with each opening and closing of the breaker. The breakers tested with 24vdc does in no way compare to breakers commonly operationing at 120, 208, 240, and 480v to 600v with often times hundreds of amps.
The test as you suggest is not a test that a manufacturer recognizes. Field testing procedures are as included is NEMA AB4. As far as a microhmeter is concerned it has never been applicable in any breaker testing procedures even in the breaker testing procedures as commonly conducted in our warranty returns facility.
My objective is to stay focused of accepted industrial/commercial breaker testing procedures. Any contact resistance results I would consider as a symptom and not something that would determine to be a reason for rejection as there are no breaker contact resistance standards.

 

[zog]

Regardless, what do the results even mean? Since I was one of 2 beaker applications engineers for a major breaker manufacturer and no where did we have a procedure for measuring breaker contact resistance as an acceptable test for breaker "health." As such where are these contact resistance test procedures coming from? It isc not a legitimately recognized test by the manufacturer that I worked for.
Resistance=heating. Also, contact resistance will in fact change with each opening and closing of the breaker. The breakers tested with 24vdc does in no way compare to breakers commonly operationing at 120, 208, 240, and 480v to 600v with often times hundreds of amps.
The test as you suggest is not a test that a manufacturer recognizes. Field testing procedures are as included is NEMA AB4. As far as a microhmeter is concerned it has never been applicable in any breaker testing procedures even in the breaker testing procedures as commonly conducted in our warranty returns facility.
My objective is to stay focused of accepted industrial/commercial breaker testing procedures. Any contact resistance results I would consider as a symptom and not something that would determine to be a reason for rejection as there are no breaker contact resistance standards.

Um, I was directly quoting from NEMA AB4 (They say DLRO in leiu of microohmeter but DLRO is a brand name for Meggers microhmeter), but as stated earlier contact resistance is just one of many tests to be used to determine condition. Same test is also part of the NETA/ANSI standards.

6.4 INDIVIDUAL POLE RESISTANCE TEST (MILLIVOLT DROP)
See Figure 7 and Figure 8 for typical test set up.
NOTE?The circuit breaker should be removed from the equipment for this test. In cases where the circuit breaker can be safely
isolated as installed, the test may be performed with the circuit breaker in its equipment.
6.4.1 Purpose
To assess the electrical integrity of internal connections and contacts in a circuit breaker. This can be done
by conducting a millivolt drop test across the line and load terminals of each pole with the circuit breaker
contacts closed.
The millivolt drop (resistance) of a circuit breaker pole can vary significantly due to inherent variability in the
extremely low resistance of the electrical contacts and connectors. Such variations do not necessarily
predict unacceptable performance and should not be used as the sole criteria for determination of
acceptability (See 6.4.5).
6.4.2 Equipment
6.4.2.1 This test should be conducted using a 24 volt, or less, direct current power supply capable of
supplying the rated current of the circuit breaker. For circuit breakers rated higher than 500 amperes, the
power supply should be capable of delivering no less than 500 amperes.
6.4.2.2 If the above equipment is not available for field tests, a Digital Low Resistance Ohmmeter (DLRO),
or 4-point tester, capable of 10 to 100 amperes (DC), may be used.
NOTE?Use of a multimeter or low current ohmmeter in place of the power supply will not provide an accurate or reliable
measurement of millivolt drop and should not be used.
6.4.3 Procedure

6.4.3.1 After being properly isolated, remove the breaker from the enclosure. In cases where the circuit
breaker can be safely isolated/disconnected as installed, the test may be performed with the circuit breaker
in its equipment.
6.4.4 Test
NOTE?If the circuit breaker is equipped with an under-voltage trip release, energize the trip release to allow proper operation of the
circuit breaker (See 7.3 for proper procedures).
6.4.4.1 The test is performed as follows:
6.4.4.1.1 Apply test current across a pole equal to the breaker rating (or 500 Amperes minimum for
breakers rated in excess of 500 Amperes). Record the millivolt drop and the test current. Do not maintain
current for more than 1 minute. If this equipment is not available, use the following test.
6.4.4.1.2 Apply test current across a pole of 10 Amperes, or the Ampere rating of the breaker, for breakers
rated less than 100 Amperes. For breakers rated more than 100 Amperes, apply a test current across a
pole of 100 Amperes. Record the millivolt drop and the test current, or resistance. Do not maintain current
for more than 1 minute.
6.4.4.1.3 De-energize the test circuit. Manually operate the breaker to the OFF and then ON positions.
6.4.4.1.4 Repeat steps 6.4.4.1.1 and 6.4.4.1.2 for a total of three readings on the pole being tested.
6.4.4.1.5 Repeat steps 6.4.4.1.1 through 6.4.4.1.3 for each of the remaining poles of the circuit breaker.
6.4.5 Results
The results of test will vary according to the breaker frame type, ampere rating, and manufacturer. The
manufacturer should be consulted to determine the maximum allowable voltage drop. If the average test
values of any pole of the breaker exceed the maximum allowable drop, the circuit breaker may have
reached the end of life and additional tests may have to be conducted.
NOTE?Inconsistent readings could be the result of oxide films or foreign material on the contact surfaces, depending on the service
history of the breaker. If high millivolt or high resistance readings are detected, it may be advisable to do tests in section 6.6, which
may help clean conta

 

[templdl]

"acceptable Zog,
Yes, that is a lengthy test to finaly get to the results and what they mean.
" The manufacturer should be consulted to determine the acceptable level for the breaker under test."
That's nice in theory by the reality is that having 18 years directly selling and 5 years as an applications engineer supporting molded case circuit breakers for one of the largest manufactures we never had any "acceptable level" for a breaker user test. It is an impressive but the results had really no relevance in determining the health of a breaker and certainly was not a factor to determine that a breaker was defective and would be covered under warranty. That certainly wasn't in my warranty department manager's basis for a warranty replacement. In my trips back to our engineering facility where our warranty dept. was located it have the opportunity to run factor tests on some of the breakers that were returned an this test was not a part of determining if a breaker was to be replaced under warranty.
And for sure any contact resistance that is meaningful would result in heating on the breaker which may escalate to weakening the contact spring tention which weakens the contact tention to a point where the contacts can arc or weld or be destroyed and on rare occasion it does happen. An infrared scan of a breaker that is in device will show a noticeable temperature difference should a pole be failing.
Even though we did not have any magic contact resistance values that we could provide from a field testing point of view it is a tool that never the less that can be uses to determine a beaker's heath. If it is worth the time to do this test and the relevancy of the results is up to the tech but from a manufacturer's point of view that in and of by itself was not any basis of a warranty replacement.