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Contact Resistance vs. Millivolt Drop
by Dan Hook
Advanced Electrical Testing, Inc.
I
have had the opinion that the millivolt drop test was better than a digital low
resistance ohmmeter (DLRO) check for the simple reason that the millivolt
drop method is an alternating current (ac) test at rated current whereas a
typical digital low resistance ohmmeter test is a low value direct current (dc) test.
This opinion stemmed from a gut feel that an ac breaker would be better tested
with ac, rather than a complete review of the tests on my part. Recently I had a
customer with the will, and the money, to delve deeper into the contact resistance
vs. millivolt drop controversy than ever before in my experience.
During a recent acceptance testing and commissioning job, a low-voltage
power circuit breaker, 800 ampere frame with 400 ampere plug, was fully tested.
The millivolt drop method at the long time pickup value of 400 amperes was
used during the primary current injection testing in accordance with the testing
requirements set forth by the customer. The results were 88 mV, 86 mV, and 97
mV for phases A, B, and C respectively, well within the +/-50 percent specified
in NETA standards.. An “Acceptable” sticker was attached to this breaker and
all was well until testing was performed on the switchboard and bus duct two
months later.
Phase connections were being tested between two 8000 ampere bus duct services connected to a double-ended switchboard. During testing with a DLRO,
C-phase showed an unacceptable deviation in resistance as compared to the other
two phases. After further investigation and switchboard breaker manipulation,
the high resistance connection was narrowed down to be internal to the power
circuit breaker previously thought to be acceptable. Additional DLRO tests on
the breaker revealed results of 29, 23, and 96 microhms, for phases A, B, and C
respectively.
Why the difference in the test
results? The difference lies in the
character of the test. As mentioned
above, the millivolt drop method
uses a low voltage ac signal to
generate high current through each
pole of the breaker; voltage drop is
measured using a multimeter from
line to load. This millivolt drop is a
function of the ac resistance of the
material as well as the inductive
reactance of the phase under test.
Both of these factors combined
represent the impedance of the
circuit. The DLRO method uses a
small battery pack and generates
10 amperes dc flowing from line to
load on a single pole of the breaker.
The readout is in ohms. When the
reading is stable it accounts for
the dc resistance of the current
path from line to load. Figure 1
below shows that if the ac inductive
reactance is large enough relative
to the dc resistance, the resultant
impedance magnitude can mask
the presence of an unacceptable
deviation in dc contact resistance.
Where:
R=dc resistance component
X=ac inductive reactance
component
Z=resultant impedance vector
www.netaworld.org Summer 2006 NETA WORLD
Figure 2 — Obvious imbalanced heating between phases (front view
of breaker contacts).
Although the dc resistance component increases by 50
percent, the resultant vectors only vary by approximately
12 percent.
The breaker was deemed unacceptable and a warranty
return to the manufacturer was attempted. The manufacturer
was unwilling to provide a replacement or warranty repairs
based on the above test results. They were of the opinion
that neither of these testing methods was as good as a full
load heat run to determine the thermal performance of a
circuit breaker. After approval, we proceeded with a full load
heat run lasting two hours.
The thermographic images show obvious contact resistance discrepancies on C-phase. The manufacturer agreed
to repair/replace the defective breaker.
During the course of this project I learned that contact
resistance is a superior and sure-fire way to verify acceptable
contact condition for low-voltage power circuit breakers.
Needless to say my company will not use the ac millivolt
drop method as the sole criteria for acceptance of new or
used equipment.
Editor’s Note: Dan has brought up a subject that has been
a problem for many testing firms, particularly with respect
to molded-case circuit breakers (MCCBs). ANSI/NEMA
AB 4-2001, in clause 5.4, “Individual Pole Resistance Test
(Millivolt Drop)”, requires a direct current power supply
of 24 volts or less and a current equal to the rating of the
circuit breaker or 500 amperes, whichever is less. NEMA
members usually do not recognize warranty claims based
on a 10 ampere DLRO test, yet many NETA firms have
experience indicating that a poor result from the 10 ampere test usually is indicative of a problem. In power circuit
breakers, the problem can usually be resolved by removing
the arc chutes and cleaning the contacts. In MCCBs, it is
common for the manufacturer to void the warranty if the
breaker is disassembled to examine the contacts or internal
connections. Thus, the testing firm must provide expensive
additional testing to confirm a problem.
Heat run tests are not difficult and can be done with the
same primary injection equipment that is used to test the
breaker’s trip unit. However, the test is time consuming
and, therefore ,expensive. A 24 volt dc power supply with
an adjustable current output of 500 amperes is considerably
more expensive than a DLRO and, therefore, not usually an
item on the equipment list for a NETA member’s service
vehicle.
As Dan points out in the article, an ac millivolt drop test is
not an adequate substitute for the dc test. This is unfortunate
since many ac primary injection test sets have metering to
measure the ac millivolt drop. Also, NETA Maintenance
and Acceptance Testing Specifications do not describe the
source to be used in the pole resistance test requirement.
This, I am sure, is an oversight and not an intention to allow
either ac or dc millivolt drop tests.
Dan Hook is the President of Advanced Electrical Testing in Pacific,
WA. He holds a Bachelor’s Degree in Nuclear Engineering and a Masters
Degree in Electric Power Engineering, both from Rensselaer Polytechnic
Institute in Troy, NY. Dan has over 12 years of experience in the electrical industry. He served in the US Navy performing maintenance and
testing, and also teaching nuclear submarine electric power generation
and distribution systems.
Figure 3 — Increased heat dissipation into C-Phase primary disconnect (rear view of switchgear).
NETA WORLD Summer 2006
www.netaworld.org