What is acceptable for voltage drop across a suspected fault? And is it dependant on the voltage in the system? 120V vs. 480V

I read somewhere on this site about "anything over 500 mV in a 480V system is a significant fault". Is that published anywhere?

Today, I found a 200 amp main breaker in a 120/240V circuit breaker panel that has one leg pretty hot (185 deg. F). The other leg is at ambient. So there is a 120 degree F delta T. I'm messuring 1.1 volts dropped across the fault. For 110 volt potential, is this acceptable? Current is comparable (36 & 42) between the two, and no where near full load.

I'm sure it is a lot hotter inside where the actual fault is. Any thoughts?

Panel_0-46.doc (430 Kb, 56 downloads)

FWIW, we have enough info to calculate watts under the assumption that the voltage drop you measured is resistive (sounds like a good assumption to me).

Watts = volts times amps (where volts and amps are rms... how most meters read).

1.1 volts times 42.6 AMPS ~ 47 watts.

It strikes me as pretty high (half a 100 watt lightbulb dissipated as heat in that little area).

I'm not sure what it adds to the diagnosis. I don't know of any limit although one would probably approach it the same as delta-T (compare to other phase).

You say that's a breaker? I haven't seen one that particular shape. I tend to think the hotspot is at the connection rather than the internal contact since the heat shows so much more distinctly at the top than the bottom.

this is a subject in which I have interest. regarding your comment Pete: "I'm not sure what it adds to the diagnosis." I discussed this with my Therm II instructor recently and he was very skeptical, and even said that others who have attempted judging severity by watts have been unsuccessful. I haven't seen any data.

The assumption is that anomalies have a
linear progression to failure..I'm not sure how safe that assumption is, but I'm sure it gets less predictable as temps increase.

I have a 1400 amp bus, with an anomaly which heats the bus for 20 feet on either side of the anomaly. These questions come to mind: How many watts does it take to heat such a mass? How much electrically conductive path is left?

Certainly a 100 watt anomaly on such a circuit doesn't represent a very high percentage of the overall current. So I think in terms of watts used by the anomaly versus entire load.

It is an interesting angle to think about, but until we have a viable technique, I think I will stick with Delta Ts.
Attached is simple spreadsheet that I have been using to fiddle with these calcs.

Watts_calcs.xls (36 Kb, 39 downloads)

If we assume that purpose of IR imaging in a 3 phase system is to detect an anomaly between phases then, if temperature difference has been detected, the mission is accomplished. The only thing which can be a matter of discussion is the delta T treshold which is set as an alarm for specific ranges of load.

There might be also a situation when all 3 phases show no difference although all 3 connections are poor to generate enough heat to bring the temperature above recommended for a specific wire. The temperature may or may not correlate with voltage drop.

But why should we be bothered by voltage drop all together? Isn't preset absolute or difference temperatutre of a connector sufficient and convenient way to make a call? One of the reasons to have a low treshold setting is the fact that if left uncorrected then even a slight problem in connection will deteriorate over time.

David

David,

Where I find voltage drop to be useful is in a situation where the IR camera can't see all of the components and/or all the connections. Say for instance, I can see the 3-phase wires going into a molded case switch, and all 3 coming out of the starter. But what I can't see is the contacts in the molded case switch, the fuses (cause their behind plexiglass), the vacuum conacts in the starter, and all the bolted wire conections between there. So, if I suspect a problem, it's nice to take a voltage drop reading from top of molded case switch thru ALL components, and out of bottom of starter.

I am still curious, what is an acceptable amount of voltage drop?

I can see some value in voltage drop in some circumstances as well. If you are lucky to have access to multiple points, you might be able to narrow down the exact source of the hotspot which may or may not be obvious from the image if the heat spreads out and the emissivities are unknown. For example let's say you have a a lugged connection landed on a terminal on a fuse clip and the image shows heat on the lead and the fuse clip. Check voltages from copper lead (protruding through center of lug barrel) to lug; from lug to terminal; from terminal to fuse clip; from fuse clip to fuse ferrule; from fuse ferrule to opposite fuse ferrule. etc. We expect they should all match pretty closely to the other phases except the one where the problem is. Now you know EXACTLY where the problem is (you might have a pretty good idea looking at the image, but it never hurts to have a confirmation) and have a better idea at what part of your craftsmanship/processes/materials needs to be improved.

Also, assuming we have current measurements as well, the voltage drop tells us the resistance which is a more direct indication of the deviation than the temperature rise. Extreme examples include indirect heating as Bill said.

I am not saying that voltage drop method is useless. This is just another method. It is completely different from IR from the practicality (you know all the dangers) point of view in using it as a PdM tool for surveylance.

A voltage drop measurement is meaningless by itself. Voltage drop is a function of the current flowing across the resistance.

Ohm's Law states that voltage drop is proportional to the current flowing through a circuit.

E=IR
or in your case with I and E known
R=E/I

I believe you should consider your open circuit voltage, which, at the main would be 240 volts.

R=1.1/36=.03 ohms

You can also calculate your total circuit resistance.
R=240/36=6.666... ohms

You can consider a bad connection a resistance in series with the load; a series circuit where:
RT=R1+R2 and
IT=I1=I2 and
ET=E1+E2

Kirchoff's law states that the sum of the voltage drops across a circuit must equal 0.

To further calculate math checking the load resistance with the voltage drop factored in:
240-1.1=238.9 volts
238.9/36=6.636 ohms

Now you have R1 and R2 so:
R1+R2=RT
.03+6.636=6.666 ohms

This confirms the total circuit resistance calculated above.

I suspect the voltmeter probes were not in exactly the right place to measure true voltage drop across the resistance. Your temperature delta between comparible circuits indicates a higher resistance than 0.03 ohms, which I consider a good connection.

As a very general rule, when a connection resistance approaches 0.5 to 1 ohm, I get concerned. But that depends very much on how much current and voltage the circuit carries. A high current (200A)and voltage (480V) connection should be lower than say, 0.3 ohms. A 20 amp 110 volt circuit can have a higher acceptable resisance. Acceptable voltage drop on a connection is very dependant of both current and voltage of a circuit because of Ohm's law proportional statement. On a pushbutton contact carrying 24 volts to a PLC input at a currnet of milliamps, the acceptable resistance might be several ohms- 5 or 6 ohms might be all a perfectly good contact with very small surface area might be capable of.

Remember that your mechancial connection of a large terminator has more surface area and a larger conductor so its resistance will naturally be less than a small terminator and small conductor with less surface area.

I tried to put this down in a message last night and gave up. I had to sleep on how to explain it. I hope this helps. More so, I hope I got my calculations right.

J-

This message has been edited. Last edited by: Wally,

I agree 100% with everything that was said, although I don't have as much experience to call out specific resistance levels.

But I also believe that if we verify the currents are reasonably balanced, then comparison of voltage drop among phases is in itself a useful measurement even without calculating resistance.

Calculating resistance provides a way to compare across a wider spectrum of equipment, provided we recognize the limitations. First and foremost as was said, depends on the size of the connection.

Just for discussion, there are a number of standards involved in design and manufacture of conductors that suggest that for a good new connection, the contact resistance of a connector associated with a conductor of diameter D should be less than the resistance of a section of the same conductor with diameter D and with length also D.

For copper 10AWG wire, diameter is 0.1", resistance is one milliohm per foot, the rule would suggest a contact resistance of 0.1/12 = ~ 8 micro-ohm at 25C

Comparing 16AWG wire to 10AWG wire, the diameter is one half (0.05") and resistance is 4 times (4 milliohm per foot), the rule would suggest a contact resistance for 16AWG wire of 4*0.05/12 ~ 16 micro-ohms.

These are very very low values. If we corrected to a higher temperature the resistance will increase somewhat (around 40% higher at 100C than 25C). The measurement technique will be important in these cases.... for example how close to the probes get to the actual contact. Also those are pristine factory contacts....I'll bet there can be quite a bit of increase before we begin to get concerned. I certainly defer to Jeff's values in terms of what he has seen in the field using field measurement techniques.

This message has been edited. Last edited by: electricpete,

I would say the initial problem was more than likely just a bad connection. However the temperature levels reached does mean that the materials are begining to break down, this includes the internal components as well as the cable. At temperature levels like this you should be looking at replacement rather than repair anyway, so I dont really see what resistance adds to the analysis. Sure it does confirm that internal damage has begun, but you should expect it at this temperature anyway.

I would just replace the breaker and the damaged cable and forget about over analysing it.

Here is another problem we recently had (see attached image). This is a 100 amp disconnect for a 75HP Fan. It's 480V three phase. Current measured 55 amps (left, A-phase), 64 amps (center, B-phase), and 71 amps (right, C-phase). Actual voltage measured was 441 (A-B), 456 (A-C), and 454 (B-C). Voltage drop measured from copper wire going into the lug at the top of the disconnect, to copper wire coming out of the lug at the bottom of the disconnect (to take into account ALL connections within the disconnect) was .113 VAC (A-phase), .587 VAC (B-phase), and .233 VAC (C-phase).

As you can see from the image, the B-phase knife connection was 358 degrees F, while the C-phase wasn't far behind at 329 deg F. It had less than half the voltage drop, but was carrying more current. Does this make sense?

This disconnect was just changed out two days ago (thank God), but it ran like this for almost 2 months. The insulation on the wires was burned back a couple inches, and the knives were toast as you'd expect.

Mod_3_Disc.doc (430 Kb, 21 downloads)

Again, at that temperature you should be scheduling replacement ASAP anyway. The resistance add little to your overall conclusion.

Resistance measurements are valuable, in the right circumstances, but do not overuse them.

Perhaps, but the original question was about voltage drop. I think someone with the desire to gain knowledge is worth an explanation. Voltage drop is misundersood enough in how it can be applied.

Bill, that voltage imbalance at the supply side of your disconnect raises my concern. How far back in the system does the imbalance go? The supply voltage imbalance would cause phase current imbalance in the motor.

Wally,
That kind of voltage imbalance is not uncommon here. We have 10 large single phase induction furnaces for melting iron that are 4.5 MW each. As they cycle up and down (in 15 minute cycles) they screw up not only our voltage, but half of northeast Wisconsin's. In fact a few years ago, Wisconsin Public Service put a bunch of power factor correction capacitors (?) in our substation and it helped everyone else up and down the line from us, but our plant still has a lot of issues. On non-production weekends its common to have voltages pushing 500 VAC, and then be down in the 430's or even 420's during full production and the heat of summer. And of course when the voltage goes down the current goes up....

Bob,
I see a lot of stuff that is that hot. It all gets reported and scheduled for replacement ASAP, but we don't always have the components to replace them with. So then what, punt?

In this industry we often suffer from analysis paralysis, we simply cannot move forward as we are spending too much time analysing things.

Like a delta T a resistance measurement is not much use without direct comparison or historical data. When components reach very high temperatures like these two components, they will almost certainly begin to fail. I always recommend replacement on compoinents that have reached over 100C, as very few normal items are rated for this high temperature. As deterioration begins the components will usually begin to anneal and its resistance will increase, as will its temperature. The annealing process can occur below 80C (for this reason I normally recommend replacement of cables above this level), and as the resistance increases, so will the temperature, and there will be more annealing. So there is a snowballing effect that can grow very quickly. If you already know that your components have reached a very high temperature, then you can assume that annealing has taken place already and therefore they should be replaced. Doing resistance measurements without a comparison or historical data will add nothing to the analysis, sure it helps with your knowledge, but not much else. If you do not have replacement parts then you are indeed in some difficulties. I personally would never rely on those measurements to make a decision to leave a component in service that I know has reached a very high temperature level and has begun to deteriorate. I suggest you consider replacing the component with something else, that is available.

I know we always want to know more about what we are looking at, but sometimes in our quest for more knowledge we forget to ask ourselves if that information is relevant.

I was reading and saw something that reminded me of this thread. I'm not saying it's right or wrong, just a data point. Take it for what it's worth.

From "Electrical Contacts : Fundamentals, Applications and Technology" by Milenko Braunovic´, Valery V. Konchits, and Nikolai K. Myshkin (2006, Taylor and Francis)ISBN:9781574447279

quote:

7.2.3.2 Corrosion
Many power connectors in service are subjected to relatively severe outdoor environments. As a result, corrosion is recognized as one of the most significant reliability concerns. The corrosion problem is particularly severe in some areas, since connections must often be made between dissimilar metals and in particular, aluminum to copper. In recognition of the long-term effects of a marine/coastal, subtropical environment on the performance of power connector systems, a program aimed at comparative evaluation of compression, bolted, and fired wedge connector systems was conducted by Tyco in cooperation with Battelle. The test samples included compression, bolted, and fired wedge types of overhead power connectors. These connectors are schematically illustrated in Figure 7.19. The sample size was 50 connectors of each type. The connectors were installed at the site using the tools and practices recommended by the respective manufacturers. Prior to the assembly of each connector, the surfaces of all conductors were wire brushed. Separate wire brushes were used for the copper and aluminum conductors. After brushing, the connections were typically made within a period of 3–5 min. Specimens corresponding to a given connector type were connected in series on a single line, so that three separate lines were assembled and connected in a continuous series loop. A connector was deemed to have failed when the resistance between the two corresponding equalizers increased by 1000 microohms above its initial value. This represents 4–5 times the typical initial resistance value across the connectors. This criterion represented a threshold beyond which connector failure was typically rapid and catastrophic, for all connector types.

I guess you might read this to imply that a voltage drop 4-5 times greater than a similarly loaded sister connection means the connection is near failure... at least if the degradation mechanism is corrosion.

Seems like a pretty broad and bold generalization to me, though. (I'm a little skeptical)

Another quote from the same reference:

quote:

The advantages and superiority of resistance measurements over IR thermography in detecting the signs of early deterioration in connections is clearly shown in Table 11.4, which is based on the results reported by Snell and Renowden.

I am lumping comparative resistance measurements and comparative voltage drop measurements together... even though one is energized and one not energized, they can give you very similar information (neglecting change in contact resistance with temperature). This author seems to lump them together as well.

I'm not going to copy the whole table.... you'll have to get the book or else get the reference which is cited as the source for the table: "Estimates of likelihood of detection by IR thermography", IEEE ESMO 2000 paper 28C-TPC 17 by Snell J. and Renowden J

Maybe John Snell will weigh on this thread, since he is a contributor here.

It does make some sense that comparative resistance (or related parameter comparative voltage drop) is more directly tied into what's going on in the contact than comparative temprature rise and less influenced by loading and heat transfer considerations. But most of us don't have as much experience with voltage drops and there are not as many standards to reach for. Then again, those temperature rise standards imo only provide a false appearance that all connections can be neatly/easily categorized by one or two simple rules.