I took some thermographs a week ago of a fuse block with a thermal anomaly on "A" phase. Because of some additional questions we went back today to take follow up shots. Now both A phase and B phase are hot.

This is a 10 HP VFD driven AHU, supporting a highly critical area "Must not go down"

I have never seen an anomaly change like this so quickly, particularly jumping to another phase. It is not a load issue, it is specific to the fuse block.

I find this alarming, but I'm having trouble accounting for the change. Anyway, the customer wants to limp into shutdown 3 months from now, I have to make the call. I'll be monitoring it very closely to try to understand what is happening, any suggestions would be helpful.

Obviously "meltdown" is a possibility but I would expect higher peak temps in that case. I don't think it is that extreme... I could be wrong.

Shifting_Anomaly.doc (216 Kb, 108 downloads)

Also:
number 10 wire, fuse block is 30 max, load is 10.5-11.5 amps, well below what these components can carry.

That's an interesting one. Just out of curiosity, do you have a visual image? Are both images on the same temperature scale? Is this on the input or output of the vfd? What's going on with that blur just below A and B phases on the 2nd (rightmost) image?

Have you checked harmonic loads? What kind of temperatures are you getting? Clearly, until you understand the nature of the problem, it would be wise to increase the frequency of inspection and do additional testing. Checking the voltage drop across all parts of the circuit would be very useful and indicative.

1st Loads:
A 10.6 B 10 C 11.5

1st Voltage drops: A .4 B .4 C .3

The blur:The right most image is a close up, and the depth of the Fuse block (4-5") precludes being able to focus on all areas at the same time. If you are asking "why is there heat coming from there?" That is the problem. The construction of this fuse block is such that we don't know what is between the terminal of the wire and the fuse block clip. It is some unseen 4" long conducting element, making this an "indirect thermograph" and thus raising the risk of assesment.

John: We are going to daily infrared monitoring until we understand why this is changing. We are going to be monitoring loads to check for variations. It has turned out that the criticality of this unit is extreme, I don't know if they will even let me bring it down if it comes to that.
Re: harmonic loads : would not all 3 phases be affected, and would not the entire length of the conductor be affected, if not why?

We always take Voltage drops and loads for any anomaly found.

Pete: If you go in the doc, and click on the right hand pic and reduce its size, you can see the other scale.
I believe it is on the output of the VFD.
The peak temps are 56 C, but it is 25C ver "C" phase, which is a significant differential.

I've been trying to consider whether doing a live motor circuit analysis on this unit is worth doing. Load variations due to motor winding problems should affect the entire conductor, not just the fuse block.

Martin,

Is is possible that a wire was tugged as you were taking load readings? It is very strange to me that the center of the block is warmer too, perhaps suggesting heat coming from behind.

good point. Interesting to note if you look at the top of the left picture that apparently the current does flow down below the surface or behind a block to get to the top terminations.

It is tough to get a handle on where the heat is really coming from in either picture. Although I can pretty well tell where the components are, I am spoiled by having a visual image always available in our reports generated from Mikron Mikronscan 7200V. I see you're using Mikron... yours doesn't have that?

If you'll forgive a question on a tangent - do you find much value from the millivolt drop measurements? We don't do those... they seem to be considered a little too risky by our plant folks, especially for critical equipment. But I would sure like to have that extra information. In this fuseblock it looks like you might have 4 or 5 possible places to contact each phase. In coming lug (If you can get around behind) to incoming terminal. Incoming terminal to top fuseclip. Top fuseclip to top ferrule. Top ferrule to bottom ferrule. Bottom ferrule to bottom fuse clip. Bottom fuse clip to bottom terminal. Bottom terminal to bottom lug. With that set of measurements on all three phases, there MIGHT be something more significant to jump out. But if I asked for it at our plant, they'd call it R&D, point out the risk, and point out that repair will likely address all those possibilities anyway.

Check out the attached image...
As far as MV measurements are concerned.. the jury is still out on that, I think I need a more precise meter.. I'm a bit skeptical sometimes. We will still do it, but at some point I am going to compile the info that I have to see if it is helpful. My instinct at this point is that it is not adding much value.

John, as I'm sure you know, it is not atypical for a device to be warmer in the center, all else being equal. Of course in this case we are talking about a bit more heat than normal. The attached doc has a better view from the bottom on the infrared, and the visual image was left this time also.

Shifty_Anomaly.doc (168 Kb, 45 downloads)

It is possible that you have a weak spring inside the contactor or bad contacts. I have seen simular to this before and thats what it turned out to be. This can mean that sometimes you get a better connection inside than other times.

Bob:
This is just a fuse block, I don't believe there are any springs inside, but I have been wrong before.

OK Correction: It is not just a fuse block, in the back it has a disconnect, so the spring theory is in play. Temperature read 85C yesterday.

Martin,

Given that you are probably dealing with a large thermal gradient, i.e. the difference between what you see/measure on the surface and the probable, internal heat source, I would urge you to be very cautious. Not fully understanding how the component is constructed suggests the same.

If possible to do safely, can you blow air over the component? This will not fix the problem but will help you manage it and reduce the chance of what appears may quickly become a "runaway" condition. A barricade and a fan would probably go a long way toward keeping things cool until you can get into it.

cooling with a fan is a good idea as a temporary measure. Make sure and position the fan at a distance that it will not fall inside the cabinate if it does happen to fall by accident.

We have had higly variable ambient temperatures lately, We are taking 3 shots today, one early in the cool morning, one midday when the sun is directly in the unit, another late in the afternoon, each with a good ambient reference.

We did the repair, and the answer was this:

There really were 2 Anomalies, one on A phase, the worst one, and one on B ohase. Both anomalies are on the "stabs" from the fuse block into the disconnect switch.

Thanks for "the rest of the story." Given than you now can see the damage that was done, I'd be curious to hear what you think about the surface temperatures you were seeing and the idea of trending the changes?

Trending the changes was good for us because it let us see what a huge impact ambient temperatures had on the peak temperatures. In fact, I'll attach the spreadsheet. There is a trend which basically shows the progression rate of the anomaly minus ambient influence. We ended up taking our thermographs just after sunrise each morning, and that settled down the swings and let us get a better idea of the real rate of change.

3024_comparisons.xls (24 Kb, 21 downloads)

Another follow up note:
In the reports, I had used the nominal "C" phase as a reference point for this wire gauge and load. The peak fuse temperatures were based on the readings from the area of stab bolt at the fuse block.

The post repair "nominal" Shot shows that these fuses are normally a bit warm..They are slow blo fuses 45Cbut certainly not the cool 27 C of the reference "C" phase.

The problem of course was that the heat conducting from the Anomaly areas affected the temperatrure of all three fuses, so C phase fuse was not a good reference point, but niether was C phase wire, because it was not like for like. Indeed i had to get 1 foot down "C" phase wire before I got to a region unaffected by the heat conducting from the anomaly.
There may also been so emiss. differences between the Wire insulation and the fuse block plastic, although I don't expect that to be much.

Why did peak temps shiftback and forth?? hmm...
Maybe the anomaly was changing condition, maybe the way air was flowing (convection)inside the fuse block.. there were open holes and slots.
It could be something nobody has thought of yet.

Let me preface this with - we only have one vfd on site ...

Can the shift be related to the vfd controller operation?

We have pressurizer heater circuits that are three phase, but operate in a 'single phase' mode during some operations...