Panel cover temperature

twm22

Member
I have a client who has implemented a program that measures panel cover surface temperatures using simple infrared temperature detectors. They are asking for my input and I have played down the effectiveness of the program, only because I believe they are concluding that what's going on inside the panels is OK based on the external panel cover readings. To re-emphasize, they are NOT using thermal cameras to detect bus hotspots. They are using simple surface-reflect thermometers.

The simplest argument against this program is that the panel covers are 90F when the shop ambient temp is 90F, and 95F when the shop ambient temp is 95F, but there are supporters of the program that believe that we will catch the instances where a panel cover is at a runaway high temperature. One might say, what the heck, do the measurements, it can't hurt, but that's not really the case. The temperature measurements are taking time and creating a sense of security that panel hotspots will be caught.

Would anyone be willing to comment? Thanks
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
You are right that the program is probably giving a false sense of security. That said, there are multiple issues to consider in classified locations and temperature is only one of them.

Section 500.8 reviews those "multiple issues" and a thorough understanding of the whole Section may offer a resolution. Subsections 500.8(C)(4) and (5) and 500.8(D) deal specifically with temperature considerations.
 

Chamuit

Senior Member
Location
Texas
I agree with your train of thought. I would ask, where are you going to take the temperature and why? Arcing events can come and go along with the heat they produce.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
Temps of outside of cover is worthless. They need to open the covers, leave the deadfront on, and read the temps of each individual breaker.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Temps of outside of cover is worthless. They need to open the covers, leave the deadfront on, and read the temps of each individual breaker.
Actually, depending on the type of equipment, opening the covers could initiate an event if the internal tempertures exceeded those of Tables 500.8(C) or 500.8(D)(2). Using proper Protection Techniques [Section 500.7] eliminates the need for the program described in OP.
 

paulengr

Senior Member
First off the comments on bus bars are about as silly as most thermography claims. A general design philosophy for switchgear and MCCs is no exposed bus. Busbars are in a separate generally inaccessible compartment with bolted covers. The key area (bus connections) are not accessible. In fact the bolted connections though important are far more accessible than stab connections which are always behind panels. Thus IR thermography at best picks up failures indirectly anyways in these areas.

In terms of component issues it is far more successful there but generally we are looking for temperature differences of a few degrees (2-5) at most concentrated in a small area. This is along conductors where the source is much hotter. It dissipates within the cooling air within the enclosure and there is no way to see it at the outer door unless the issue is severe.

As to setting off an arc flash opening a door show me one OSHA documented case. Closing sure i can buy that but after combing 10 years of OSHA investigations looking at arc flashes, there was a single case of disturbing a defective elbow connector in a trench but nothing else. Failures on closing or insertion are another matter entirely.

So if I’m reading this right, it’s some kind of electrical gear in a hazardous location. Motors have a T code and a special design to maintain a maximum surface temperature even if a fault occurs internally using a flame path (labrynth) for venting. General enclosures do not have a T code but the general design concept is to prevent a flame from within the box from propagating. This is the purpose of the conduit seals, NEMA enclosure requirements, etc. But there is no way to control any of this once the door is opened because the enclosure integrity is compromised.
 

twm22

Member
First off the comments on bus bars are about as silly as most thermography claims.
Hi paulengr - Your first line was of great interest to me. It seems like you don't have high regard for thermographic testing. I'm going to take this thread to a more appropriate part of the forum and pick your brain on it. If thermo testing isn't all it's cracked up to be, I'd love to hear your opinion, and would really love to be pointed to any documentation. I see too many clients spending thousands of dollars on it
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hi paulengr - Your first line was of great interest to me. It seems like you don't have high regard for thermographic testing. I'm going to take this thread to a more appropriate part of the forum and pick your brain on it. If thermo testing isn't all it's cracked up to be, I'd love to hear your opinion, and would really love to be pointed to any documentation. I see too many clients spending thousands of dollars on it
I don't think there is anything "wrong" with the basic idea of thermal testing. The problem I see is that it is a nuisance to do right so people do it wrong and basically just waste the money spent doing it.

I think down the road we will see switchboards, panelboards, MCCs and the like with thermal sensors embedded inside so the scanning can be done on a continual basis and easily correlated to load and ambient temperature.

These days the cost of doing the scanning is not really worth it for what little good it does in most cases.
 

paulengr

Senior Member
Hi paulengr - Your first line was of great interest to me. It seems like you don't have high regard for thermographic testing. I'm going to take this thread to a more appropriate part of the forum and pick your brain on it. If thermo testing isn't all it's cracked up to be, I'd love to hear your opinion, and would really love to be pointed to any documentation. I see too many clients spending thousands of dollars on it
It’s not that it doesn’t work, far from it.

To begin at the joint bolt pressure and joint compound and sanding if used crack or remove the oxide layer forming alpha spots. Higher pressure creates more and mushes the metal of already formed spots. These are cold welds. Even as preload releases you have to almost totally lose contact pressure to get any appreciable increase in resistance. If you have access to one try it. Bike together two bus bars then loosen the fasteners while watching a micro ohmmeter.

Basic fastener theory shows why if the initial installation is done correctly the joint is good for life. There are always signs of corrosion, stripped threads, and other obvious issues with failing joints. Those are just as obvious on initial installation. And if you find one there are always more. It’s a static load. Fasteners self loosen via the Jost effect. Those conditions do not exist on bus bar connections and rarely even in mechanical power transmission. Google it and Junkers machines.

Second issue is the 800 lb. gorilla in the room is constriction resistance. Going back to alpha spots less than 10% of the total joint area is actually in contact. At low currents this is not an issue. So if I have a 1” piece of busbar and I put a 7/8” notch in it how much heat do I get with 10 A? None. If I go to 1000 A how hot will that notch get? That’s constriction resistance. The joint is only going to overheat when you need it most under near capacity loads. An IR rule of thumb is 25% of capacity. With a typical MCC spec of 800 A horizontal 300 A vertical are you really going to get 200+ A on the horizontal splice plates? I can easily rig a breaker tester to produce 800 A of current and do a millivolt drop test in the shop but it’s not practical. But overall it does pick up early on issues.

Third issue is design. Once upon a time we built outdoor overhead substations. We used large open frame knife switches. Protection was mostly a lot of chain link fences and danger signs. Those days are long gone. Now we build equipment where the controls, switching equipment, and bus are in physically separate compartments. The only energized access is controls. Access to components requires tools and is interlocked so that it often can’t be accessed energized. Even in de energized testing often I have to disassemble key interlocks. Bus compartments are bolted and impossible to access if the panels are back to back or against a wall. Contact fingers are always obscured by the equipment. So as a consequence direct measurement is usually only practical in uncommon cases. You can add windows but these are limited. It takes multiple large windows to be effective, if it is even possible.

There are three methods of detecting loose connections typically mentioned. The first is torque testing. Every fastener engineering book will clearly state not only the fallacies of this method but why it’s a terrible idea. Within even minutes after tightening a bolt the bolt relaxes and preload drops. Reloading overstretches the bolt. And 75% of torque is overcoming thread friction. Dirty fasteners, improper wrenching techniques, improper installation (flat washers between bus bars...don’t laugh I’ve seen it) stripped or galled threads. The issues are there on day one.

Micro ohm readings measure DC resistance using up to about 10 A on a DLRO or Ductor. So it directly measures resistance but since we need constriction resistance and alpha spots tend to remain until right at failure it doesn’t change.

Threw other methods not often used or mentioned are fiber point sensors, distributed fiber measurement, and millivolt drop. Fiber point sensors run several hundred dollars each. They are rugged and have no issues with voltage limits. The downside is the overall cost. Distributed temperature uses nonlinear properties of single mode fiber plus time of flight to measure temperature at any given distance on a single fiber but the technology costs hundreds of thousands. Millivolt drop just measures the voltage across the joint. It is very simple and limited only by access and safety issues. It has kind of fallen out of favor.

So coming full circle IR scans of bus are looking for an issue better served by other methods. It is the exception not the rule that the bus is visible. Fortunately copper is so thermally conductive issues can be detected indirectly: But the issue stems from initial installation issues. Self loosening is a myth, doubly so in electrical pressure contacts where alpha spots cause hysteresis even if loosening occurs. IR scans so frequently detect loss of spring force in fuse clips and breakers, overheated bearings and motors, worn contact tips, and many other issues compared to rare bus bar incidents.

IEEE 493 gives fm failure rate data on disconnects of 10E-12 and similar data exists to the point that IEC 61511 for process safety systems and similar standards simply ignore bus and wiring failures. While I believe they exist human caused failures dwarf any age related issues.

Going further I reviewed every arc flash incident in the investigation database OSHA maintains from 2009 to 2014. There was a single incident though it supports my point. At a parking deck an electrician covered up a lighting panel opening by sandwiching loose metal between the covers. When an attendant later operated a breaker the metal fell into the bus initiating an arc flash. So bus was involved though not loose bus and poor workmanship was the cause.

Thus it is far more likely that issues will b found with contact resistance tests. They are just as effective on spot checks on say annual PMs. By nature you get a visual inspection for free: They can be trended. They are done safely offline. They are not affected by load. I am a strong believer in IR just not specifically for bus.

So I think it’s tilting at windmills. It’s not really the best way to detect the concern and the concern is not very likely. As to the OP in a division 1 area energized work is definitely a bad idea. I’m really not worried about equipment temperatures internally but the fact that human error rates are vastly higher. Taking a cue from two large LNG storage facilities in my area locate all starters and switchgear outside the division 1 and preferably 2 areas. Purge and pressurize instrument panels. Open only after sniffing and only if necessary when there is no sign of gas present. At that point you can have an overheating contact but it doesn’t ever become a hazard. Infrared cameras for this are a solution looking for a problem.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
For what it’s worth I do IR a lot and I agree with the comments here.
But in a utility setting they are exceptional. I have shot some IR in mills that have problems, more as a courtesy than anything.

I have found bad bearings on motors, loose connections, and other small items that were installation errors.

I can’t get to all the spots in enclosed switchgear, nor do I intend to.
And as stated earlier, panel temperature is worthless. If the actual connection point isn’t visible with load, your wasting your time and money.
I can shoot a group of panels and see one warmer than the other ones, but all it means is the bus is behind that panel. Doesn’t mean there is a problem.
Panels with controls are cooler, as they should be.
 
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