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If you have mild overloading/overheating, especially if the overload protection isn't set up correctly, the highest resistance connections are at the lugs/joints and damage from overheating will start there and then work progressively back up into the cable body. After this occurs the first couple feet or so of insulation will lose it's flexibility and either crack on its own or if you so much as touch it, it will crack and fall apart in your hands. If you catch it in the act, it will be literally smoking but won't actually burst into flames or anything like that. This can occur without seeing any ongoing temperature issues and without discoloration occurring at the lugs and the flaking insulation might be your one and only sign that overload protection isn't set up right.
This doesn't mean you have an ongoing problem. Although all standards and manufacturers specifications say that you need to change the cable and/or remove the obviously defective portions, the reality is that this usually takes a lot of labor to remove and replace cable so it doesn't get done...after all, the insulation is clearly obviously still there. And many times it gets overlooked since it just looks like the black rubber/plastic stuff is on the cable like it always is and cracks on something black are hard to see unless you are looking for it. I don't condone the practice of leaving it when it is obviously defective but I definitely understand the reasons that this becomes a problem that may take years to address even if maintenance departments know about it and are truly committed to trying to do something about it. As a field service engineer I'll point it out and document it which often gives the maintenance department ammunition to try to justify replacement but often the fact is that it's hard to convince someone that it is important. They don't recognize how much cables will actually jump during a fault and that all that insulation will fling off as the cable suddenly flexes.
It's also definitely possible to find corrosion or similar damage occurring from other sources but the difference is usually obvious because you'll see a whole lot more than just damaged cable if it's a corrosion issue due to corrosive atmospheres.
As to the moisture problem...first off the anti-condensate heater idea works well IF you have a reliable source of power and if you are going to PM the heaters once in a while. There are two versions of these. One is a strip heater that is literally exactly what it sounds like and constantly puts out heat. A local motor vendor should easily be able to put you onto a source because they use them for customers that request them in motors, especially medium voltage motors. Second type are the ones that have a small thermostat that turns on and off as required and might even have a small circulation fan depending on the power output. Pretty much any MCC or switchgear catalog will list these in the "accessories" section. Obviously the reliability of a device that has more than just a strip of metal will be less...so along with the strip heater you will be committing yourself to regular PM's to check them. In today's paranoid world of so much as opening a cabinet, you can see the obvious issue. Many plants get along just fine with a simple small DIN rail circuit breaker and the previously mentioned "dumb" strip heaters, turning them on in Autumn and turning them off again in the Spring..
The second moisture solution is to install an HVAC unit on the electrical room. This has the major advantage that not only does it control temperature but it also dries everything out by removing moisture from the air and keeping it warm in winter (prevents contacts from freezing up) and cool in the summer (prevents overheating issues). But then again if the room is not really built for it (insulated, relatively well sealed) or the equipment is outdoors, this might not be practical. Next I know this is going to sound like an episode of Mike Holmes (and I got the idea before his show) but in practice I've converted at least one room by purchasing a couple buckets of isocyanate spray foam insulation and spraying all the walls and ceiling with it. This works a lot better than ratrofitting fiberglass or rockwool batting and as a bonus, it forms an air tight seal on all the walls over all the holes. It does come in a "closed cell foam" version which means that even if you get future leaks, unlike the fiberglass and rockwool it won't absorb moisture and mold or fall apart, and you can also get fire retardant versions.
Third issue is to make sure to keep everything white glove clean as possible. This isn't always all that practical. Most of my experience comes from working in foundries, steel mills, chemical plants, wood plants, and mines. Cleanliness of the type required for medium voltage equipment standards in particular (clean, dry, cool...yeah right) is an exercise in futility and you might as well accept the fact that routine cleaning with vacuums and rags is simply part of the business. But going away from the extreme cases, google the words "dry banding". Moisture by itself is NOT a cause of electrical failures (the old adage water and electricity don't mix). If that were the case, then every power line in the world would be destroyed by rain storms. Contamination (dirt) by itself usually does not cause failures either unless the equipment is so buried that it becomes thermally insulated (back to the cable issue). BUT when you mix water and dirt, you get a highly conductive film that especially in medium voltage switchgear will naturally seek out the weakest insulation and blow your equipment to pieces.
This still might now sound helpful but as an example I recently rebuild some medium voltage switchgear that came from a paper pulp mill. The outside looked terrible and the normal doors that are used to access the components wasn't a whole lot better when I started. This particular design had a "3 compartment" design...there was the usual low voltage controls section, then the component section, and then the bus bars were all mounted in a separate cabinet that was sealed by bolted panels from the back that depending on how it was installed, might have never been opened. After probably 20 years of service based on the age of the equipment, there was hardly even enough dust in that back busbar compartment to even wipe down the bus bars and it looked like the original manufacturer had even used a piece of kiln dried wood instead of insulators for what looks like some kind of expansion/contraction reasons.
Finally the last solution with regard to condensation is to live with it. If you keep the walls primed and painted and use an epoxy paint if you have to make repairs, you won't have rust or other contaminants. As soon as you see surface rust, get in there and totally remove it (maybe even consider one of those rust neutralizer products) and then paint back over it with an expensive ($60-$100/gallon) epoxy paint to not only provide a rust free surface but some of the paints impart a degree of hydrophobicity (water hating) properties with cause it to bead up and come off. If you have medium voltage switchgear, I've found that you need roughly 6-8" tall insulators instead of the usual 3-4" tall that you get on custom equipment to prevent tracking from just condensate and dust in mining operations. Two sources for epoxy insulators that I've used are Reuel in Goldsboro, NC, and Federal Pacific in Bristol, TN. Both have insulator shops that can basically make any shape you want and Reuel has a great catalog online. Most of the time if you are buying them from the big names (Square D, ABB, etc.) it is really coming from one of these smaller shops. This is also a time to seriously rethink busbars. The major advantage of busbars is that when you can use them (engineered routes in cabinetry), they are a lot cheaper and the current ratings are usually much higher than what you can do with cabled connections when you are designing and building equipment...and I've even had local machine shops build equipment from my designs with bus bars for space reasons. But cables have fewer problems with condensation when designed and installed correctly...not a problem for low voltage but with medium voltage, tracking and voltage stress always have to e considered. Vertical up and horizontal busbars are generally not a problem but when you have busbars running in a horizontal direction that are oriented vertically (one on top of another) the insulators seem to be more prone to contamination from rust and condensation. That being said of course typical cabinet layout is either that the busbars running from end to end are either along the floor/roof or along the back wall, and the back wall orientation is the one to watch out for.
Also I've found in practice for medium voltage equipment in mines where I couldn't do a whole lot about contamination or moisture that the Federal Pacific Auto-Jet disconnects have a lot more distance to them and the shape tends to avoid shorting out for whatever reason so I've had a lot fewer condensation-related failures. I've had pretty regular failures in very moist and cool summers with ABB, Toshiba, and Powercon disconnects and pretty much anyone else that sells a similar brand name. I've even gone so far as to switch back from enclosure-mointed main disconnects in the 25 kV class back to pole mounted gang-operated switches or fused cutouts mounted outdoors on a steel frame because even if you have a lot of heavy truck traffic kicking up dust, mud, etc., it tends to hold up and it's self cleaning whenever a rain storm comes by. This is an extreme case of where the trend towards enclosed equipment is actually a bad idea and I've gone back towards "open" overhead style equipment.
Some of the mining equipment I've looked at has small vents on it and there are some anti-condensate vents that you can buy from the usual suspect for enclosures (Hoffman). The idea sounds OK but in practice unless you go all the way to buying desiccant dryer breathers and routinely replacing the silicone beads in them as it gets used up (again...maintenance issue), just a simple vent has almost no impact on condensation whatsoever. Also stainless steel NEMA 4X stuff is actually a bad idea when it comes to condensation...it is actually worse for the reason for the vent things. No matter how creative you try to be with all the little vent things unless you are using a chiller, air conditioner, or dessicant dryer, or it is hermetically sealed, sooner or later the inside of the enclosure is going to be the same atmosphere as the outside. You are much better off (and it doesn't hurt the budget so bad) going with a NEMA 12 enclosure and simply siliconing the holes in the bottom if you insist on "NEMA 4X" sealing. Also watch out for solar heat gain. In summer those pretty stainless and aluminum panels get so hot that they can easily exceed the temperature rating of the equipment all by themselves and then if you have a humid day like we do in the United States Southeast, guess what happens at night when the temperature falls and the air density goes way up, especially if you have this now very cool surface? In practice I've tried to design NEMA 4X installations with "lean to's", "overhangs", and other similar shading from summer sun. It makes a major difference not only in overheating issues (back to the cable damage) but also in actually reducing the amount of condensation.
And with regard to medium voltage equipment, solidly insulated epoxy equipment, rubber coated, and SF6 insulated switchgear is totally sealed against these kinds of problems. What you want to look for is the stuff that is purposely designed to work in underground vaults. It can actually work while submerged for an indefinite period of time and once you are already buying equipment for the 15 kV and 35 kV classes, the overall cost actually stays about the same because with this kind of equipment you can just mount it on a wall and forgo any need for any kind of a "building" more than maybe some kind of roof to get it a little sunlight protection. I know of at least one marine base that uses the S&C SF6 sealed interrupter type switchgear and many utilities that use the Elastimold version. In particular the city of Charleston, SC, uses the Elastimold stuff and due to the fact that the whole city is only a couple feet above the water table, all of their vaults are flooded except when they pump them down for maintenance and they've been using the stuff for decades, and it's now the standard across the board for Duke (largest utility in the world) Power for their underground installations. GIS gear has always been available for quite some time and similarly has a design where you basically install it and then go in an inspect/grease the (external) linkages once every 5-10 years and test it to make sure that it is still functioning correctly, and that's it for the full 40+ year life. Think about what this is saying...doing preventative maintenance maybe 3 or 4 times over the entire life of the equipment. That's a far cry from the annual stuff that we did in the past on metal clad draw-out style switchgear although in some ways it's really just proof that you really can do almost no maintenance on electrical equipment but only if you are willing to pay for it. SF6 insulated vacuum gear is really, really expensive and the underground switchgear stuff is a practical alternative.
Anyways...maybe just to give you some ideas to think about. All my experience in the past 20+ years has been in jobs where I work as "chief troubleshooter" about 50-95% of the time and the rest of the time I'm doing project engineering and other activities that other engineers do. Right now I'm in a field service group that covers NC, VA, SC, and sometimes beyond those areas. I go to a lot of customer's plants where the staff maintenance and engineering has given up on trying to get it fixed so I don't get to work on the "simple" stuff very often. Most of my work experience has been working in humid environments. Often the office is the pickup truck and I rarely get to work where it's clean, dry, and temperature controlled. I'm very knowledgeable in most of the Codes out there, but as Mike Holmes says, "minimum Code specification" is just that...minimum. My methods might be a little unorthodox but come from a world of field testing. If you're in the NC/VA/SC area and want to get into specifics of your situation, PM me. General advice is free of course.
Also if you are in a "H2S industry" (wood plants, waste water plants, oil refineries, phosphate mines, sulfide mineral mines, etc.), be aware of "silver whiskers" (google it) and silver sulfide corrosion. This is NOT caused by condensation. It only takes about 0.1 ppm to damage silver plated equipment, and almost all the manufacturers these days use silver coated bus bars and components unless you specify otherwise (and you can't get away from silver contact tips in many cases), and no thanks to idiot regulators in Europe now all the electronics comes with silver solder instead of lead-tin. It is a major problem in these industries and unlike everything else in the plant just because you can't smell it doesn't mean that the levels aren't unacceptably high as far as electrical equipment goes. Similar to condensation...usually you have to live with it and simply be aware that this is a major problem that you can't easily get away from. Newer equipment is much more susceptible thanks to the idiots that came up with RoHS, so don't be surprised to see failures in new equipment compared to old.
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