A MIDWEST Thermographer found an interesting problem with a 208 volt, 3 phase, 75 kva dry type general purpose transformer. The Infrared Scan showed the enclosure of a Square D transformer was much warmer on one side compared to the middle and other side. It was an older transformer and had been in service for many years. An enclosure must re-emit enough heat from the air cooled transformer and the difference in heat from one area of the transformer to another must be substantial enough for the Thermographer to see the difference in heat pattern. But when you’ve scanned 1000s of these dry type transformers over 15 years, you have a pretty good idea of what’s normal or not. A load check revealed one phase was at 220 amps, another at about 115 amps, and the third phase 90 amps. They definitely had a large load imbalance on their old transformer. The maintenance man said they had been having nuisance tripping on the output breaker, but when he checked the load, there was less than 100 amps, 50% load. Unfortunately he only checked the load on one phase and got unlucky enough to pick the lightest loaded phase. But it was also very easy for our Thermographer to see the load imbalance on the circuit breakers on the load and line side of the transformer. It is amazing how tough these old air cooled transformers are. 10 kva to 500 kva, general purpose dry type transformers are found everywhere. Some are real dinosaurs, beyond old, but not obsolete because they are still running. Some over 70 years. A couple other things about these really old dry type transformers. They were so overbuilt that they can be very forgiving of overloading and they can be really loud.
Here is another MIDWEST transformer horror story. This involves a 1000 kva indoor dry type power transformer, 13.2 Kv to 120/208 volts. The first time we saw this transformer, we thought it was just a spare new transformer core and coil being stored in one of the customer’s equipment rooms. It looked just like a replacement transformer because the core and coil were not in a metal enclosure. It was out in the wide open area of the room with no protective enclosure or barriers, nothing to protect it. To our great surprise, we soon realized that this old transformer was energized. The hmmm and the exposed wires connected to it were a big tip. One had to walk around the transformer to get to one corner of the room. Beside electrical conduits, there were other overhead pipes in the room, plus communication cables. There were electrical panels and non electrical equipment in the room. The electrician said he was told it was okay because it was a locked door and nobody was suppose to go in there. He told his boss that he thought it was very dangerous, but he was told it met code. He thought the whole thing was nuts. We asked where the key was secured so that unqualified personnel could not enter the room, which they called a vault. The key was hung on a nail over a nearby doorway going into a mechanical equipment room. This was considered secure because nobody knew what the key was for, except the ‘right’ people. We explained the extreme shock hazard and arc flash hazard to anyone, even qualified personnel, entering the room and the fact that unqualified personnel may acquire access to the room because of the key location. In plain English, this set up was crazy. To this day, nothing may have changed, because someone decreed “it met code.”