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What to Inspect in Electrical Substations and Lines

While many know how critical it is to identify heat-related electrical problems prior to failure, too often they don’t inspect some of the most important equipment in a substation. That is what I’d like to cover this week: what to inspect and what might you find.

This OCB bushing shows a classic signature associated with the connection in the head. Repairs made before failure will have a huge return.


Typically we focus on the most critical equipment in a substation, equipment that, if it fails, will result in a wholesale outage. Transformers not only fit into this group but also are very susceptible to heat-related failure. Remember to inspect all the critical parts of the transformer:

  • Bushings: look for heat in the external connections as well as the more difficult to see internal connections in the bushing head and—as seen subtly at the base of the bushing—connections to the coils
  • Surge protection: look at segmented signatures with small rises in temperature as indicators of serious problems
  • Cooling systems: blockage or low oil in radiators will show up as cool tubes. Also, look at the fans after they have operated for 15 minutes or more. Failures in the cooling system are very costly because they rapidly result in damage to the transformer.
  • Tap changer tanks: normally these run at or below the temperature of the transformer. If a bad tap is carrying load, it will heat the changer tank but, beware, it may not be energized when you are inspecting!

Four of the six primary contact point on this set of disconnects are showing abnormal heating.

Oil-filled circuit breakers (OCB) and voltage regulators are also critical to operations and can suffer heat-related failures similar to transformers.

Too often various disconnects and switches are neglected during an inspection. While their failure may not seem crucial, if a circuit needs to be opened and a disconnect is welded closed, there will be a serious problem! This equipment is very susceptible to damage because the localized heating in the jaw or latch end typically damages the metal and the trend to failure grows dramatically more rapid. All these devices depend on the spring tension of the metal to maintain viability of the electrical contact, and at temperatures as low as 200oF (93oC), annealing begins to occur and performance degrades.

Take your time, working with good conditions, to inspect all the equipment in your substation. You reap big returns on your investment and get through the peak season with improved reliability.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

5 comments to What to Inspect in Electrical Substations and Lines

  • we always use the Koten brand of circuit breakers at home and they are good’`:

  • our circuit breaker is always manufactured by General Electric and they last very long,~.

  • dustin

    thanks John, good info!

  • Aru

    1. When we inspect outdoor electrical substation, does it matter when you carry out the inspections? If we carry out the inspection in day time, will solar radiation affect my findings in a hot day? Is it advisable to carry out the inspection at night? Please suggest!!

    2. Also should I set the relative humidity in the camera as found in weather website?

    Thanks in advance for your reply!

  • Fluke Thermography

    1. Yes. From my experience, it usually does have an impact on the quality of your inspection work. The conditions can be very important to whether or not you see an issue or you do not see an issue. The sun, and it’s enormous infrared energy, can completely mask the thermal signatures of equipment. Inspecting on an overcast day, or before the sun has some up is always preferred, if possible. Otherwise, just be aware of the potential issues caused by solar loading. Also, avoid conditions where there is significant wind or breezes. This too, can mask potential problems. Convective cooling and convective heating can greatly affect not only the temperature measurement calculations, but the infrared image itself. Precipitation, such as rain, snow, or ice, can also have a negative effect on your thermal imaging work. It is best to conduct inspections when it is dry and calm. If you cannot properly inspect equipment because of any of these factors, make sure that you note the conditions during the inspection. As a matter of best practice, it is a good idea to record date, time, ambient air temperature, weather conditions, wind speed and wind direction (if any). Know the limitations of the technology, and the limitations that “Mother Nature” imposes upon you.

    2. Typically, the relative humidity setting that is offered by some camera manufacturers is not really all that important for most inspection work… unless you are inspecting in the middle of the dense fog… or you are inspecting from a VERY long distance. It is usually not even worthwhile to spend much time with this setting. Having proper focus is far more important to image quality and temperature measurement calculation than relative humidity.
    The important thing that you must remember about most portable infrared cameras (thermal imagers) is that they are primarily designed for “qualitative” inspection work… work that allows you to see what is different and unexpected… so that you can document and investigate further. Doing “quantitative” analysis… analysis that involves the actual temperatures is something that often takes a significant amount of more time, care, and effort, with very little difference in action. The thermal imagers do not “measure” temperature. They measure apparent surface infrared energy coming off of an object and then “calculate” an apparent temperature. For a variety of reasons, what the imager tells you may not be the “actual temperature” of the surface of the object… and most certainly will not usually be the temperature on the inside of many objects.
    Emissivity, reflected background temperature, and transmissivity are all factors in getting “more accurate” temperature measurement calculations. Unfortunately, it is often very difficult to know these exactly… regardless of what is printed on a piece of paper, the internet, or even in a look-up table within the imager.
    My advice is this: look for the thermal differences that indicate a potential problem. Use the temperature measurement calculations as one possible gauge of severity (according to how the equipment is supposed to operate), and then plan accordingly to either monitor or schedule further inspection and repair of the issues found.
    Questions such as these are important to know the answers to. I always strongly recommend proper formal training for those individuals who do critical inspection work, or who will be doing this work on a regular basis. The Snell Group (www.thesnellgroup.com) is an internationally recognized thermography training organization, with significant experience in both training and field work. They offer infrared thermography training that is surpassed by none in the marketplace. They are independent of any manufacturer, and are primarily concerned with making sure that the students who take their courses are “qualified” to do infrared inspection work… regardless of the make or model of imager that you are using. The don’t just want to give you a piece of paper that says you paid money and sat in a class. They want to ensure that you are using the infrared camera to its fullest extent, you are using it properly, and that you get a good return on the investment that you have made in both the camera and the training. Training such as this can have a significant positive impact on your ability, and credibility, as an infrared thermographer.

    Hope this helps.
    Fluke Thermography

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