Insulation Resistance and Moisture Protection – The First Safety Check

Even if a cartridge heater seems brand new, it might still be harmful to use. Damage that you can see doesn't always mean there is a risk. Sometimes it lurks inside, in the white magnesium oxide powder that keeps the resistance wire from touching the sheath. That powder takes in water from the air. A heater that is kept in a moist warehouse or left on a shelf during the humid summer months can get enough water to create a fire hazard.


One easy test stops most moisture-related problems before you plug in any cartridge heater. Using a megger to check the insulation resistance shows you how well the internal insulation is working. The lowest allowed value is 2 megohms. If the number is lower, it signifies that moisture has weakened the dielectric strength of the magnesium oxide. If you run a heater like that, the current could flow from the resistance wire to the sheath. The sheath gets power. Someone could get shocked if they touch the mould or the equipment. The circuit breaker could trip at any time. Or the heater can just short out and stop working.

The adversary is high humidity. When the relative humidity surrounding the termination end goes above 85%, moisture gets into the heater through the lead outlet. When the air is cold, condensation collects on surfaces and gets sucked into the powder. After being in a moist place for a few weeks, even a heater that was completely dry when it left the factory can become dangerous. That's why how you save things is just as important as how you install them. To keep the insulating quality, heaters should be stored in a dry environment, preferably in sealed packaging with desiccant.

If the insulation reading is low, the heater doesn't have to be thrown out. Conditioning, which is also known as "baking out," helps get rid of the moisture. When you apply a lower voltage, usually 20 to 30 percent of the specified voltage, for 30 to 60 minutes, the heat is moderate and the water evaporates. As the water departs, the insulating resistance goes up. You should test the heater again after it has been conditioned. The heater should not be used if the reading stays below 2 megohms. This means that it is permanently broken.



It's a good idea to megger test heaters that have been in storage for more than six months before putting them in. The test doesn't cost much compared to the expense of a broken heater, broken equipment, or an accident. This test is a common part of many facilities' pre-startup checklists. A few minutes with a megger will give you the confidence that the heater is safe to turn on.

The test isn't the end of moisture protection. The lead exit location should stay dry after the heater is in place and working. If the application involves water cooling, washdowns, or excessive humidity, using a heater with a sealed termination or placing a protective boot over the leads prevents further moisture entry. A heater that starts out dry but gets wet while it's running will break down much like one that was wet when it was stored.

The first step in staying safe is also the easiest. Check the resistance of the insulation. Don't let the heater get wet. If necessary, condition it. It is safe to power a cartridge heater that has strong insulating resistance. A low-resistance one is a warning indication that you should never disregard.