Dealing with moisture inside electric heaters

Those who work with electronics usually equate water and electricity as a bad combination – and for good reason. Labels can be found on many kitchen appliances and bathroom electronics such as electric knives and hair dryers, warning users that a shock can occur if the device comes into contact with water.

By John Pape, Watlow March 15, 2008

Those who work with electronics usually equate water and electricity as a bad combination — and for good reason. Labels can be found on many kitchen appliances and bathroom electronics such as electric knives and hair dryers, warning users that a shock can occur if the device comes into contact with water.

It stands to reason any electronic device should be handled with care when water is present. But how does one deal with electronic devices when water or moisture is inside the device and yet needs to be energized? Examples include industrial heaters used in plastics processing, foodservice equipment, packaging, telecommunications and semiconductor processing industries, to name just a few.

Heater manufacturers hear a variety of issues in the field such as:

When the power is turned on, the heater smokes

Heater does not pass high potential (hi-pot) test

Heaters trip Ground Fault Interrupter (GFI).

These complaints typically can be attributed to moisture inside the heater. While this can seem daunting and even irritating to the new user, inconveniences can be minimized with proper precautions.

Why some heaters have moisture

Many industrial heaters are manufactured with a compacted insulation called magnesium oxide (MgO). MgO is hygroscopic, meaning it can readily take and retain moisture out of the environment. The heaters must go through a bake-out process prior to final assembly. At times when the heaters are exposed to high humidity environments after shipment from the factory, the heaters can absorb moisture. Additionally, when the heater sits idle on a machine at room temperature, occasionally they can take up moisture.

For example, take into consideration heaters installed in high humidity environments on equipment that travel overseas. Moisture in these heaters could cause nuisance tripping of the GFI. When the heater is powered, it smokes or outgases. This outgassing is a result of the moisture exiting the heater as it reaches its operating temperature and is actually quite common in most heaters.

Additionally, there are various levels of binders along with the moisture that can burn out from the heater when it is energized. The user should not be concerned when the moisture in the heater outgases. For more information regarding the binders and other ingredients that make up part of the out-gassing, see the product’s related Material Safety Data Sheet (MSDS).

An important requirement for industrial heaters is they must pass an electrical specification called a dielectric strength test or hi-pot test. The hi-pot test measures the heater’s ability to restrain leakage current at a given voltage. A breakdown in insulation or a faulty assembly can cause a hi-pot failure. Additionally, a high level of moisture can also indicate excessive leakage and/or a hi-pot failure. When moisture is in the heater, it serves as a direct path to ground, which will cause the heater to fail hi-pot.


There are several steps that can be taken to avoid issues related to moisture. They include:

Field bake-out — It is often inconvenient for the user to send heaters back to the factory to be dried. However, one approach to ridding the heater of moisture in the field is placing the heaters in an industrial oven overnight at 121-204 C (250-400 F) depending on the size of the heater. When the application requires heaters quickly, a field bake-out can be an option. This will ensure no nuisance tripping of the GFIs and will permit the heaters to pass hi-pot tests.

Low-voltage controller bake-out — In applications where SCR power controllers are used, a way to optimize heater performance is to use the low-voltage start-up feature, if offered. Some controllers have an option called heater bake-out. During the heater bake-out step, the controller gradually increases voltage to the heater while monitoring the output current. When the heater achieves full output before the bake-out time is completed, the user then knows the heater is dry and it can be put into regular service.

The key difference between standard soft-start and the bake-out option is since soft-start is active for only one second, for example, there is not enough time to use this lower voltage to bake out any moisture inside the heater. If an MgO-insulated heater element is shut down for a long period of time (or sits in storage), it can absorb moisture. If full power is applied, excessive current can blow fuses and cause irritating tripping of the GFI.

The heater bake-out feature soft-starts the load to remove moisture from the heater and prevents blown fuses. The bake-out option on some controllers offers additional user-enabled programming options such as allowing the user to input parameters based on application needs, including a specified bake-out time period. Occasionally due to high moisture content, the bake-out cycle time may not be long enough. If the output current reaches a user-specified trip point during the bake-out — as it would if arcing occurred in the heater — the SCR shuts off the output and activates an alarm. After the heater bake-out, the SCR automatically switches to the user-selected control mode (phase-angle or zero-crossing).

Special insulation — While it is common to apply low power to the heater to dissipate the moisture from the MgO insulation prior to assembly, this is a short-term solution since the MgO can readily wick in moisture if the right conditions exist. Another choice is to opt for an insulation that can better handle the moisture. A feature on various heaters involves a treatment process that coats the MgO insulation. The treated insulation fends off moisture when sitting for long periods of time. It is also ideal for heaters that are exported to end users around the world, which is when moisture tends to penetrate the heaters.

Align testing procedures — When a heater fails a field hi-pot test with excessive leakage, it might be attributed to moisture in the heater. UL and CSA have guidelines for testing heaters prior to shipping. This test criterion was established to protect the user and the user’s equipment. If additional internal hi-pot testing is required at the end user’s site, they are encouraged to align their test procedure to that of the UL testing standard. The hi-pot test parameters listed in the table are typical for testing strip heaters.

Safety: ground the heater

Do not forget to ground the heater. Any electrical device — including heaters — can leak current to its sheath or short circuit to the sheath. Leakage current is a small amount of electric current that passes through the electrical insulation of heaters, motors or other electrical devices. The leakage current may be strong enough to cause a shock or cause sensitive electronic devices to not operate properly.

An electrical or safety ground is used to protect people from being injured as well as keeping equipment from being damaged (See Figure B). As the name implies, it grounds electrical current in case of a short circuit. It is a wire attached to the metal part of an electric device and connected to a metal rod that is driven into the ground. The ground functions as an alternate return path for electricity. The ground path effectively drains off any leakage current or short circuit current into the earth.

Heaters can be manufactured with a ground wire or a ground terminal as an added safety feature. The third lead or terminal on the heater is where the connection is made. Additionally, when measuring resistance between the terminals (or leads), the ground will measure open or a very high resistance between one terminal and the two other terminals.

It is important to fully understand the application when using electric heaters. In addition, it is always worth the extra steps to ensure safety and protect expensive equipment. Whether it involves grounding the device, baking the moisture out of the heater, choosing the proper insulation or proper testing, the peace of mind of knowing the right steps where taken specifying and applying heaters can go a long way.

Heater voltage
One-second test voltage
mA setting

* Maximum allowable leakage (compensated) shall be 4 mA per 100 square inches

Less than or equal to 250 V
1,250 Vac

Greater than 251 V
Two times rated voltage + 1,000 V, 20%

Author Information

John Pape has worked for Watlow Electric Manufacturing Co. for 18 years. Pape earned a Bachelor of Arts degree in Business Administration at Lindenwood University and is a member of the Society of Plastics Engineers. His current duties as a technical support specialist include account management, pre- and post-sales support, project management and the sales training coordinator for Watlow’s St. Louis heater division.