Electric heaters are subject to a number of failure modes that can seriously affect any system in which they are a part. This article discusses the major failure modes, including catastrophic shorts, and proposes solutions for prevention of these causes.
Control Mode FailuresControl mode failures involve failure in some part of the control system that precedes the final element or power module.
Primary Element or Sensor Failure. Sensor failure can take the form of either a sensor break or short. In either case, the normal result is that the controller will turn full "on." (For example, in the case of an RTD, the controller would assume zero resistance and turn output up to 100% to compensate.) The solution to this failure mode is simple: Most modern controllers have lead break and short detection. Using a controller with this type of protection, if an output relay closes or opens, the control will annunciate or shut down the process, or go to an intermediate mode.
PID Runout. In some applications, the control loop may be tuned improperly. This can result in improper control and cause the process to turn on 100% power for a long period of time, leading to process overheating. The simple solution to this problem is to properly tune the loop. Most modern controllers have an autotune circuit that will automatically select the best PID settings. Another protection against PID runout is to establish a high limit alarm, which can activate the proper response.
Power Mode FailuresThese types of failures are more serious than control mode failures. They can involve electrical shorts and even heater burnup or meltdown.
Inadequate Controls. Occasionally, a processor's equipment may use a non-IEC contactor to control heating loads. Such an old-style contactor can, on occasion, weld the contacts together and cause heater runout. If contactors are used for power control, the best plan is to substitute an SCR control module, which will protect against this type of failure. SCR power controls provide close process control and ensure long power module and heater life.
While highly dependable, SCRs sometimes can fail in the "on" mode. This can lead to heater runout if not properly handled. An SCR short detector, used in addition to the control module, will trigger an output if it detects a condition indicative of a short such as a failure of forward voltage drop across the SCR.
One question often asked is how to reliably shut down the process once a power mode failure is detected or annunciated. There are several ways to do this, but all must address interruption of power going to the heater. This differs from the control mode failures, which can be interrupted by breaking the control signal output.
Interposing IEC Contactors. Using a modern IEC contactor interposed in the load line, in a fail safe mode, is a solution that offers fully automatic heater shutdown. In this mode, the contactor goes into a NC configuration when the process starts up (fails safe). Then, if an SCR short detector indicates that a short condition is present, the contactor automatically shuts down the power lines associated with that SCR.
In processes where the heater load draws no more than 16 A per phase, there is another alternative: a high limit thermostat, which can operate reliably as a high limit control up to 1,100oF (593oC). One drawback is that a thermowell or hole is needed to mount the thermostat's capillary bulb. The bulb should be positioned as near to the heater as possible.
Many heaters exceed the 16 A rating, however. In those cases, using the shorted SCR detector is the preferred alternative -- it offers the simplest installation when combined with proper fusing. The shorted SCR detector can be wired to annunciate a failure or to cause power interrupt through the proper power device.
Water is another cause of shorts in heater systems. If the heater is operated in a high humidity environment or if water lines are a part of the heater system, be sure to specify that the heater manufacturer use waterproof varnish on all internal, end terminal connections. The magnesium oxide insulating compound used inside the heater element is very hygroscopic, so any moisture exposure can quickly lead to heater failure. Good system design also dictates a water line layout that ensures that if the lines are compromised, they cannot leak on electrical terminations or hot heater systems.
If a short occur within the heater, there are several options. If an SCR is a part of the heater control package, be sure to select one with a properly sized I2T quick-acting fuse. If a short occurs, the SCR's electrical limit will be exceeded, blowing the fuse and protecting the SCR and equipment. (It should be noted that if the SCR short occurs in the "on" condition, the I2T fuse will not be blown, so an I2T fuse cannot provide complete heater and equipment protection.)
Of course, there are different types of shorts. For example, if the short occurs to a ground that is protected using circuit breakers (a common engineering practice), the breaker will trip the circuit and shut down the heater, provided there is a high enough potential. Heaters also can develop internal line shorts that overload the heater and SCR. A fast-acting I2T fuse can protect against this type of failure. The fast-acting fuse also can act to prevent damage in a line to ground short, if the potential is high enough.
Another recipe for catastrophic failure is an oversized SCR combined with an oversized fuse in a control system that is not protected by a shorted SCR detector. This control configuration is particularly prone to failure when used for large, high watt density, cast-in heaters. If an SCR short occurs, enough energy can be delivered to the load to melt the heater's aluminum casing. To prevent this, consult the manufacturer or its representatives about properly sizing the SCRs and fuses to the load.
Heater failure also can occur in systems with zero-fired control, where full current typically occurs at full voltage to the SCR. Therefore, if a shorted SCR condition exists, the fuse will not be cleared if it is properly sized. But, the potential still exists for heater runout and zone overheat -- this is why a shorted SCR detector is so important.