Heater Elements

Do you know how an overload protects a motor ?

Do you know how a heating element works ?

It doesn't seem so.

Protect it from what? A motor overload does just that... protect a motor from an overload. How do you overload a heater element?

I'd think it could act as its own overload.

I guess the answer is NO

Yes, if you powered it from Russel's pinwheel device.

Au contrare' mon ami! No TOL will lead to disaster..... If the thermal output of the heating element is constrained, thermal runaway will occur. As the temperature rises, element resistance will increase, and lower current will flow, as the temperature continues to rise.....if lucky, the element melts, shorts out to ground, and blows fuse/CB. If unlucky....FIRE! The current needed to melt the element will be well below the fuse/CB rating needed to protect the supply wiring!

Au contraire au carre...

How is the overload relay expected to detect the problem if the overheated heater's resistance increase and the current decreases? The overload relay will not react.

The only protection here is a temperature sensor or an element resistance sensor. As for the protections when the element fails, fuses are usually the best to limit the collateral damages.

Pretty much precisely what I stated. The TOL here is a simple thermal fuse, or more often a Klixon thermal breaker that self resets. Standard stuff in HVAC electric heating design, clothes dryers, hair dryers, Mr Coffee, etc etc etc!

In addition the motor overload generally controls a contactor coil to disconnect the motor load rather than breaking the full motor load so you would also need a contactor (unless possibly the heater is really low current).

Just another reason why the answer is no.

In my case it often happens that the motor burns up to protect the fuse. It usually works with expensive electronics equipment, too.

Generally the main components of the motor starter i.e SCPD (fuse or MCCB), contactor and thermal overload relay are properly selected to comply with the requirements of Type 2 co-ordination as per IEC. The curve of the thermal overload relay should be below the hot withstand curve of the motor. if the commponents are properly cordinated then the TOL will de-energise the contactor before the motor burns due to overload.

In case of short circuit the fuse/ MCCB will protect the motor as well as the contactor by disconnecting the power circuit. Contactors are not suitable for breaking higher current (typically 10 times of the rated current). Thus the fuse/ MCCB rating is so selected that in case of short circuit the SCPD will operated much faster than the TOL.

Not according to British Standard 7671.

You could IF YOU KNOW how to select it and HOW to wire it!

AP-1,

Short answer--YES and NO.

Explanation--

• The cold element, like a non-rotating motor, draws more current than a warm element (or rotating motor). It may take somewhat more time for the element to get to its operating temperature (and lower current draw) than it takes for a motor to get to speed. But, the "inrush" current for the element is much closer to the final current for the element, so the longer time should pose no problem. (This is the YES portion of the answer.)
• The assumptions inherent in motor overload selection for a given motor application are fairly well-known, with many installation instructions available, so problems can be avoided easily by a competent person. The assumptions inherent in applying a motor overload to a heater are less clear, so errors and accidents would be more likely. You can use trial & error to come up with the proper motor overload selection or setting that would provide steady-state protection against overheating of the element, but I know of no instruction manuals that would apply. (This is a MAYBE portion to the answer.)
• If the heating element is with a fan to force air past it, and the air flow fails, it is very unlikely that the motor overload would ever sense this problem and provide protection prior to the burning-out of the heating element. Other methods are readily available that can provide such protection, such as a temperature-sensing probe. I know of no codes to which such a use would comply. (This is the NO portion of the answer.)

My overall opinion--don't do this one; it has too many problems and even liabilities.

--JMM