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Insulation resistance is a key parameter for any electric heater to operate safely. All Watlow® heaters undergo insulation resistance testing before leaving the manufacturing facilities. However, during shipping and storage, the mineral insulation (magnesium oxide or MgO) material used in metal sheath heaters can absorb moisture rendering the heater unusable until the moisture is purged from the heater.
Moisture impacts the functionality and performance of mineral insulated heaters and increases the likelihood of a short-to-ground episode. Although all heaters should have their insulation resistance tested before installation, heaters held in storage for long periods of time or used in high humidity regions are especially vulnerable to absorbing moisture from the ambient environment and should, therefore, always be tested before use. When too much moisture has been absorbed from the surrounding atmospheric humidity, the heater can become “wet” and must be purged of all moisture prior to use.
An insulation resistance test, or megohm test, is required to determine the quality of the insulation within the heating elements. This test is easy to perform and should be completed before putting any electric heater into service. This critical step will help avoid the installation of a “wet heater” that will have a higher chance of causing damage or shorting out.
What is Insulation Resistance?
For an electric heater to work correctly, current needs to flow directly through the coil wires within the heating element and not be able to short to the outer sheath through the absorbed moisture. The greater a heater’s insulation resistance, the greater the heater’s ability to handle the voltage and operate correctly. As current travels through the heating element, low insulation resistance can allow the voltage to arc across the dielectric material and ground out to the sheath, which can destroy the heater or cause broader system damage.
Magnesium oxide (MgO) is typically used as a dielectric material to insulate the element wire from the heating surface or outer sheath. As long as the insulation resistance of the MgO remains high, the heater can function correctly.
While MgO is an excellent high-temperature insulator, it also is hygroscopic, meaning that it likes water and actively draws moisture out of the surrounding atmosphere, effectively lowering the insulation resistance. If too much moisture is present when the heater is fully powered up, the current will find the shortest path out by arcing to the sheath and shorting out the heater.
A heater arcing and grounding out can have a range of consequences, from a nuisance trip of a breaker to an explosive event. Depending on the voltage and type of material being heated, a short to ground can result in a more significant explosive event generating damage, injury or even death, so understanding how to perform a megohm test properly is essential to safe operation.
Understanding the Value of a Megohm Check
Not all heaters have the same megohm rating. Depending on the heater’s construction, operating environment, type of seal, etc., a heater’s megohm value may be higher or lower. Knowing the acceptable insulation resistance for your heater is critical to maintaining a high-performing environment. Although each Watlow heater runs through rigorous testing before it leaves the factory, the insulation resistance of a heater can drop over time, especially if the heater is offline or spends time in storage. MgO is hygroscopic, meaning it is sensitive to the moisture in the air. Over time, the MgO will absorb more water from the air, which lowers the resistance of the MgO.
A megohm check confirms that the insulation resistance is still within an acceptable range. If the value is too low, the heater is considered a “wet heater.” As a general rule, if a heater has an insulation resistance of 500 megohms or greater at 500VDC, then the heater is acceptable to be put into service.
While using a multimeter to test for insulation resistance is possible, it is essential to always check for insulation resistance using a proper megohm tester.
What if the Megohm is Too Low?
So what if you measure your heater’s insulation resistance and the megohm is too low? Time to trash that heater? Luckily, no. Users can restore wet heaters to their proper working condition.
Wet heaters should never be put into operation until their insulation resistance has been increased. The heater will not be able to contain high voltages and high currents. Putting a wet heater into service runs a high likelihood of a direct short to ground occurrence.
Because the moisture in the magnesium oxide is lowering the insulation resistance, to return the heater to its proper form, the water will need to be “baked out” to purge the internal moisture. As magnesium oxide is exposed to water, it becomes magnesium hydroxide. Magnesium hydroxide has a lower resistance and does not work as well as an insulator.
To perform a heater bakeout, the heater is typically placed in a special processing oven to dry out the heater and remove all remaining moisture. A bakeout temperature of the heater at 120 degrees Celsius is required for at least six hours. The warm air is enough to convert the magnesium hydroxide back into magnesium oxide and water. If a bakeout oven is unavailable, some Watlow controllers feature a heater bakeout setting to accomplish the same outcome before powering up the heater for full operation.
After performing the bakeout process, check the heater’s insulation resistance again with a megohm tester. If the insulation resistance still remains too low, perform the bakeout procedure again. In extreme examples, the bakeout process may have to be performed for up to 40 hours to remove enough moisture from the heater.
What if the Megohm is Too High?
So you know what to do if the megohm reading is too low, but what if it is too high? What do you do then? Celebrate. There is no such thing as a megohm test or reading being too high. A high insulation resistance means the heater is in proper working order and can safely be put into service.
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A wet heater does not mean you have lost that heater for good. Conditioning your heater through a proper bakeout process and confirming that the required insulation resistance has been restored through a megohm test will have your heater online and working as required.
Keep your work environment safe by performing an insulation resistance test before every heater install.
What are the key characteristics and specifications that affect the choice of resistor? Factors that should be taken into consideration include initial tolerance and value selection. However, the tolerance or variation of the value of a resistor is affected by multiple parameters, as explained below.
This is a measure of the variation of the nominal value as a result of temperature changes. Generally quoted as a single value in parts per million per degree centigrade (or Kelvin), it can be positive or negative. The equation for calculating the resistance at a given temperature is:
Rt=Ro[1+α(T-To)]
Where Ro is nominal value for room temperature resistance, To is the temperature at which the nominal resistance is given, T is operating temperature and α is the TCR.
Put simply, a 1 MΩ resistor with a TCR of 50ppm/K will change by 50Ω per 1 degree of temperature rise or fall. This may not sound like much but consider if you were using this resistor as the gain resistor in a x10 non-inverting amplifier circuit with 0.3v on the + input. The worst-case change in output could be as much as 7.5mv which is equivalent to about 5LSBs in a 5v 12-bit ADC circuit. This kind of change can be quite noticeable in precision design. Remember also that the TCR is quoted as ±x ppm/C so it is feasible, although unlikely, that the second resistor in the circuit could change in the opposite direction hence double the possible error. Finally, it’s worth noting that some precision resistors quote variable TCRs over the temperature range the circuit is operating in, and this can complicate the design process significantly.
Ageing and stability are a complex amalgam of multiple changes to the value of a resistance value over time and are the result of temperature cycling, high-temperature operation, humidity ingress and so on. Typically, the value will lead to an increase in resistance over time as conduction atoms migrate within the device.
The thermal resistance is a measure of how well the resistor can dissipate power into the environment. In practice, engineers use thermal resistance to model the heat dissipation for a system – it is thought of as a set of series ‘thermal resistors’, each representing one element of the heat dissipation of the system.
This is mainly important if the design means the resistor is running at or near its maximum value and can significantly affect the long-term reliability of the system. An example of where this parameter could be used is to calculate the size of a PCB pad or ground plane requirement that would be used to keep the resistor’s value and operating temperature within acceptable limits.
All resistors come with a maximum power rating, specified in watts. This can be anything from 1/8th watt right up to 10s of watts for power resistors. In a first pass analysis, the engineer would check that the resistor is operating within its rated value. The equation for calculating this is P=I² R, where p is the power dissipated in the resistor, i is the current flowing and R is the resistance. Sadly, things can be more complicated than this; for exact work, the engineer needs to take account of the thermal derating curve for the resistor. This specifies the amount by which the designer needs to de-rate the maximum power dissipation above a given temperature.
This might seem theoretical as often the de-rating kicks in at quite high temperatures, but a power circuit in an enclosed housing in a hot region can often exceed the cut in point and the maximum power dissipation will need to be reduced appropriately. It’s also worth noting that the maximum operating voltage of a resistor is de-rated with power dissipation.
Any electronic component that has flowing electrons is going to be a source of noise, and resistors are no different in this respect. In high gain amplifier systems or when dealing with very low voltage signals, it needs to be considered.
The major contributor to noise in a resistor is thermal noise caused by the random fluctuation of electrons in the resistive material. It is generally modelled as white noise (i.e. a constant RMS voltage over the frequency range) and is given by the equation E=√4RkT∆F where E is the RMS noise voltage, R is the resistance value, k is Boltzmann’s constant, T is the temperature and Δf is the bandwidth of the system.
It is possible to lessen system noise by reducing the resistance, the operating temperature or the system’s bandwidth. Additionally, there is another type of resistor noise called current noise which is a result of the electron flow in devices. It is rarely specified but can be compared if the standard numbers using IEC60195 are available from the manufacturer.
The final challenge to consider is the high-frequency performance of the particular resistor. In simple terms, you can model a resistor as a series inductor, feeding the resistor which has a parasitic capacitor in parallel with it.
At frequencies as low as 100Mhz (even for surface mount resistors which have lower parasitic values than through-hole parts) the parallel capacitance can start to dominate, and the impedance will drop below nominal. At a higher frequency still, the inductance may predominate, and the impedance will start to increase from its minima and may well end up above the nominal value.
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