heating element resistance

[powerplay]

When an 240 "Oven" type heating element is deteriorating, what kind of resistive value does one see if it is burning/burnt out? I just got the call from an sausage maker, and he had asked if an higher reading on an element means it is ok and still part of the circuit...on the way to check out the voltage on the Contactor and get the thing going.

 

[GoldDigger]

If the resistance increases above the normal level it indicates that either there is a terminal connection problem or that a thin spot has developed in the resistance wire.
The result of the latter is that less heat is generated by the whole element but a higher concentration of heat is being generated at the thin spot, leading to even faster deterioration. Like a light bulb filament burning out.
If it is a visible Calrod type element you may see a "hot spot" in the area of the developing failure.

Tapatalk!

 

[dploof23]

If you take the voltage squared and divide by the wattage of the heater element, you will get the expected resistance for the heater. The manufacturer of the heaters I use say the heater is good as long as it is within a +5% or -10% resistance tolerance.

Hope this helps,
Dave

 

[GoldDigger]

If you take the voltage squared and divide by the wattage of the heater element, you will get the expected resistance for the heater. The manufacturer of the heaters I use say the heater is good as long as it is within a +5% or -10% resistance tolerance.

Hope this helps,
Dave

That is a good place to start, but if you actually measure the resistance when the heater element is new and then you suddenly see the resistance increase, that is a bad sign regardless of the percentage, as long as your measurement is accurate (repeatable) enough to really see the difference.

 

[broadgage]

In practice, IME heating elements show a nearly constant resistance until the point of failure.
A slight thining at one point will only increase the total resistance very slightly, probably not enough to measure, and any significant thining will lead to prompt failure.

 

[powerplay]

... after metering out the mercury switches that control the heaters, I am informed there is an electrical panel mounted to the back of the unit.... it was an unlabelled breaker he assumed was on but was off. I was easy on the Invoice and he knows I'll spend it on deli goods anyways!

I was wondering if regular lighting contactors would stand up to controlling the heaters adequately, and if mercury switches were really necessary?

 

[powerplay]

... after metering out the mercury switches that control the heaters, I am informed there is an electrical panel mounted to the back of the unit.... it was an unlabelled breaker he assumed was on but was off. I was easy on the Invoice and he knows I'll spend it on deli goods anyways!

I was wondering if regular lighting contactors would stand up to controlling the heaters adequately, and if mercury switches were really necessary?

...the switch was labelled mercury, but i was informed they were liquid filled to quench the arc... not sure how readily available they are, or how long to get in.

 

[hurk27]

Many times we will fine elements wired in parallel, and a higher resistance on one set will indicate that an element had failed, while the unit will run it will take longer to reach temp.

A three phase bank of heaters wired in a WYE with the WYE point floating as many 480 volt furnaces are if you loose one element not only do you loose the heat from that element but the other two elements are now running at a lower voltage (240 instead of 277 to be exact) so you loose a lot of heat output, this also happens of a 208/120 system wired in a floating WYE, and you get 204 volts instead of 120, but WYE wire heaters at 208 are not as common as the 480.

The above only happens if they wire them in a WYE with the WYE point floating.