A newly wired furnace suffered burnout of all six of its silicon carbide elements, at a cost of some $9,000. The controllers were in good order and showed normal working temperature, though the furnace was clearly much hotter. The type R thermocouples were good and located properly, but somehow the controllers were deceived. What caused the failure? The extension cables used were Type J, but they should have been Type R. Worse yet, the red was connected as positive (+) and white as negative (-), meaning that the wires crossed at both ends. This frequent and costly error occurs because red as the positive lead is a logical assumption, given that red is positive everywhere except on North American thermocouple wires.
Knowledge of extension cables and their color codes could have prevented this incident. In this example, for a while after the furnace came up to temperature, there was no problem. But, as the thermocouple head warmed up in relation to the controller terminals, the extension cable injected a negative signal equal to eight times that temperature difference into the controller, causing it to severely overheat the furnace.
Figure 1 covers extension cables for the six most commonly used thermocouple types and five varieties of color codes used in various countries, plus the IEC codes. The IEC codes represent a single world standard and are showing up on European equipment. They will eventually replace the currently used codes, but don't expect it anytime soon.
Three kinds of infractions will continue to plague industry for a long time yet. They are, in increasing order of severity:
If copper extension cable is used, part of the wiring fails to generate its share of the signal. The shortfall from the true indication is roughly equal to the temperature difference between the thermocouple head and the controller.
2. Correct extension cable used but polarity reversed at both ends.
If the correct extension cable is used but polarity is reversed at both ends, the wiring generates a subtractive signal. In this case, the shortfall is twice that in first case.
3. Incorrect extension cable used.
If the incorrect extension cable is used, the signal is unpredictable; therefore, the indication could be too high or too low, according to whether or not correct polarity is observed and type of extension cable used.
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