4 Dec 09 14:45
Are you sure that the devices are RTDs and not thermistors?
Running with 5 devices when many motors run with three and many many more run with none is a question for the loss prevention department.
If you have never or almost never had a valid temperature trip you are probably safe using five devices.
If the motor is pushed to the limit, run hot and frequently overloaded the question becomes more complex.
If the RTDs usually show the same temperature (even heating) that's a vote to go with five devices.
If the RTDs show different temperatures, (hot spots) and the RTD that failed was the device that ran the hottest, it becomes more worrysome to run with one less device.
How well is the motor protected by the conventional overloads? Are the conventional overloads set properly or are they set high in the expectation that the RTDs will provide all the needed protection?
What is the estimated cost of repairing the RTD and how does that compare with the cost of a rewind?
Bottom line, for most applications I would run with one device missing.
If the motor is frequently overloaded and abused and overheat trips are common I may decide differently, but without seeing the actual site conditions it depends.
"Why not the best?"