51 (time overcurrent) can be used for other things besides overload protection of a motor. So roughly speaking overload protection is a subset of the functions that can be performed by 51.

For motor overload protection, I'm used to seeing 49 when there is an overload heater to trip a motor starter and 51 when there are CT's feeding a protective relay. It sort of makes sense that 49 senses temperature (even though that temperature may be a result of current through a heater) while the 51 senses current and uses non-thermal means to develop a time/current trip characteristic.

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(2B)+(2B)' ?

RTD's protect winding against temperature rise due to overcurrent as well as insufficient cooling (clogged filters, non-working external blowers, insufficient cooling water etc.). The electrical protections like instantaneous OC/OL and IDMT OC will not sense insufficient cooling. I consider RTD's protection as a must, especially in MV machines.

Muthu
www.edison.co.in

Modern digital motor protection relays have sophisticated stator and rotor heating models that should provide superior motor protection compared to a simple time overcurrent (51) function.

In many of these relays, the motor model (49) can be biased by RTD inputs if available. This will help handle long term overloads and high winding temps - the RTDs make the motor model more accurate. But the RTDs are relatively slow to respond and won't be fast enough for some motor conditions. So I would not rely solely on RTDs. There should also be an instantaneous overcurrent (50) for short circuit protection.