protection curve but only a point (overcurent 51)

[godel123]

hi
overcurrent (51) protection.
When we see 100 A 0.3 s.
so why we talk about curve, but there is only a point

 

[Krustabas]

At 100 A you see 0.3 s. If you test the relay at 150 A, it will operate faster. With lower current the operate time will be lower. That is the reson why we talk about curve (inverse, extremely inverse and so on) in contrast with the definite time overcurrent protection (50) where the operate time is fixed and does not depend on current level.

 

[godel123]

how to "choose" the seting values to protect a motor, i mean is there a genral considerations or we have to make calculations?
exemple of a motor : 50 Kw, 400 V.

 

[ScottyUK]

For a 50kW 400V motor just buy an overload relay to suit the motor full-load current. Normally a Class 10 overload is fine. If it is a load with unusually severe starting conditions, and if the motor is designed to cope with it, then you might look at a Class 20 or even a Class 30 overload. I've never come across a Class 30 relay in real life, even on conveyor duty, but they exist in catalogues. Standard IEC motors simply won't survive a Class 30 start, it would have to be a 'special'.