QUESTION:

Overload relays 3RB10 were equipped with 'phase failure protection'.

Why has this feature been replaced by 'phase failure sensitivity' on the replacement series 3RB20 and 3RB21 ?

ANSWER:

The phase failure protection feature in 3RB10 devices resulted in many cases of nuisance tripping in unstable networks with subsequent complaints from customers.
This is why the new 3RB20 / 3RB21 range has phase failure sensitivity which results in time delayed tripping (> 3 s) in the event of phase failure and current imbalance > 40 %.