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The PR521 unit is an integral part of HAD medium voltage fixed or withdrawable circuit-breakers, on which it operates by means of a demagnetisation release (Y03), of which there is only one for the three poles and which acts directly on the operating mechanism of the apparatus.
The PR521 unit can carry out the following ANSI (50-51-51N) protection functions:
| Description | IEC | ANSI |
| Protection against over current with definite time-delay | I> | 51 |
| Protection against short-circuit with adjustable time-delay | I>> | 50 |
| Protection against instantaneous short-circuit | I>>> | 50 |
| Protection against homopolar fault towards earth | Io> | 51N |
The protections can be made in two-phase or three-phase mode, depending on whether two or three current sensors (hereinafter indicated by C.S.) are connected.
The unit is self-supplied and ensures correct operation of the protection functions (50, 51 and 51N ) when there is a current greater than or equal to 20% of the rated value of the C.S., circulating in at least one phase.
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