def METHOD_NAME(self):
    """
    Retrieves the serial number of the device
    Returns:
        string: Serial number of device
    """
    serial = "N/A"
    attr_path = PSU_DIR+self.psu_serial_attr
    attr_rv = self.__get_attr_value(attr_path)
    if (attr_rv != 'ERR'):
        serial = attr_rv
    return serial 