Limit sys login trigger cannot raise the error-CodePudding

Current because of the need, to limit sys login
And then through the logon the trigger to achieve
But in the trigger triggered raise the error is thrown out

Would you please tell me what the method the great god not

 
The CREATE OR REPLACE the TRIGGER LOGONCTRLTRIGGER 
AFTER LOGON ON the DATABASE 
Declare 
A int:=0; 
The BEGIN 
If ora_login_user='SYS' then 
RAISE_APPLICATION_ERROR (20202, 'PRIVS_ERROR, both PLEASE CONTACT ADMINISTRTOR'); 
end if; 

END LOGONCTRLTRIGGER;

But if you use the sys execution procedures section is to be able to raise the