privillages for pg_class

Hello.
I use following statements for temporarily disable triggers and enable them again:
--Disable triggers.
UPDATE "pg_class" SET "reltriggers" = 0 WHERE "relname" = 'orders';

--Enable triggers.
UPDATE pg_class SET reltriggers = (
SELECT count(*) FROM pg_trigger where pg_class.oid = tgrelid)
WHERE relname = 'orders';

It works when connected as a superuser.

But, when I try to execute it as a normal user, not enough privillages, although I granted all privillages to pg_class and pg_trigger.

Do I miss something?

Thanks.