Error size varchar

Hi!!
I got error about a length field varchar. I have a table with a field type
varchar(20) but if I try to set to this field more than 20 characters I got
error.
I did a function to control the length of data and put it on trigger but
when it ocurrs I got the error anyway and the trigger not works.
This error is over than trigger execution??

This is the error

ERROR:  value too long for type character varying(30)

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*-Developer DataBase
*-JQ Microsistemas
*-809-747-2787
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