$ isql -ch utf8 localhost/3330:empty30
SQL> set list on;
SQL> select a.mon$character_set_id,r.rdb$character_set_name 
SQL> from mon$attachments a join rdb$character_sets r
SQL> on a.mon$character_set_id=r.rdb$character_set_id 
SQL> where a.mon$attachment_id=current_connection;

MON$CHARACTER_SET_ID            4
RDB$CHARACTER_SET_NAME          UTF8                                                                     

SQL> with c as(
CON>   select rpad('', 8, cast('абырвалг' as varchar(255) character set utf8)) s1,
CON>          cast('абырвалг' as varchar(8) character set utf8) s2
CON>   from rdb$database
CON> )
CON> select c.s1, octet_length(c.s1) o1, c.s2, octet_length(c.s2) o2
CON> from c;

S1                              абырвалг
O1                              16
S2                              абырвалг
O2                              16

C:\>chcp
Текущая кодовая страница: 866

C:\>chcp 1251
Текущая кодовая страница: 1251

C:\>isql 192.168.0.201/3330:empty30 -ch win1251
Database:  192.168.0.201/3330:empty30
SQL> set list on;
SQL> select a.mon$character_set_id,r.rdb$character_set_name 
SQL> from mon$attachments a join rdb$character_sets r 
SQL> on a.mon$character_set_id=r.rdb$character_set_id 
SQL> where a.mon$attachment_id=current_connection;

MON$CHARACTER_SET_ID            52
RDB$CHARACTER_SET_NAME          WIN1251

SQL> with c as(
CON>   select rpad('', 8, cast('абырвалг' as varchar(255) character set utf8)) s1,
CON>          cast('абырвалг' as varchar(8) character set utf8) s2
CON>   from rdb$database
CON> )
CON> select c.s1, octet_length(c.s1) o1, c.s2, octet_length(c.s2) o2
CON> from c;

S1                              абырвалг
 O1                              8 
S2                              абырвалг
O2                              16

with c as(
  select rpad( _win1251 '' , 8, 'абырвалг' ) s1
        ,rpad( _utf8 '' ,    8, 'абырвалг' ) s2
  from rdb$database
)
select '>'||c.s1||'<' s1x, octet_length(c.s1) o1, '>'||c.s2||'<' s2x, octet_length(c.s2) o2
from c;