how to control aborted_clients?

Question

Hi, my aborted clients are keep on increasing. How to control it?
We are executing alter table pt_online to add index.

table size is 150gb.

(1) Restart the pt-online-schema-change online tool or alter table to add index.

    *** pt-online-schema-change failure message ***
    Table. The pt-online-schema-change created table (_phone_observations_new) has reached "data" to 84.09G and original table (phone_observations) has "data" around 150G.   

Still it is going to take time but we are good to start using phone_observations table as the data insert rate on _phone_observations_new table is very slow)

pt-online-schema-change is a tool.
It will perform adding index and inserting without disturbing other sessions.
When we are performing alter add index.it was not allowing to insert in table and aborted_clients keep on increasing.

        mysql> show global status like '%aborted%';
        +------------------+-------+
        | Variable_name | Value |
        +------------------+-------+
        | Aborted_clients | 204 |
        | Aborted_connects | 19561 |
        +------------------+-------+
        2 rows in set (0.00 sec)

        mysql> show global status like '%max_used%';
        +----------------------+-------+
        | Variable_name | Value |
        +----------------------+-------+
        | Max_used_connections | 145 |
        +----------------------+-------+
        1 row in set (0.00 sec)


        mysql> show global variables like '%max_conn%';
        +--------------------+-------+
        | Variable_name | Value |
        +--------------------+-------+
        | max_connect_errors | 10 |
        | max_connections | 400 |
        +--------------------+-------+

Please help me here.