They are realized in three ways:
1 historical type: water is not isolated from the electrical part. With this solution which is the cheaper one you can reach current up to 2800Arms
2 type: using a thermally-conductive and electrically-insulating sheet, which allow a good heat exchange combinated with an excellent insulation you can separate the cooling water from the electrical part. With this solution are reached currents up to 1800Arms.
3 type: for smaller size use semiconductor module allows you to automatically have the insulation of the cooling water from the electrical part. With this solution are reached currents up to 500Arms.