All resistance welders including spot welders are designed with industrial cooling systems. The water supplied to these systems can be supplied from wells or city water. This water enters this industrial machine with a water-cooling system which was not cleaned or designed to carry drinking water.
The used water is then discharged to a sewer system or in most cases sent into a recirculating system. The recirculating system is usually a COOLING TOWER or CHILLER designed to cool the water back to a desired temperature. These systems may be open to the environment or closed. The water may be treated with industrial chemicals to maintain its cooling characteristics and insure free flow. This water returns to the spot welder. Well or city water is used as make up for evaporation or lost water. Water in these systems is not meant for human consumption.
The medical implications and dangers of drinking welder cooling water are beyond the scope of this blog.
Reference: RWMA – Resistance Welding Manual 4th Edition