Shock Alert has three probes on the base of the device designed to detect this potentially dangerous voltage gradient(s) inwater. It provides a visual and audible alert to the existence of potentially hazardous voltage conditions in the body of water.Note that Shock Alert is a sensitive detection device that will detect small voltage gradients in water. Shock Alert can be used infresh or chlorinated water. It may also be used in salt water pools. Shock Alert will detect voltage gradients in concrete, guniteand vinyl lined pools. DUE TO THE INSULATING EFFECTS, SHOCK ALERT IS NOT FOR USE IN FIBERGLASS POOLS ANDHOT TUBS.