EMPLOYEE WAS GRINDING A WELD ON TOP OF PRECIP TANK, WEARING PROPER PPE MONO-GOGGLES, FACE SHIELD, HARD HAT, GLOVES, LONG SLEEVES. DURING WORK TOOK SEVERAL WATER BREAKS, STARTED FEELING HOT, NO TICED HE WAS NOT SWEATING. ALSO FELT CRAMPS IN LOWER BACK AND LEGS.