International Institute of Cricket Umpiring & Scoring
Caught off Keeper's helmet
"The batsman hit the
ball and it hits the wicket-keeper on his helmet and then bounces back into his gloves. The wicket-keeper appealed for the catch but the umpire called Not out..The non-striker had started to run and the wicket-keeper was quick enough to throw down the non-striker's wicket, to be rewarded by a Run out decision. Comments please"
A fair catch cannot be made if the delivery had previously come into contact with a helmet being worn by a fielder. However the ball does not become dead just because it has hit a fielder's helmet while being worn, therefore, if the non-striker was out of his ground the Run out was fair.
Read more about Law 32.3 (Caught) at the MCC website