Hi, Welcome to B:bob:B!
This appears to be a standard fan trap. It’s covered in the Designer FAQ here. Do please read the FAQs in each forum and try searching before posting a question on BOB, thanks.
Dave Rathbun 
(BOB member since 2002-06-06)
Hi, Welcome to B:bob:B!
This appears to be a standard fan trap. It’s covered in the Designer FAQ here. Do please read the FAQs in each forum and try searching before posting a question on BOB, thanks.
Dave Rathbun (BOB member since 2002-06-06)