This works ( or at least doesn't return errors ):
$AddAccessRule = New-Object 'security.accesscontrol.filesystemaccessrulE'("CREATOR OWNER",@("ReadAndExecute,Synchronize"),"ContainerInherit,Objectinherit","Inheritonly","Allow")
$objacl = get-acl C:\A
$ObjAcl.AddAccessRule($AddAccessRule)
Set-acl -PATH C:\A $objacl