Ex motors will loose certification in the eyes of the governing bodies if there is any alteration from the design the maker originally submitted to the test house for Ex testing. Any alterations from the design however minor or trivial could lead to the motor becoming a danger to property and to life. We as repairers work to IEC60079-19, the Eurostandard which covers all Explosive atmosphere equipment repairs and will only use replacement parts supplied by the maker for the application, because we know we will be liable should an incident occur. Unlike a lot of the cowboy and backstreet firms who take on Ex repairs without the knowledge to do a safe job. I am sure there are others in the forum in a similar position to myself who have "Horror Stories" they could tell.
Ex fans are non-metallic for 2 reasons, firstly as Itsmoked correctly said to prevent sparking and heat build up if the fan should come loose and secondly to prevent a build up of static which could also lead to sparking. There are still old FLP machines about made to BS229 with metal fans but it was a requirement of later standards that non conductive fans be used. Ex repairs are a potential minefield and a specialist field of motor repair. Users should not take chances and should use properly trained and competent repair companies.
Mil3?
I think you are getting your Ex concepts confused. ExD is termed explosion proof. The casing is designed to withold an internal explosion. They are made to high tolerances but they are not sealed....they cannot be!. There must be gaps to allow the motor to be assembled from the seperate parts like the terminal box lid and particularly where the shaft passes through the casing!. These gaps are machined to close tolerances and take the form of long spiggots and tight clearances. The idea being any jet of hot gas or flame released through these paths during an internal explosion is sufficiently cooled by the surrounding metal and lacks the energy to ignite the surrounding atmosphere.
