What is BS EN 3909- Test fluids and test methods for electrical and optical components and sub-assemblies about?  
BS EN 3909 specifies the list of test fluids to be used to demonstrate that components and subassemblies will not be adversely affected by contamination by fluid types that they may typically be exposed to. Aircraft and vehicles may be subjected to regional operational processes not covered by the test fluids in BS EN 3909.
Note 1: It is not possible or practical for BS EN 3909 to address all the fluids used in aerospace applications.
Note 2: It is the responsibility of the customer to ensure that the products used are fit for their environmental applications.
Who is BS EN 3909- Test fluids and test methods for electrical and optical components and sub-assemblies for? 
BS EN 3909 on Test fluids and test methods for electrical and optical components is useful for: 
- Electrical component manufacturers
- Electrical testing agencies
- Safety regulatory bodies
- Quality control laboratories
Why should you use BS EN 3909- Test fluids and test methods for electrical and optical components and sub-assemblies?  
The fluids listed are representative of those commonly used and encountered in airborne and ground operations and align with the requirements of fluids susceptibility of ISO 7137. This shall not be considered an exhaustive list and additional test fluids may be instructed in the product standard, against which compliance needs to be demonstrated.
BS EN 3909, when used in conjunction with the test requirements defined in the product standard shall be considered the starting point to test a component to determine its minimum performance capability when exposed to the fluids listed.
Test results obtained from a number of sources over a considerable period of time have shown that, in many cases, widely varying results can be obtained when using fluids that are used in service. The practice of specifying fluids based on performance criteria rather than their constituents can mean variations in test results between batches of the fluid obtained from different manufacturers, or even from the same manufacturer.
For this reason, the BS EN 3909 Standard recommends the use of "standard test fluids" which are specified by their constituents and contain the chemicals that may be found in commonly used fluids.
Where equipment may be exposed to fluid types that are not covered by BS EN 3909 or where specific test fluids are considered to be necessary, the product standard shall identify the particular fluid required. If a manufacturer chooses to include additional test fluids (e.g. to satisfy a customer requirement), they do so at their own risk.
