I have written a short article explaining the differences between bathroom extractor fans and the safety issues you should be aware of when selecting one for your own home, all extractor fans are the same size and perform the same job but they cant all be used in the same locations here is why.
The first thing you need to be aware of is a bathroom has different zones within it, these are displayed in BS.7671 the Electricians Wiring regulations and gives all the relevant dimensions that an Electrician uses to determine in which Zone your Bathroom Extractor Fan is within and why it is within that Zone.
"Basically put the nearer to the Bath or Shower the higher level of protection from water ingress is needed"
Now this is where the makes and models vary, a fan that you can purchase from your local DIY store for a very modest price are very rarely of a suitable IP rating to be close to your shower or Bath, if it was to become splashed there is nothing to prevent water getting on to live parts causing you to get a Electric Shock while standing in water.
If you have a Electrician source and install the Fan they would firstly determine what Zone your fan is located in and source a suitable IP rated Fan which is able to withstand the expected splashing the fan will get in the given area, you would also get a better quality fan which would last longer be quieter and would be only about £20 more than the basic entry level fans.
Just because a Electrical item works does not mean it is safe, always use a Qualified Electrician for Electrical Work