Explain why RCDs fitted into extension leads or incorporated into plugs are forbidden

Question

this question is inside a mentor guide at the place i work for someone to be signed off as a fully competent electrician. 
 
 none of us can think of any reason why this may be the case, can anyone else?

gkenyon · Answer

If I were advising on such a policy, I would be highly likely to begin with the opinion that BS 7071 (because of its age) would cover only Type AC RCDs, and considering the latest knowledge that we have about those devices ("state of the art knowledge" that we are required to consider in risk assessments) ... that they ought only to be used except for purely resistive loads, and even then only where those loads are "fixed", i.e. not portable, socket-outlets, etc. It's also highly unlikely products to BS 7071 alone would have undergone EMC tests for the RCD function (although it's not beyond the possibility that a responsible manufacturer would in fact run additional tests). 
 Based on that, I think it is arguably the right thing to do, to enforce this policy until BS 7071 is updated to account for Type A RCDs and EMC testing as with the other RCD standards.

Explain why RCDs fitted into extension leads or incorporated into plugs are forbidden

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