Secondary risk: You build a dam in a river to create a lake, but the water ends up going around the dam.
Residual risk: You build a dam in a river to create a lake but it floods more land than expected when it rains more than normal,;you accept this, as it does not happen every year and no houses are affected.
The big differences between the two are this. Secondary risks arise from a plan that was put in place to deal with another risk; they all pop up in response to another risk. Whereas a residual risk is a risk that is essentially left over and remains there or are just accepted.
To summarise, a secondary risk comes from implementing a plan to another risk, some kind of action is usually done for it. While a residual risk s are sometimes expected to remain-in place even after dealing with a risk, and action may or may not take place to deal with it; these can be dealt through a contingency plan if needed.
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