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I would assume that there was a waste disposal unit attached to the sink which the Acid because its metal burnt through also there was no trip device on their CCU.
Lead pipes are still to be found in old installations. Using acid to clear a blockage effectively creates a lead-acid battery. These are capable of high discharge rates. Touching one can in certain circumstances can allow the current to go through the body directly to earth creating an electric shock for the person.
A bit like dropping a switched on hair drier in the bath when you are in it.
Oh dear , that is so scary thanks for explaining it .