[allan]

Continuing the points raised by Argus25...
My day job is to repair circuit boards for lifts and these generally fall into two categories.. well designed and poorly designed. Of course both work and do their jobs but the former include loads of protection devices which are fitted to deal with almost any theoretical eventuality.. including engineer error (which is very common). So a well designed circuit has those extra diodes to deal with reversed current flow plus a multitude of TVS diodes, either C or CA. C types will prevent damage if a higher than normal DC voltage is connected and CA if the higher voltage can also be reversed. The CA types can deal with AC voltages to prevent connection of too high an AC voltage. These TVS diodes can be rated in Kwatts and cannot always be associated with a (local) fuse other than printed circuit track. A good designer will also consider varistors and thermistors.

On the topic of charged capacitors.. a good circuit will have a bleed resistor and a poor circuit will not. I suppose a good circuit suggests a future repairer might step in (health & safety). A bad circuit designer pays no thought to repairers and I include in this group the designers of most Roberts Radios, not charged capacitors.. but rat's nest construction!
Allan G3PIY