I used to know a guy back in the 70's who ran a disco. He used one of those cables with a 13A plug on each end to power his console..! These things are bad enough at 120V, at 240V they really are suicide cables 

EEK!!! 

I heard them called suicide cords before and apparently people that string their Christmas lights backwards sometimes ask for them.

I don't have a generator but if I did I'd probably install a 30A inlet on the outside wall and then wire it in to the breaker panel using a inlet breaker and one of these interlocks that Mike mentioned.

Regarding the christmas light comment, years ago when I worked in a hardware store while in high school we had a guy come in who had his fifty-foot tall pine tree decorated with C-9 style lights and the geniuses put the male end at the top of the tree! he asked us for the materials( 1 foot of wire and 2 male plugs) to "fix" the problem. we told him "NO WAY!" and explained to him that if the other male end at the top of the tree touched anything the entire tree could become live and fry someone!. he decided to call the installers back and have them install lights correctly GOOD MOVE!

if the other male end at the top of the tree touched anything the entire tree could become live and fry someone!

It never would. The tree is not good conductor, and would not pass any significant current downwards. Then even if it would be conductive, it would still form a voltage gradient along the tree height, with the high voltage available only higher up the tree

Still wrong but for other reasons :

 - The plug sticking out up there (or any plug unplugged in the middle of the string) could kill an animal such as bird or squirrel

 - If it gets wet and then dries back while power is applied, it could start tracking an arc and set fire to the tree

 - If anyone unplugs an intermediate connection between sections of the string, they expose and handle a live plug, but might not even be aware of the unusual situation because there are both plug and receptacle present



Anyway, it is possible to bring to atleast reasonable safety in 2 much simpler ways :

1. Give him a longer normal cable, so he could put it through the tree all the way up and connect it at the top

2. Give him his deadth wish cable + an ordinary extension cable receptacle + pack of zipties. The death wish cable - for use as intended. The receptacle - to insert on the plug at the top, for enclosing the plug pins (without wiring anything to the receptacle). The zipties - to secure all "wrong" connections so they dont unplug accidentally