iPad "Not Charging" with 3m USB-dock-connector cable
I know Apple's answer - 'our cable works, there is no problem'.

IN SUMMARY - you either need to use Apple's own cables or the decent quality PC World versions or Asda or something, that are not really cheap but are two thirds the price of Apple, or half or something.  

Really cheap clone cables will NOT work.  Can't explain it, but know it is fact.  My guess, they are not soldered properly inside the lightning end, and ipad depends on quality of connection being really good, maybe?  Any money, not all the connector bits on the lightning are connected physically to the other end.  ie it gets complicated.

Explanation follows:

I have found that Apples' own cable supplied (lightning) tends to kink near the ipad end, and horribly shorts out resulting in two main issues: a - ipad reboots or similar alarming thing b - it doesn't charge c- the charger that is on the plug gets very hot indeed, but the give-away is that unlike when charging the ipad properly, the back of the ipad where the battery/charging components are, remains completely cool to the touch.

So, it's the difference with your 3m cable to Apple's one.  I have bought several 'clone' cables AND NONE OF THEM WORK.  Short or long, makes no difference.  I keep them in a bag marked 'cheap useless aftermarket cables'. If you go to PC World or somewhere similar that sells cables for over a tenner apiece, they'll probably work.

If the cable costs 2.50 gbp ir probably WON't work, that's my experience.

You don't reveal where you bought the non-apple cable?  ASDA and places like PC World, their cables will work even non-apple ones.  Others might not.

Last, the earthing (ground) thing.  Generally, for signal-level power, only one end of the ground cable is earthed.  I know the house-wiring norm is for all of them to be tied together - so I guess it depends which way Apple leaned on this one.  Lightning adaptor of course, reconfigures uses of each contact, so which one is ground may not even stay the same?