Brought a street lamp fitting and it doesn't requirement an dust the existing cable have an land what should I do next to it?
Answers: Those who tell you give the brush-off grounding are wrong.
It won't hurt you to let the fixture 'float' until something
go wrong. Then it can get serious.
If your fixture "doesn't have need of an earth" , then it's
grounded through the strap you bolt it to when you
install it.
Connect your existing ground chain to that strap.
(Most will have a green grounding screw.)
DO NOT CUT IT OFF! If it is surrounded by the box, and the fixture has no where on earth for it to go, later just roll it up and tuck it surrounded by the box. If you ever change the fixture to one that wants a ground, then you'll hold it. If the fixture you have presently is metalic, then you could still use it wrapping it around a mounting screw. That is if you a short time ago want to use it.