I have 2 high pressure sodium lamps, 400 Watt and I read that parallel to the balast you should put a capacitor to keep the line clean.

My questions:

a) is it important?

b )if yes how big capacitor?

1. No, it's power factor correction, unless you're on an industrial tariff and have a lot of lights it will make no odds.
2. Read the documentation, if it doesn't tell you (or you have no documentation), leave well alone.

For a 400W lamp expect to use a 40uF 250VAC capacitor, but as I said, it will make no odds in a domestic / light industrial environment.

By the way, it goes across the supply terminals after the fuse (if fitted) not across the ballast (which will fry your lamp or the ignitor or both).

Also note that if your lamps have active or electronic ballasts they already incorporate PF correction.

1. No, it's power factor correction, unless you're on an industrial tariff and have a lot of lights it will make no odds.
2. Read the documentation, if it doesn't tell you, leave well alone.

I'll leave it alone......only 2x400 watt and we are on a normal 3 phase meter.

The instruction are approx. the size of a stamp :-)))

I wonder if it matters if L and N for the balast and starter get exchanged. (when the lamp isn't hard wired, but with plug).

In my limited understanding of electric it should be the same in AC. If there is no ground contact and you would hold the L line, you would see the N as fluctuating AC source. So flipping it shouldn't matter.

But I am not sure if I overlooked something in my limited understanding.

(my staff was amazed when I started the 400 Watt bulb on my table......I was almost sorry I didn't bought the 800 Watt)

Swapping L & N will make no odds to the lamp.