Lightning Arrester

There is no minimum.
There's not much point in having two very close, but due to the unpredictability of lightening who knows?

JMO
I'd guess that 15 meters might be a resonable figure as I couldn't see much point in having 'em closer, unless of course you have two pointy things which you are trying protect which happen to be closer.
KrisDel^TM virtual products inc' have a great range of pointy things.
Del

The more important question is what is the maximum distance. The cone of protection is 45 degrees about the apex of each arrester. NFPA has several standards that would apply to most applications, however.

Make a drawing.

Put your 40 m tower up.

Draw a cone represented by 2 lines from the top and at 45 deg from the vertical, until you reach the ground.

Position all the structures you want to protect, in relation to the tower, (distance wise, and mark their Apex points.

All these points must fall within the cone you drew.

If not, then position the next tower such that its cone will cover the apex of the unprotected structures.

This is roughly your method (simply!) .... some designs use less than 45deg, so for extra protection (and less errors) take 35deg.

I hope you got it!

you can find some information in NFPA-780

please refer BS/IEC/LPI