EN 966 specifies zones on the helmet for impact testing and each helmet is impacted in two zones (1 impact per zone) - 1 impact with a flat anvil and one with a kerbstone anvil. The energy of impact depends on the size of head form being used (the smaller the size the lower the energy).

For the smallest head form (size 'A' - 500mm) the impact energy is approx. 46 Joules and for the largest head form (size 'O' - 620mm) the impact energy is approximately 90 Joules. Five helmets are tested on the smallest impact headform for its size range and five on the largest.

Reviewing information on these standards (some old & some current):

Snell M2000 2 impacts 150 & 110 joules (motorcycling) 
Snell RS-98 2 impacts 150 & 110 joules (recreational skiing) 
Snell K-98 2 impacts 150 & 110 joules (karting) 
EN Reg.  22.4 1 impact 132 joules 
Snell B-95 2 impacts 110 & 72 joules (bicycle) 
DOT FMVSS 218 2 impacts 90 & 90 joules (older motorcycle) 
CEN 1077 1 impact 69 joules 
ATSM F1447 2 impacts 98 & 57 joules 
CPSC 2 impacts 98 & 57 joules (bicycle) and 
EN 966 2 impacts 90 & 90 joules
There are many sites that describe helmets on the web. A good one is for bicycles check: http://www.bhsi.org/guide.htm. This describes what is actually required of the helmet in a crash.

The SNELL sites describes the tests for motorcycle helmets, but is a bit dry: http://www.smf.org/testing.html, site also has ski helmet info, etc.

New research into motorcycle helmet performance can be found here http://www.cordis.lu/cost-transport/src/cost-327.htm. Heavy going for the casual reader though. This does show that bikers tend to whack their head on cars and the ground, hard, a lot. From personal experience I can confirm this.

This is a good page on bicycle helmets again: http://www.dft.gov.uk/ stellent/groups/dft_rdsafety/documents/ page/dft_rdsafety_507998-06.hcsp

Key sentence there could be "There is little evidence that helmets of different standards perform better in protecting the wearer."

I also like "The amount of the head that a helmet can protect is driven by the needs of the bicyclist. Ideally, the helmet should provide protection against impacts anywhere on the skull, including the face, but the need for the wearer to see upwards and sideways, hear traffic and be able to tilt their head back when riding because of the seating position limits the extent of coverage significantly. "Substitute" pilot as required. If you have a major high impact whack on rocks, a motorcycle helmet might be the best for you, but for most people a "wearable" safe compromise is required.

Bike helmets are designed for big whacks where ski and bicycle helmets would seem to be designed for whacks closer to the stuff us HG pilots might encounter.

The CEN 966 standard is designed for us by people who know more about this kind of thing than most. It is not the same as bicycle, ski or motorcycle helmet standards, but is similar to them all (shock absorption, penetration resistance...). They must have a clue as to what kind of crashes we have. Bottom line is probably any standard is better than none, but a big heavy helmet is not better than a lighter one of the same standard. And we all know whatever you use change it every few years.

What to I wear? Hang gliding require a cranked upward flexing of the neck muscles. Heavier road helmets soon wear out the pilot's muscles and decreasing situational awareness (you are hanging your head and eyes down from fatigue). Since most of the risk of injuries in hang gliding involves the face the extremely lightweight fullface Kevlar Charly Insider is the best choice.

Check out this link about helmet replacement: http://www.helmets.org/replace.htm and this on helmets in general http://www.helmets.org/other.htm