The answer, as far as I can work out and from what I have read, essentially is that an inbuilt harness (ie one that comes inbuilt into a child restraint) because it is part of the child restraint, spreads the load from impact differently (to an H Harness) and more evenly across the child's entire body... whereas an H Harness is attached directly to the vehicle by the tether strap/anchor point and is more rigid... this means on impact, because the child is held so tight, the load is concentrated on the neck and head, rather than being disbursed more evenly.
so (with weight, age and height in mind for the restraint)
preference (is in rear seat) in order of:
- reversible car seat with inbuilt harness (use in either position for weight approp)
- then forward facing (non convertable) car seat with inbuilt harness
- then full back and sides boosters with lap sash only
- then adult seat with lapsash
- half boosters are no longer recommended (no side impact protection), although they are not illegal and
- H Harness/HHarness with booster with anti sub clip should only be used in cases of lap belt only and where no better seating is available.
NB: Thankyou too to those on EB who first answered my questions and took the time to post links, and where almost all this link information comes from.
Results of study - why the H Harness isn't as safe as we thought
(link currently not working... will repost correct link when I have it)
The results indicate that the simple addition of a harness system to a lap belt, although reducing excursion of the head and torso, may lead to neck forces that are much higher than those seen in the lap/sash belt configuration. There are therefore grounds for concern on the performance of the lap belt/child harness configuration. The comparatively high readings for head and neck forces and accelerations indicate the need for some attention to design. Unlike the configuration in a forward-facing child seat, the shoulder straps of the child harness when used with a lap belt are anchored directly to the vehicle structure. In a child seat, the harness is attached to the seat and the seat attachments are separate. The lap/harness configuration is therefore much stiffer in its reaction to crash forces, and this may be why the dummy neck loads were higher in these tests. This suggestion is supported by outputs for chest accelerations, which were also highest in the lap/harness configuration.
Kidsafe No longer recommend H Harness - also half boosters info
Michael Paine on child safety
- Avoid using separate child harness with a booster, unless there is only a 2-point seat belt available.
- For children over 6 who are too large for a booster seat, a 3-point adult seat belt in a rear seat.
- For children over 6, where no 3-point seat belt is available in the rear seat, a 3-point adult seat belt in a front seat [IF allowed by the new laws]. A passenger airbag will provide extra protection and is not dangerous in Australia (updated URL) - provided the child is restrained by the seat belt and does not lean forward. In any case, adjust the seat as far back as possible but ensure that the sash (upper) portion of the seat belt is against the shoulder and not the neck. Never let a child stand or sit close to the dashboard - airbag or no airbag.
Comprehensive Listing of all child restraint information, explanations, when to change seats and what's safest.
Accessory Child Harness, Safety Concerns and Correct Use
Accessory Child Safety Harnesses - Do the benefits outweigh the risks?
of course, this is just my understanding and the reading I've done. If in doubt consult the relevant authorities/information in your state, do your own research and reach your own conclusion, for your Family.