What's a TS blade guard for?

OK, a TS blade guard protects the user from a spinning blade. But, I ask this because of the overarm blade guard/DC combos. Is the blade guard on those arranged with a counter-balance so that it sits on the table and rises up when a piece of wood starts sliding towards the blade? Or, is it manually adjusted before starting the saw so that it will clear the wood? If the latter and one is cutting a 3" thick piece of wood, is having the guard 3+ inches above the table an OK thing after the wood has been pushed through -- which I ask of the people here who believe in blade guards?
I am thinking of a very simple home-made device attached to my 8' ceiling to approximate the store-bought systems and if the guard does not have to auto-move as the wood approaches, then the design can be much simpler. Thanks.
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PS: Why do the guards have to be made out of polycarbonate or the like? How about "shatterproof" polyethylene -- as in a clear Tupperware-like container?
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username is amgron ISP is clara.co.uk Website www.username.clara.net
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It's for de-blooding your TS top, blade, and DC system.
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Tht is a REALLY good question since most people do not use theirs. Maybe it is just another way for manf.'s to charge us. Now, a GOOD table saw guard is worth it's weight in gold like the Brett, etc. But the OEM is junk. Hope someone here can correct me.
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