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They can be if they are ordered and manufactured with a proper physical locking system incorporated into the curtain and barrel arrangement. There are some models of electric aluminium and steel roller garage doors which have either LPS1175 level 1 or even level 2 security ratings, so it is possible to have a security rated roller garage door.
The worst kind of electric roller garage doors simply use wha can only be compared to a drawbridge for their opening and closing, where the roller door curtain is lifted and lowered but has no real form of the curtain being held down when closed. This means you can lift the curtain up with the resistance only being the weight of the curtain itself.
Another shortfall in security for a roller shutter door is having a weak, thin curtain combined with small side guides. The deflection in the curtain means the curtain can be easily pushed out of the guides and therefore entry can be gained.
A strong curtain combined with good sized and well specified guides and then a further system to properly hold the curtain down when lowered will all combine to provide a roller garage door with good security.
Best electric roller garage doors for security are from the Seceuroglide range. A range of roller doors developed for over 30 years now and with origins from security window shutter systems. These were built with security in mind from the outset.