where to attach ground wire in plastic box with heat detector

I am attaching some smoke and heat detectors to ceiling boxes. The boxes are connected by 12-3 where the red wire is for the interconnect between alarms. The 12-3 is fed off of a standard bedroom circuit.
The 12-3 also has a ground wire, of course. However, the ceiling boxes for the alarms are plastic. And, the alarms do not have screws to attach ground wires.
Where should I put the ground in each plastic box? I will pigtail feed-in and feed-out grounds in each box. Should I do more than this? And what about the ground in the final alarm box, where there is only a feed-in wire? Should I just wirenut the ground and let it sit in the plastic box?
thanks,
mh
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Mike wrote:

Note: the following is a guess ...
Do the alarms have metal ears used to secure them to the box? How about putting a ground wire under one of those ears or under the screw securing it?
--
Joseph Meehan

Dia duit
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There is no ground attachment for the smoke or heat detector. When all you have is non conconductive material, you don't connect the ground. If there is more than one cable in the box, all ground wires must be connected together

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Where I live all plastic boxes have a metal strap that has a screw for to the ground wire and connects to one of the fixture anchor screws.

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