mobwell, you mention another set of PIR controlled lights elsewhere.
What I suspect is that this FCU also powers these (via the Red and Blue in the other 3C+E). And this yellow core in both cables is a "bus cable" to link the switched live outputs of the 2 PIRs, so that when one PIR detects motion, it turns on ALL of your lights, by providing power to the switched live side of both PIRs.
Please open up the other PIR and see if that is where the other 3C+E goes! I strongly suspect this is the case!
If this is the case, you need a light fitting here that has 4 terminals - one being the switched live output from the PIR. If yours has no such terminal, then you can just omit the yellow wire, make it safe as it will be live when the other PIR switches on.
What I suspect is that this FCU also powers these (via the Red and Blue in the other 3C+E). And this yellow core in both cables is a "bus cable" to link the switched live outputs of the 2 PIRs, so that when one PIR detects motion, it turns on ALL of your lights, by providing power to the switched live side of both PIRs.
Please open up the other PIR and see if that is where the other 3C+E goes! I strongly suspect this is the case!
If this is the case, you need a light fitting here that has 4 terminals - one being the switched live output from the PIR. If yours has no such terminal, then you can just omit the yellow wire, make it safe as it will be live when the other PIR switches on.
