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Transformers for 240V mains to 12V lights: I have several dead transformers (not sure what type exactly) hooked up in parallel to a domestic mains circuit. I can't easily get at them to change them, they're in a ceiling & I don't want to rip the whole ceiling down!

However, I can add new transformers (a spot of key-hole surgery) to the circuit, effectively bypassing the dead ones, and it's easy to rewire (new) lights to the new transformers. The old, dead transformers remain in parallel with the new ones. I will put the old12 V ends into connectors, for safety and tidiness.

Q1: What are the problems of simply leaving the old ones where they are in parallel with the new - any safety issues? Q2: Could the demise of the old transformers have been caused by replacing the switch with a dimmer - they worked initially. (The new transformers are all dimmable).

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  • \$\begingroup\$ Regarding Q2: This is quite plausible. Dimmable lighting transformers were not very common until a few years ago. \$\endgroup\$ – ARF Oct 7 '12 at 8:03
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As to Q1 - since you don't know how or why they died, you have no idea whether they are still taking some power from the mains or not.

If they've blown an internal fuse or something, then they might be effectively disconnected, so no problem.

If they've broken in some other way, you don't know if they are overheating or just sitting there passively doing nothing worse than slightly increasing your electric bill.

In theory in order to be rated for buried installation they SHOULD fail in some way that is 'safe'. On the other hand, even 'safe' stuff fails unsafely sometimes.

I personally would get them out of there, or at least cut a hole where they attach to the mains and cut & cap the wires appropriately.

Also, from a 'do it for the next guy' standpoint: powered, but useless transformers seem like an invitation for something to go horribly wrong for a future repair.

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