Electrical – New light switch causing other switches to not work

It's possible you have a short. From your question I don't know if you disconnected D by removing the connection at C or D. You could try this:

I'll assume since you did all this you're basically OK working with electrical. Get yourself a non contact voltage detector and use that through this whole process to confirm things are dead before you touch them.

• With the breaker off, and the switch off, look at the wiring in the box at D. Look for any place the bare conductor of the hot wire could touch the bare ground wire, the neutral, or metal inside the box. Look for nicks in the wire, places where stripped wire extends past the wire nut, etc.

• If that doesn't reveal anything, with the breaker and switch still off, remove the last light fixture (D) and cap the hot and neutral wires with wire nuts. Turn on the breaker, turn on the switch, see if it trips. If so, there is probably a short between the hot and neutral wires or hot and ground wires in the cable between C and D.

• If it trips with the D light disconnected and the wires capped, turn the breaker and switch back off, remove the C fixture, undo the wiring and cap the wires that feed D, and reconnect C as if it was the last light in the line. Turn the breaker back on, turn the switch back on, and confirm that the breaker doesn't trip.

If it does, then something's not consistent. I'll assume it doesn't.

• Turn off the breaker and switch again, and test for continuity between the hot and neutral wires from C to D, and between the hot and ground from C to D. If you see continuity with the wires disconnected and capped at both ends, you've found a short. The wire from C to D will have to be replaced.

If this doesn't lead you to a fix, you might want to call in for reinforcements, and leave that breaker off until they arrive.

From the color of the wires I assume you are in either Australia, New Zealand or the UK if the home was built prior to 2004 which it certainly looks to be.

The red and black wires are your hot and neutral lines respectively. Since there is a big junction of three sets, I am guessing they are feeders and are always live. Your light is connected across them which is why it remains on. That being said, what is that blue wire doing by itself?

It looks like the lonely blue wire is paired with the brown wire. They look like they form whats called a switch loop. The brown wire is the feed to the switch from the red hot wires. The power is then fed back via the blue wire.

If you have a voltage tester, turn the switch on and test from the single blue wire to the black leads. If it shows power, turn the switch off and repeat the test. If you don't have power, you found the switch leg which is the blue wire. You then move the brown wire of the cable going to the lamp to that lonely blue wire on the terminal strip.

If you don't have a voltage tester, don't worry. Just move the brown lamp wire to that blue wire and turn your switch on and off. Even if its the wrong wire, nothing should blow up.

Edit: added a picture for clarity