Electrical – Why would an iMac computer cause lights on a circuit to flicker when it goes into sleep mode

electrical

Background

  • My wife has an iMac computer that is approximately 3 years old. When the computer goes into sleep mode the lights on the circuit the iMac is plugged into flicker.

  • My house was built in 1963 in Missouri. The house has 100 amp service all wiring is copper and grounded properly.

What I have done

  • Originally I didn't realize the iMac was causing the lights to flicker. So I looked at all the wiring on the circuit looking for a loose neutral. Nothing was loose anywhere.

  • I looked at the breaker in the panel to see if it was loose. It was not loose but I replaced the breaker anyway.

  • I tested the voltage on the panel and it appeared to be between 119 and 120 so I assumed the box was ok.

  • The flickering still persisted so I called an electrician. Unfortunately the flickering was not happening we he arrived (at this point I still didn't know it was the iMac). He looked for loose neutrals and checked the panel. He said the panel and circuit looked fine.

  • One morning my wife walked into the office and started using her computer. I was in the bathroom watching the lights flicker (and losing my mind) when the lights stopped flickering. Alas, I figured it out and could reproduce the issue. Whenever the iMac when into sleep mode the lights flickered. Typically my wife would be using the computer and let it sleep overnight. This is why I notice the circuits flickering only at certain times!

  • I plugged the iMac into other circuits, set it to sleep and the lights on the other circuits flickered as well! So it was definitely the iMac…

  • I take the iMac to the local Mac shop. They think i'm crazy as they have never heard of such a thing. The run it through all their tests and it passes w/ flying colors. They contact apple. apple declares it impossible and it must be something w/ my wiring. They said they could replace the power supply in the iMac (around 150 or so, not steep) but it is probably not the issue.

I posted this question on apples forums and didn't hear anything. I found someone on google complain of the same issue but didn't see a resolution.

Any thoughts from the electrical experts? I'm going to run the iMac over to the neighbors house and if his lights don't flicker i'm going to assume it is my panel, as it causes flickering on all my circuits in the house.

I do know that some of the circuits in my house have new copper wiring where another light fixture or two was added. Newer copper wiring shouldn't have any problems with older copper wiring should it?

Best Answer

A few things that can cause this:

  1. Faulty iMac supply.
  2. "Hot fault" in the wiring... A bad junction somewhere, or a physically damaged wire, or wires that are too small for the current draw (and the fuse or circuit breaker!).
  3. Exotic interference. Is there an electronic switch or timer, or a low-quality UPS, or one of those infernal "SpySmart Meters" somewhere?

Note that for items 1 and 2, you should see a voltage drop -- measured at the outlet that the iMac is plugged in to. A drop from 120V to 110V yields roughly a 16% drop in incandescent brightness. A drop from 120V to 100V, causes about a 31% drop in brightness.

What iMac model is it? What specs does it list for power? iMacs usually run at about 80W to 200W, but I read once that they can have short, peak draws of 1000W.

A surge protector will do nothing in this case, but a battery backup with brown-out protection will protect the computer and may lesson the flickering.

If it is a "hot fault" type problem, there is a very real danger of fire. A fuse or circuit breaker cannot guard against such faults either, but you should immediately verify that the circuit breakers are properly sized for the wire gauge(s) of each circuit (the smallest wire gauge, if more than one was used).

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