A few things that can cause this:
- Faulty iMac supply.
- "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!).
- 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).
Arc-Fault Circuit Interrupter (AFCI)
An arc-fault circuit interruption device is designed to detect dangerous arcing within the protected circuit, and open (turn off) the circuit to prevent damage caused by the arcing. It does this using special circuitry to analyse the electrical characteristics of the circuit, looking for characteristics that match specific pre-programmed values. If the AFCI detects suspicious goings on, it opens the circuit.
AFCI breakers are typically combination devices, meaning they also provide similar overcurrent and short-circuit protection to a standard breaker.
Installing a combination AFCI breaker on a circuit containing knob and tube wiring would be a great idea, and could potentially prevent a fire.
Ground-Fault Circuit Interrupter (GFCI)
Ground-fault circuit interruption devices are designed to detect ground-faults, and open (turn off) the circuit when a ground-fault is detected. They work by using a current transformer (CT) to detect current imbalances between the ungrounded (hot), and grounded (neutral) conductors of a circuit. This blog entry might help you understand how GFCI devices work.
GFCI breakers are typically combination devices, meaning they also provide similar overcurrent and short-circuit protection to a standard breaker.
Installing a GFCI breaker on a circuit containing knob and tube wiring, probably won't provide any benefit. GFCI devices are designed to prevent electrocution, not to protect the wiring.
Combination AFCI GFCI Circuit Breakers
Circuit breakers that provide AFCI, GFCI, overload, and overcurrent protection are becoming more widely available. If you can find them for your panel (and afford therm), these would be the best option.
Best Answer
The circuits are “crossed” at one, or possibly more, points. Be very careful when sorting this out.
This is how people get killed by electricity as they think they have isolated something but it is still live.
The method to find it is to disconnect at each junction until the relevant part goes dead and label wires to work out what goes where.
Had a similar problem where a site had been wired with additions over 30 years and most was not labelled... the law changed and it all had to be identified. Relatively easy as it was between buildings, switch of a breaker and 15 seconds later the phone went : “we have no power.. oh, where are you?” That was that breaker labelled... Good fun, but we did not mention those power cuts were deliberate... :)