Short circuit on AC power inlet

Question

iMac Intel 27" EMC 2429

Modello A1312 / metà 2011 / Processore Core i5 da 2,7 e 3,1 GHz o Core i7 da 3,4 GHz, ID iMac12,2

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Marcel @marcelzee

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Postato: 8 mar 2020

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Short circuit on AC power inlet

I have a 27 inch mid 2011 iMac (EMC 2429). Yesterday I came home to find the UPS that the iMac was plugged into in an alarm state, en the AC circuit that powers the UPS tripped. It turned out to be the iMac. Every time I plugged it into a wall socket, the AC power tripped. I removed the power supply unit following the instruction on the iFixit website. There is nothing visibly wrong with the PSU, and when I measure the resistance between live and neutral on the AC power inlet cable, I get about 1000 Ohm. I would like to order a new PSU, but before I do, is there anything else I can do to convince myself that a new PSU will solve the problem?

I have verified that the fault is not anywhere in the cables before the PSU. The fact that the resistance measured 1000 Ohms on a DC multimeter was a piece of misleading information. The impedance presented to a 50 Hz AC current was obviously very low, because it causes the circuit breaker to trip every time. So I am going to go ahead and order a new PSU from iFixit.

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Dan @danj Rep: 478,9k Self employed contract worker 35 329 970 · Answer 1 · 2020-03-08T09:20:57-07:00

A tripping breaker is a telltale sign there is a short. So the pathway from the outlet to the power supply as well as the supply is where you need to focus. Try a second line cord, Check the systems socket to make sure its not damaged or has a short. Using a Ohm meter measure the resistance from the prongs of the socket and the pigtail which goes to the power supply. You should have good continuity along each leads pathway and nothing across the the leads.

While you weren’t home did your house loose power from the grid (outage) or encounter a lighting strike? While you had a UPS did you have any peripherals plugged into a non-isolated outlet of the UPS? As an example you have a wired network connection or have a printer plugged in on a different outlet. If you do a surge could have traveled along their pathways back to your system.

I would still check my outlets to make sure they are properly wired and grounded. Go to the hardware store and get a simple outlet checker as the easiest way to quickly test them. Also check your ground line from your breaker panel to the grounding point either a metal water pipe leaving the building or one or more ground rods. Make sure they are in good shape and the metal to metal contacts are not green with corrosion. If you do you may have a bad device which is mis-wired leaking power to ground.

You’ll still need to replace the power supply in the end ;-{

iMac Intel 27" EMC 2429

Short circuit on AC power inlet

Update (03/09/2020)

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