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I have a ~20 year old BOSCH TKA8011 coffee maker. (Of the filter coffee kind.) It is fairly simple: it has an on/off push-button, a decalcify push-button, and a low-water-level indicator which triggers a slow brewing function. That's all; no delayed start function, no LCD display, no bells and whistles.

Actually, I would be much happier with it if it only had a mechanical on-off switch, but apparently a mechanical on-off switch is a low-end-appliance-feature-only, so if you want a high-end feature like a removable water tank, then you have no option but to buy an appliance with a whole bunch of useless electronics. But I digress.

Some time ago while washing it I accidentally poured copious amounts of water onto it, which very possibly got inside. It stopped working. It did not trip any circuit breaker, it was just dead. I dried it for several hours using a fan, then it started working again. A few weeks after that, I left for a couple of months. The day before I returned a cleaning lady had visited my apartment, I have no idea what she may have done. When I tried to make coffee, half-way through the brewing process, the safety circuit breaker dropped.

Since this "safety circuit breaker" thing might have different names in different countries, let me clarify what this is: it has nothing to do with excess current, it breaks when it detects a leak of electricity to the ground, probably even a very tiny leak. It has a button that you can press to test that it works.

I experimented enough to know for sure that what triggers the safety circuit breaker is the coffee maker and not any other appliance or the wall outlet.

I gave it another round of drying, but the situation remained the same: the safety circuit breaker would drop every time I tried connecting the plug of the coffee maker to the wall outlet.

I opened up the appliance, (why do they make it so hard to open them up?) and examined it. The 230V go directly onto the PCB, while the ground goes everywhere else. On the PCB they have an incredible amount of electronics for a coffee maker; I saw electrolytic capacitors, ceramic capacitors, a transistor, a zener diode, a bunch of blocks that are probably power diodes, several resistors including a power resistor, and a bunch of surface-mount components that I could not identify. It has a tiny heating element that is mounted against the hot plate using heat-conductive paste like the one used between CPUs and heat sinks. The paste was probably dead after 20 years, but I am pretty sure that's not the problem.

Believe it or not, the mere act of opening up the coffee maker, without actually doing anything else with it, improved its condition. I was now able to plug it in, and the safety circuit breaker would not immediately drop. It would only drop if I went ahead and pressed the "Start" button.

I removed the PCB from its place and I applied a generous amount of WD-40 on both sides of it; no further improvement.

I disconnected the heating element; still no further improvement.

So, reluctantly, I resorted to the one thing I knew would solve the immediate problem of not being able to make coffee: I disconnected the ground. Of course, with the ground disconnected, the safety circuit breaker is not triggered, because there is no ground-leak for it to detect.

Here are my questions:

  1. How unsafe is this? (Is it okay as long as I am present when the appliance is operating? Is it a "better solve it within a week" type of thing? Better solve it within a month? Evacuate apartment building immediately?)

  2. Any ideas / hints / guesses as to what may be wrong and what I could do to properly fix it?

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    You have a 230volt machine with a possible(very likely) voltage leak that could kill/maim someone and defeating the safety controls to prevent it from happening. You probably got more than your moneys worth from having that machine 20 years. Water and electronics never play nice together.
    – crip659
    Mar 14, 2022 at 17:36
  • @crip659 yes, I know. But at least I can make coffee while shopping around for a new coffee maker. My questions are taking for granted the fact that this is unsafe.
    – Mike Nakis
    Mar 14, 2022 at 18:47
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    Would at least leave it unplugged, except for when making coffee(no leaving it on to keep warm) and do not touch any metal parts when plugged in, hopefully it is all plastic.
    – crip659
    Mar 14, 2022 at 19:02
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    Instant coffee is cheaper then replacing you!
    – Gil
    Mar 14, 2022 at 20:53
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    "And the nominees for this year's Darwin Award winner are..."
    – FreeMan
    Mar 15, 2022 at 11:59

2 Answers 2

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Your coffeemaker has current leaking to ground. This is an incredibly dangerous situation. Full stop. Ground fault circuit interrupts save human life, bypassing it endangers everyone who interacts with the faulty device.

This is incredibly unsafe. That the device creates a ground fault the moment it is plugged in, but once disassembled it can be plugged in without creating a ground fault means you have to assume that when it is plugged in the chassis creates a ground fault. Let me repeat: since removing the chassis from the PCB eliminates the ground fault you have to assume any metal parts of the chassis are energized when it is plugged in.

Of course, this may not be the root cause. Maybe the PCB is old and cheap and the water damaged it in such a way the the ground plane can make contact with an energized plane when it is squeezed by one of the screws connecting it to the chassis. Maybe the water corroded something, maybe it left mineral deposits that created a short. But until you can ascertain the root cause with certainty you should treat this appliance as a hazard that can kill or maim anyone who touches it while it is plugged in. It won't burn down your apartment, but it might kill someone who doesn't know it is dangerous.

Off-topic, but it's easy enough to make coffee while shopping for a replacement. Simply boil water, take it off the heat, let it sit for ~30 seconds, and then add 40-60 grams of ground coffee per liter of water. Stir every now and then for 4 minutes and pour it through a filter in the basket of your (unplugged) broken coffee maker. It's how we would make coffee when in the outdoors, and it's better than injuring yourself with electricity.

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    Good complete answer with temporary alternative way.
    – crip659
    Mar 14, 2022 at 22:09
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    Another easy alternative (and what I do using a cone in my campervan) is to mimic the machine, but using a kettle - put grounds in a filter paper in the basket, then pour near-boiling water over, repeating until the jug is full.
    – Chris H
    Mar 17, 2022 at 12:00
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How unsafe is this? ... Is it a "better solve it within a week" type of thing?

It's potentially lethal and you've removed an important safety measure.

It's an unplug it and cut the mains plug off NOW "type of thing" so that neither you nor anyone else kill yourselves with it.

(I'm a coffee drinking electrical engineer.)

[Edit]

this "safety circuit breaker" thing might have different names in different countries

US: GFCI - ground fault current interrupter

UK: RCD - residual current device

Dutch: aardlekschakelaar (or so Google tells me)

Greek: ρελέ ηλεκτροπληξίας, αντιηλεκτροπληξιακό ρελέ or διακόπτης υπολειπόμενου ρεύματος Take your pick (also Google). British English also has various names for these devices. RCD is the current name.

Whatever it's called, if it trips it means an unsafe amount of electricity is leaking from your coffee maker. As you've disconnected the ground wire that electric current is no longer flowing relatively safely to ground and is now available to dangerously flow through you or anyone else who handles the coffee maker.

with the ground disconnected, the safety circuit breaker is not triggered, because there is no ground-leak for it to detect.

You haven't solved the problem; you've just stopped it being detected.

Any ideas / hints / guesses as to what may be wrong and what I could do to properly fix it?

As crip659 noted, you've had your money's worth and it's time to replace it.

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    Seconded by another electrical engineer
    – gatorback
    Mar 15, 2022 at 0:50
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    Disconnecting the ground to stop a breaker tripping is one of the most dangerous things you can do to an appliance, so this answer is spot on. If a coffee-requiring engineer wouldn't use it, you know it's bad. Use the non-electric parts to produce a temporary solution and get something else - and I say that as someone inclined to make do and mend.
    – Chris H
    Mar 15, 2022 at 10:50
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    LOL, I'd give you an extra vote if I could for the translations. :D
    – FreeMan
    Mar 15, 2022 at 16:17

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