Why does my washer trip circuit breaker on sanitize cycle?

Question

Bought this machine used and the seller failed to mention this to me or this is a problem with my wiring.

All cycles complete 100% on this machine (LG model WM2277HW), except for the aforementioned "Sanitary" cycle. That cycle is one of the reasons my wife wanted to purchase this.

Anyway, I've done general troubleshooting and I'm somewhat lost. I've got 123 volts at my outlet, which is a dedicated line straight to the breaker box with a 20 amp breaker (I have swapped around breakers and their location within the box). I replaced the old outlet (I could feel it getting hot) to no avail.

LG service manual says I can check water temp by pressing and holding the Wash/Rinse button. During the sanitary cycle, the display shows the water temp slowly rising, as there is an independent water heater inside this unit (which I believe is only used during the sanitize cycle). At some point that I'm not certain of, the breaker trips. If I hear it I can run over to the breaker box, reset it, turn the unit back on, start the cycle again and it will trip it at the very beginning of the cycle instead of halfway through. If I wait a couple minutes, it will run the cycle again without tripping in the beginning. If I open the door up, the water has definitely been heated and I can see steam.

I've run tests (very high RPM spin speed, water cycling, heater on/off) as indicated in the service manual and nothing trips the breaker. All this leads me to believe I've maybe got some exposed insulation in that line and after the washer has been pulling a lot of amps for a while the heat is causing expansion and then a short develops. Or both of the breakers I tested are faulty.

Does this sound feasible or is this more than likely a control board/ heater element issue in the washer? I know I need to test it on another line, but I don't have another dedicated line anywhere close. I could plug it into the other bathroom circuit, remove all load and test there. Will that work?

My thought process:

• Heater element is working long enough to raise temp considerably, indicating non-faulty.
• Tumbler works on regular cycle, indicates no-fault
• Drain pump works on regular cycle, no-fault
• Circulation pump works (though rather loudly) on regular cycle, no-fault
• This leaves the outlet, wire, or breaker as most likely culprit

Answer 1

As mentioned by @Steven. You'll want to get yourself an ammeter, and figure out how much current the appliance is drawing. While shopping for the meter, you'll want to look for a "Peak Hold" feature (though it might go by other names depending on the manufacturer). This feature will allow the meter to retain the highest measured value. That way you don't have to monitor the meter.

Remember when using the meter, you have to clamp it only around one of the circuit wires. If you clamp it around the cord; for example, you'll always get a reading of 0. The best way to do this, is to clamp the meter around the ungrounded (hot) conductor in the panel where it connects to the breaker.

Circuit breakers include overload protection, which is also known as thermal protection. This is typically a bimetallic strip, that pulls the contacts open if it gets too hot. This type of protection is time delayed, since the device has to physically heat up. If the current through the breaker is really high (but less than what trips the magnetic protection), the device will heat up quicker and react more quickly. In your case, however, it's more likely that the current is just high enough to cause the breaker to slowly overheat. This would explain why letting it cool down (waiting), would allow it to run longer once reset.

Your intuition could also be correct, and a loose connection could be exacerbated by the heating due to high current draw.

In either case, an ammeter will be a useful tool to start diagnosing this issue.

NOTES:

• If the breaker is a GFCI breaker, the sanitize cycle could simply be leaking too much current.
• If the breaker is an AFCI breaker, the sanitize cycle may be doing something strange that the AFCI is picking up.
• Thermal protection in circuit breakers can be affected by the ambient temperature, and the temperature inside the panel. So if the panel is in a really hot location, it could trip before it typically would/should.

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Update:

Now that it's clear that the circuit is protected by an arc-fault circuit interrupter (AFCI) circuit breaker, I'd like to change my answer. While an ammeter is useful, and testing the max. current draw might be useful. I suspect that the machine is doing something during the sanitize cycle, that the AFCI doesn't like.