Electrical – Could a disconnected ground wire in the breaker box have caused any problems

There are only a few possible causes:

• The newish switch was not wired correctly. I think this is the most probable cause: anything recently changed and not working should be reviewed. Maybe there is a stray strand of wire between the switched terminal and ground, or some wiring fault in the lights. If any of these are the problem, turning on the switch will trip the breaker even if the dehumidifier and other devices are off. That is, turning on only this switch is enough to blow the breaker.
• The breaker has derated itself—that is, it is marginally defective. This happens sometimes, especially with heavily loaded circuits. Breakers get hot, or if they are tripped many times they can experience something similar to heat fatigue.
• The load has increased: more bulbs or more wattage, or the humidifier is using more power. Many electrical systems do not provide steady voltage. If it has increased recently, from say 114 to 122 volts, that might be just enough to increase the power to where the breaker should trip.

If it were me, I would

1. Shut off the circuit
2. Pull the "new" switch and examine it for physical problems.
3. Use a DMM to check for continuity between the black wire and ground: there should be none—that is, the ohms should be infinite.
4. Disconnect the white wires (should be a wire nut in the switchbox) and check for continuity between white going to the lights and ground: should be none. The ohms between the white wire going to the breaker panel and ground should be close to 0.0.
5. Check the resistance of the light circuit with the DMM between black and white. For 120 watts incandescent @ 120V, should be 8–10 ohms (cold tungsten has its resistance of about one fifteenth its hot temperature).

If the problem had not been found after that, I would reconnect the white wires (undo step 4), turn on the breaker, and put a clamp-on A.C. ammeter (actually an analog meter would be better for this test) to measure the current on a black wire, and turn the switch on. It can either spike strongly or not at all. Strongly is bad and means there is a fault in the wiring or fixtures. Not at all means more tracing of wire, perhaps from the circuit breaker panel is needed.

When an appliance has an internal shorting or arcing fault, or electrical wiring has is shorting hot or neutral to ground, that is called a "ground fault".

You have a "Whole-house RCD" - aka GFCI - it looks for current leakage from ground faults. SOP in Europe-influenced electrical installations is to fit a 30-35ma (milliamp) RCD that protects the whole house. The goal is to protect the house from arcing fires; 30ma can kill you. This unit is typically combined with the main circuit breaker.

Beyond the whole-house RCD, you then have branch circuits with circuit breakers. Most of them will be one size such as 13 amp or 16 amp. Take note of that.

North American practice is to protect each branch circuit with its own GFCI (RCD) device (breaker). The goal here is personnel protection, so only bathrooms, kitchens etc., and a lower detection threshold of 8ma.

My suggestion is to do the American thing - and replace one of your branch circuit breakers with an 8ma personnel-grade RCD, of the amperage rating of most of your branch circuit breakers. This will have a lower threshold than the main, so it "should" trip sooner. If the problem is on that branch, the trips will mostly now occur on this breaker. If that branch circuit is not the problem, then swap the breaker to another branch circuit, and try it again. Eventually you will narrow it to a single branch.

Then unplug all the appliances and test again. If that does not clear the problem, then disassemble all the junction boxes in that branch and look at the wiring. If unplugging the appliances does clear it, then return one appliance a day and when the problem returns, that's the one. Have the appliance serviced.

This method won't work for the few odd-sized breakers you may have for stove, water heater, air conditioning, etc. Simply switch those off for a day (when feasible) and see if the problem goes away when they are off.