Story time: Last week my furnace failed. The blower motor run capacitor went toast, so the motor wouldn't spin. The furnace is on a multi-wire branch circuit and shares its neutral with the basement. When the furnace motor failed, other devices in the basement went up in smoke (literally). A catalytic cap on the control board of my brand new water heater exploded and a ceramic cap in one of my computer UPSes also exploded.
The question: I want to understand, on a very technical level, how exactly the blower motor was able to damage other devices that were solely connected over the neutral wire. My research has pointed to things like "imbalanced load" and "dips or surges", which of course is correct but completely devoid of the electrical detail my nerdy brain is craving.
- What happens electrically at the motor terminals when the motor run cap fails and the motor stalls?
- How does this affect the 120v AC between the hot and neutral on the furnace's branch of the circuit?
- How does this affect the alternate phase 120v AC between the hot and neutral on the basement's branch?
- Does the phase difference between the two branches of the circuit play a role in this failure?
- Why did existing protections (such as fuses on the hot and neutral power wires inside the water heater, or built-in surge protection on the UPS) not prevent the damage?
- Other than running new copper for a dedicated furnace breaker (which I'll probably do anyway), how can I protect my home and devices from damage from these kinds of failures in the future?
I'm not an EE, but I am a hobbyist and enthusiast and it's the technical details of how the motor failure cascaded across the neutral wire that I'm confused by. I thought about putting this on DIY SE but since it's the electrical technobabble I'm interested in I decided this was the best place.
Because someone does not seem to understand, I will try to make this abundantly clear. This is NOT a question about doing my own electrical. I am NOT asking for advice on how to wire my house. I am NOT asking about building code or furnace ratings. This is NOT a DIY question.
This IS a question about how the design of a multi-wire branch circuit allowed a (failed run capacitor on an induction motor) failure on one leg to propagate over the neutral wire to damage equipment on the other leg. Or, generally, how could any electrical failure on one circuit affect another circuit over a shared common? Does the phase difference between the two circuits play a role in how a failure could propagate? If so, how? This IS an electrical engineering question.
According to my understanding of electrical circuit design, this kind of failure propagation should not be possible. Which means that either the circuit is incorrectly wired or that my understanding is flawed. I assume the latter because the circuit has passed multiple city inspections over the years and has passed wiring tests before and after this failure. I am here to improve my understanding of how electricity works.