How can a failed furnace blower motor damage other equipment on a shared neutral? [closed]

Question

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.

So

• 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.

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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.

Answer 1

The issue can be explained if you have a failed neutral connection. In some locales, the electricity is split 120 VAC | 0 (neutral) | 120 VAC, with the two 120 VAC 180° out of phase. All three wires go to the outside power transformer. That means there is 240 VAC between the two "hot" wires, and 120 VAC between either and neutral. This allows both 120 VAC appliances (e.g., lamp or radio) and 240 VAC appliances (e.g., electric dryer or stove) to run from the same entrance panel.

If the neutral wire breaks, there is still 240 VAC across the outside wires, but 120 VAC appliances are effectively paired in series across the 240 VAC. For example, if you have just two appliances on, one in each half of the circuit, and one is a refrigerator drawing 5 amperes (600 W) and the other is a lamp drawing 0.1 A (12 W), about 235 VAC would be across the lamp, and 5 VAC across the fridge. poof! Lamp is torched.

Not only is that a fire hazard, but the neutral wire in the house now has 235 VAC on it. This means the shell of some two-wire appliances can be a serious shock hazard.

The issue was not the motor, but the missing connection between the house neutral wiring and the center-tap of the outside power transformer. Consider it luck that the house did not burn down nor was someone electrocuted. Fuses did not blow because the circuit current rating was not exceeded. The appliance voltage rating was exceeded because of defective house wiring, or defective wiring between the outside power transformer and the house entrance panel.

This is not a DIY project. Get a certified electrician, who will contact the electric company, to fix this ASAP.