Tripping Out: A Field Guide To Circuit Protection

Jul 20, 2023

My introduction to circuit protection came at the tender age of eight. Being a curious lad with an inventive – and apparently self-destructive – bent, I decided to make my mother a lamp. I put a hose clamp around the base of a small light bulb, stripped the insulation off an old extension cord, and jammed both ends of the wires under the clamp. When I plugged my invention into an outlet in the den, I saw the insulation flash off the cord just before the whole house went dark. Somehow the circuit breaker on the branch circuit failed and I tripped the main breaker on a 200 amp panel. My mother has never been anywhere near as impressed with this feat as I was, especially now that I know a little bit more about how electricity works and how close to I came to being a Darwin Award laureate.

To help you avoid a similar fate, I’d like to take you on a trip (tee-hee!) through the typical household power panel and look at some of the devices that stand at the ready every day, waiting for a chance to save us from ourselves. As a North American, I’ll be focusing on the residential power system standards most common around here. And although there is a lot of technology that's designed to keep you safe as a last resort, the electricity in your wall can still kill you. Don't become casual with mains current!

What saved me that long-ago day was a circuit breaker. In its simplest form, a circuit breaker is just an electromechanical device designed to protect circuits by turning off the juice when the current flowing through it gets above the design point. Breakers need to sense the flow of current and turn it into mechanical action so that contacts can be physically separated quickly and safely. The most common mechanism are electromagnetic, where more overcurrent creates a magnetic field to pull apart contacts, and bimetallic, where an overload heats up a bimetal strip and bends it, activating the switch.

Residential breakers in North America come in a couple of different flavors. The branch circuit breakers are used to protect each branch circuit – the outlets, light fixtures, and appliances that are connected in parallel back to the main panel. Each branch circuit is typically rated for either 15 or 20 amps, and unless it's running a large load like an electric dryer or a well pump, it’ll be a 120-volt circuit. In addition to the branch breakers, a panel will have a main breaker to catch any faults that a defective branch breaker misses, like what happened to me. It's usually a 4-pole affair – one for each hot line, one for neutral, and one for the ground. A main breaker also acts as a switch to de-energize all the branch circuits, making it safer to work in the panel – but beware the lugs feeding the main breaker! Those are always hot, and the next breaker up the line is probably a big disconnector on a power pole.

Main breaker panels around here have an interesting arrangement that allows for both 120- and 240-volt circuits. Power is distributed in a split-phase arrangement, where the transformer on the pole has a center-tapped 240-volt secondary winding. This results in two hot legs each at 120 volts relative to the neutral, or 240 volts across the two hots. Inside the distribution panel, the two hots are connected to bus bars with fingers that interlace. This allows installation of either single-pole breakers, which are connected to a single 120-volt hot leg, or double-pole breakers, which bridge the two hot legs for a 240-volt circuit.

If you ever come across a problem where every other single-pole circuit in your panel is working and none of the 240-volt circuits are working at all, you’ll know that one of the hot legs is not energized for some reason. You’ll probably want to call the power company in this case.

On the first day of a college EE lab course, the instructor gave us a sobering safety lecture entitled, "It's the Current That Kills." The figures he quoted about how little current it actually takes to kill staggered me – how had I not gotten at least 30 heart-stopping milliamps through me at some point with the various line voltage shocks I had experienced? Turns out that I probably got way less current than that, and that it likely didn't pass through my heart, but still, it's nice to know that many circuits in a modern residential service panel are protected by ground-fault circuit interrupters. Known as GFCIs or GFIs in the States and residual-current devices (RCDs) in the UK, these devices are capable of cutting off the flow of current if as little as 5 mA leaks from the hot line. And it does so very quickly – about 25 to 40 milliseconds, which is less time than it takes a 30 mA current to put a human heart into ventricular fibrillation.

GFCIs work by sensing the current imbalances between two conductors using a current transformer. Both the hot and the neutral conductor pass through the current transformer's toroid. Normally the currents cancel each other out, but if there's an imbalance due to leakage, a current is induced in the transformer which can be used to trip an electromagnetic circuit breaker. That there's a chip to take care of most of the sensing and tripping is not surprising; what is unexpected is the fascinating story of how the first GFCI chip, the LM1851, came into being in the mid-1970s. Spoiler alert: many graduate students were electrocuted in the making of the first GFCI.

In the US, GFCIs are required by code anywhere there's the slightest possibility of water and electricity mixing. Originally intended for bathroom and kitchen outlets, GFCIs are now also found in basements, garages, and in any outdoor outlets, especially for pools and spas. While the most common form factor for GFCIs is built into a duplex outlet, some circuit breakers have GFCIs built in. And of course there's the wall wart GFCIs that now grow out of the power cords of every hair dryer and curling iron manufactured.

A more recent circuit protection modality is the arc-fault circuit interrupter (AFCI). Intended to prevent fires due to high-temperature arcs, AFCIs have been required by code since the early 2000s for branch circuits servicing bedrooms. Early AFCIs were subject to nuisance tripping from "normal" arcs, like the brushes in a vacuum cleaner motor. As the technology improved and nuisance tripping was reduced, code requirements were rewritten so that AFCIs are now required on all branch circuits, servicing just about every room in the house.

While AFCIs sense current anomalies like GFCIs do, the similarities pretty much end there. AFCIs also need to monitor voltage and to analyze the waveform of the circuit under monitoring for telltale signs that potentially destructive arcing is happening. In addition to recognizing arcs between the hot line and neutral or ground, AFCIs need to detect series arcs, which might happen when a hot wire wiggles free within a loose terminal. All these arcs have characteristic waveforms that a microcontroller analyzes to determine if a fault exists while ignoring arcs from equipment operating normally. It's complicated stuff, and it's no wonder it took a while for manufacturers to get it right. A good treatment of the specifics of the detection algorithms can be found in this paper (PDF link).

Of course there are plenty of other devices working to keep your electrical supply clean, like surge protectors, noise filters, and UPSs. But these are mainly for keeping your devices healthy and happy. Breakers, GFCIs, and AFCIs are are life safety equipment, whether by protecting the structure around you from catching fire, or by preventing you from getting bitten by many Angry Pixies when you drop your hairdryer in the toilet. But remember that they’re the last line of defense — at the end of the day it's mostly up to you to make sure you don't do something to as dumb as I did way back when.