Bench Degree·PLASMAchapter

Chapter 8: The Arc

The hardest part of switching off a large current is putting out the arc you started by trying to switch it off.

Take a small mains-powered vacuum cleaner or a power drill, run it, and switch it off at the wall while watching the switch in a dark room.

There is a flash inside the switch. A brief blue-white spark, every time, at the moment the contacts part. You have been making that flash all your life and never had a reason to look at it.

Now do it with a flashlight running off a battery and a simple switch. Same thing, smaller. Do it by touching and separating two wires from a car battery, which is the crudest possible version, and it is violent: a bright bead of light, a bang, and a small pit burned into the copper.

That flash is an arc, it is a plasma by every criterion in Chapter 2, and it is created by the act of trying to stop the current. The whole of high-voltage switching engineering exists because of it.

ON THE BENCH: Watch a switch arc, and then measure one

Parts: a 12 V car or lawnmower battery; a car headlight bulb as a load; two heavy wires; a length of thick wire wound into a coil of fifty turns around a nail, as an inductor. Safety glasses. Cost: nothing if you have a battery. Time: 20 minutes. Hazards: a car battery will deliver hundreds of amps into a short and can start a fire or burst. Always have the bulb in circuit as a load, never short the battery directly. Wear safety glasses: the arc spits molten copper. Remove rings and watches. Method: run the bulb from the battery, then break the circuit by pulling one wire off, watching the contact point. Then replace the bulb with the nail coil in series with the bulb and break it again. What you should see: a small arc with the bulb alone, and a noticeably longer and brighter arc with the coil in circuit. That difference is the point of the experiment, and Section 4 explains it: the inductor is refusing to let the current stop, and it will generate whatever voltage it takes to keep it flowing, including enough to sustain an arc across a widening gap.

Section 1: What Changes When a Glow Becomes an Arc

Chapter 6 put the arc at the far right of the current axis. The transition into it is not just more of the same, because the mechanism at the cathode changes completely.

In a glow discharge, the cathode supplies electrons by being hit. Positive ions accelerated through the cathode fall slam into the surface and knock electrons out mechanically. It is a cold process, it is inefficient, and it requires a large voltage drop right at the cathode to give the ions enough energy. That drop is most of the few hundred volts across a glow discharge.

In an arc, the cathode supplies electrons by being hot. The current density has become high enough to heat a small spot on the cathode to thousands of degrees, and a metal that hot emits electrons all by itself, in enormous numbers, the way the filament of an old radio valve does. This is thermionic emission and it is a far more generous supply.

Once the cathode is doing that, the discharge no longer needs a large voltage to run. The voltage across an arc collapses to a few tens of volts while the current climbs into the amps and the kiloamps.

That is the arc’s signature and it is worth committing to memory, because it is the opposite of a corona in every respect:

Section 2: Welding, Which Is an Arc Used as a Heat Source

An arc between an electrode and a workpiece delivers an enormous amount of heat into a very small area, and that is the entire basis of arc welding.

The arc is not doing the joining. It is a heater. It melts a small pool of the parent metal, filler is added, the pool solidifies, and the two pieces are now one piece of continuous metal. The plasma is a tool for delivering concentrated heat, and every welding process differs mainly in how it manages the pool and keeps air away from it.

Why keeping air away matters, which is where Chapter 3 pays off. Molten steel is chemically ravenous. Given access to atmospheric oxygen it forms oxide inclusions that make the weld brittle; given nitrogen it forms nitrides that do the same. So the pool must be shielded, and the shield is either a gas or a flux that melts into a protective slag.

And the shielding gas is chosen on the ionisation table. Argon at 15.8 eV strikes and holds an arc reliably at ordinary welding voltages, is chemically inert, and is heavier than air so it settles over the pool and stays. Helium at 24.6 eV needs a higher arc voltage, which puts more heat in, which is sometimes exactly what is wanted for thick aluminium or copper. Carbon dioxide is cheap and gives deep penetration but a harsher, spattier arc. A welder choosing a gas bottle is making a plasma physics decision, whether or not it is described that way in the shop.

The three processes a reader is most likely to meet:

Stick, or manual metal arc. A consumable coated rod. The coating melts to form both a shielding gas and a slag blanket. No gas bottle needed, works outdoors in wind, tolerant of rust and dirt, leaves slag to chip off.

MIG, or gas metal arc. A continuously fed wire through a flashlight with argon or a mix flowing around it. Fast, easy to learn, poor in wind because the shielding gas blows away.

TIG, or gas tungsten arc. A non-consumable tungsten electrode, argon shielding, filler added separately by hand. Slowest, most controllable, the choice for thin material and for work that will be inspected.

Robert Murray-Smith recorded two useful sessions on getting a first arc going, Very Very Basic Arc Welding (youtu.be/Jr5Zqds46-A) and Tips That Helped Me Become a Better Arc Welder, video 1224 (youtu.be/8WMN0doNBJg), and they are worth watching before a first attempt because both show the failures as well as the successes.

Section 3: The Plasma Cutter, Which Is a Different Thing

A plasma cutter looks like a welder and is not one, and the difference is worth understanding because it explains what it can and cannot cut.

The arc is forced through a small nozzle, which constricts it. Constricting an arc raises its current density and therefore its temperature dramatically, to around 20,000 K, and turns a broad soft arc into a narrow violent jet. Gas is blown through the same nozzle at high velocity.

The cut is made by melting and blowing. The jet melts a narrow line of metal and the gas flow physically ejects the molten material out of the far side of the plate.

Which explains the practical differences from an oxy-fuel torch. An oxy-fuel torch does not melt steel; it burns it. It preheats the steel and then blasts oxygen at it, and the iron oxidises exothermically, so the reaction supplies most of its own heat. That works beautifully on carbon steel and not at all on aluminium or stainless, because both form a tough oxide layer that refuses to sustain the reaction.

A plasma cutter does not care. It is melting, not burning, so it cuts anything electrically conductive: aluminium, stainless, copper, cast iron. It needs the workpiece in the circuit, which is why it cannot cut wood, glass or plastic, and why the earth clamp is not optional.

Section 4: The Arc You Did Not Want

Now back to the switch, and to the problem that makes this chapter matter more than welding does.

Opening a switch does not stop a current. It creates an arc.

As the contacts separate, the last point of contact carries the whole current through a shrinking area, which heats it enormously. The metal there vaporises, and metal vapour is easy to ionise. Now there is a conducting plasma bridging the gap, and current keeps flowing through a switch that is mechanically open.

With an inductive load, which is most real loads, it is far worse, and your experiment showed it. An inductor stores energy in its magnetic field and physically opposes any change in the current through it. Try to stop that current and the inductor generates whatever voltage is needed to keep it going, which can be hundreds or thousands of volts from a 12 V supply. That voltage is more than enough to strike and maintain an arc across a widening gap. The arc is the inductor’s stored energy finding a way out, and it will keep burning until that energy is spent.

This is why motors, solenoids and transformers are hard on switch contacts, why relay contacts weld themselves shut, and why every switching circuit in electronics has a diode or a snubber across the inductive load to give that energy a harmless path.

How the industry actually puts arcs out

Alternating current has a gift built into it, and this is the single most important fact in switchgear. An AC current passes through zero a hundred or a hundred and twenty times a second. At that instant there is no current to sustain the arc, so it goes out on its own. The breaker’s only real job is to make the gap non-conducting fast enough that the arc does not restrike when the voltage comes back. Everything in an AC breaker is designed around that few-hundred-microsecond window.

The methods, in rough historical order:

Air blast and arc chutes. Blow the arc with compressed air, or drag it into a stack of metal plates that splits it into many short arcs in series, each needing its own voltage to sustain. Common in low-voltage breakers, and the reason a domestic breaker is bigger inside than the contacts alone would need.

Oil. The arc decomposes surrounding oil into hydrogen, which conducts heat well and blows the arc out. Effective, and it puts a large quantity of flammable oil in a substation.

Sulphur hexafluoride. Extraordinarily good at capturing free electrons, so it quenches an arc superbly, and it dominated high-voltage switchgear for decades. It is also a greenhouse gas with a global warming potential in the region of 24,000 times that of carbon dioxide and an atmospheric lifetime of millennia, which is why the industry is now actively replacing it. Anyone reading this in the trade will meet both SF₆ equipment and its replacements.

Vacuum. Straight out of Chapter 5’s Paschen curve: pull the contacts apart inside an evacuated bottle and there is almost nothing to ionise, so the arc cannot be sustained once the metal vapour has dispersed. Compact, sealed, maintenance-free, and now the standard for medium-voltage distribution. A vacuum interrupter is Paschen’s law sold by the thousand.

SLOW DOWN. Check Your Understanding: Direct current is dramatically harder to interrupt than alternating current at the same voltage and current, and DC breakers are correspondingly larger and more expensive. Why? Think about the previous section before reading on.

Because there is no current zero. An AC arc extinguishes itself a hundred times a second and the breaker only has to stop it restriking. A DC arc has no such moment: the current never stops on its own, so the breaker must actively force it to zero, either by driving the arc long enough that the supply cannot sustain its voltage, or by injecting an opposing current. This is not academic. It is why battery storage systems, solar arrays and electric vehicles all need purpose-built DC protection that cannot be substituted with an AC breaker of the same rating, and it is a live engineering problem in exactly the technologies expanding fastest.

Section 5: Melting Steel With It

The largest arcs on earth doing useful work are in electric arc furnaces, and they are worth a paragraph because the scale is hard to imagine.

Three graphite electrodes, each up to about 700 mm (28 in) across, are lowered into a bath of scrap steel and struck. Currents run to tens of kiloamps and the furnace draws in the region of 100 MW. A modern furnace melts a hundred tonnes (110 short tons) of scrap in under an hour. Most recycled steel on the planet goes through one.

Two consequences that reach beyond the mill. The graphite electrodes are consumed and must be fed in continuously, which makes electrode graphite a globally traded commodity with its own price cycles. And the load is violently erratic as the arcs strike and break against shifting scrap, which causes voltage flicker on the local grid severe enough that arc furnaces must be connected at high voltage with compensating equipment, and are a standard case study in power quality.

Section 6: What This Chapter Established

An arc is a thermal plasma with a hot cathode, and its signature is low voltage at high current, which is the mirror of corona.

Used deliberately it is a heat source of unmatched concentration, which is welding, cutting and melting, and the gas you choose comes straight off Chapter 3’s ionisation table.

Created accidentally it is the central problem of switching, and every technique for putting one out is a way of denying it either the charge carriers or the voltage it needs.

And Paschen’s law sells switchgear. The vacuum interrupter is the left-hand branch of the curve in Chapter 5, in a bottle, in every substation.

Next: the same discharge used not for heat but for light, which is where the fluorescent tube, the neon sign, the orange streetlamp and the most efficient lamp ever manufactured all come from, and why every one of them is now losing to something that contains no gas at all.

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