How to Spot a Psychopath

I just noticed this MAKE: blog piece (which links to a site that's currently down) about using outdoor sensor light fittings to give other gadgets motion triggers. They're talking about using this trick in a haunted house context, given the impending arrival of this year's Satan Day/nocturnal orgy, but you could plug in anything you wanted.

Well, within the current capacity of the light fitting and its wiring, anyway. A motion-sensitive bathroom heater, for instance, might be a neat idea but could easily pop a fuse or smoke some insulation.

The Edison-screw-to-standard-receptacle adapters they use are, I note, neat little off-the-shelf items - just screw 'em in to turn any light socket into a power socket. Apparently you can even get three-pin versions, with the earth pin unconnected.

And fair enough too, I say. What kind of crappy haunted house lets all of the kids come out alive?

The other day, all of my UPSes started beeping.

This did not surprise me. I live in Katoomba, New South Wales, and mains power is a little flaky up here. That's why I've got so many UPSes, plus a vintage big-ass power conditioner I bought cheap on eBay - it's currently sitting on a shelf, but will no doubt come in terribly handy Real Soon Now.

What was odd this time was that the mains power was, as the UPSes were telling me, off. But the lights were still on.

That, obviously, meant that a breaker or RCD or something had tripped in the house's fuse box.

So I went out there, and observed that everything was fine. Breakers all on, master switch on, giant safety-wired council fuses still intact.

(Which fact I, of course, discovered by cutting off the safety wire and levering the big porcelain fuse blocks out with my pocket knife, while wondering whether I'd actually manage to jump all the way over the fence if this obvious suicide attempt came to fruition. My feet weren't bare, it wasn't raining, and the lid of the breaker box stayed up by itself so I didn't have to balance it on the top of my head, but this was otherwise as silly-looking a thing as I've ever done while standing in front of a breaker box.)

Lighting circuit and its spinning-disc usage meter: Working fine. Power circuit and its separate meter (a clue!): Dead as a stone.

Long story short, I called an electrician, and while he was glumly inspecting the Edwardian-era wiring on the back of the breaker panel, a nice lady came down the street to ask whether there was a blackout.

For lo, her house had no power at all.

At this point, both of us highly technical manly men twigged to the fact that this house has two separate power feeds coming to it from the pole outside. Those two feeds are from two of the three phases carried by the pole. One of those phases, the one that feeds the house's power circuit, had dropped out. The phase feeding the lighting circuit was still up.

Three-phase power uses three separate live conductors, each of which carries an AC waveform 120 degrees out of phase with either of the others. Various commercial and industrial premises have a full three-phase hookup; three-phase is useful for driving some kinds of high powered motors. It's perfectly normal for street mains wiring to be three-phase too, but ordinary houses only need single-phase power. So, usually, each house in a street is hooked up to one of the three available phases.

But for some reason, some ordinary houses get more than one.

If one phase is knocked out on the average street, therefore, only the houses hooked up to that phase will go dark. That's likely to mean about one third of them. This odd-sounding situation is actually quite common, though people often don't notice, because they don't walk up and down the street to see if everybody's lost power when their own lights have gone out.

In my house's case, loss of one phase gives me a two-thirds chance of losing lights or power, but no chance of losing them both.

The power cut lasted a few hours, long enough that I started to worry about the food in the fridge. Plus, I wanted to eat some of the food in the fridge, but opening the fridge door during a blackout is not a great idea. Unpowered fridges can stay cold for quite a long time, but not if you open the door.

I, accordingly, made this.

On the other end of this magnificently wrong-looking object (hey, it could be worse) is the refrigerator. Which ran fine from light-socket power for the, oh, ten minutes or so before the bloody power came back on anyway and I plugged the fridge back into the wall.

In the olden days when mains electricity was a new idea, getting the 'lectricity on meant you had light sockets installed, and nothing else. Wall sockets were rare, because there wasn't much you could plug into them. If you wanted to run some appliance other than a light, you just plugged it into a light socket, using a cable that terminated in an Edison screw or bayonet plug.

If you hack the plug end off an extension cord, it takes only a couple of minutes to replace it with a light-socket plug, which you should be able to buy from any decent hardware store. That's what I did.

The very existence of light bulb connector plugs invites you to do this sort of thing, which could explain why their packaging (here in Australia, at least) is printed with dire imprecations against fooling with this stuff if you are not a qualified electrician.

Which I, gentle reader, am not.

The warnings are not kidding. There are several entertainingly dangerous ways in which a cord like this can go wrong.

The only contacts available in a light socket are active and neutral. So you can't, in this case, make a cable that creates a circuit from active to earth (which will instantly trip an RCD "safety switch") or neutral to earth (which will probably also trip the RCD, in a slightly more roundabout way, if anything else is running on the same circuit at the same time). You certainly can, however, make a cable that swaps active and neutral. Indeed, it's impossible not to if your cable terminates in a bayonet connector - you can plug those things in either way around.

Reversing active and neutral is a land-mine of a mistake. It can sit there harmlessly for years, and then interact with another individually innocuous mistake to kill yo' ass dead.

The lack of an earth also means... there's no earth. Whatever I plug in via my magical mystery light socket extension cable will now be un-grounded.

This, also, is generally harmless. Completely harmless, if whatever I plug into the cable isn't grounded in the first place (look for "double insulated" on the appliance label, or just a two-pin plug).

But the primary purpose of earthing is to protect you from being zapped by a faulty appliance. If you plug, say, a shiny chrome toaster into an ungrounded cable (which you probably shouldn't do anyway; a light bulb socket will become warmly unhappy if you ask it to run a thousand-watt toaster), and a live wire comes loose inside the toaster and touches the chassis, then the toaster will sit there with a live chassis and wait for you to touch it.

(It'd be great if you could be touching the sink with your other hand when you do that, by the way. Thanks.)

If the toaster's earth wire is connected, the above failure will trip an RCD in milliseconds, or blow a fuse/trip a circuit breaker in only slightly more time. With no earth wire, the small load created by a dying human body will not bother your house's breaker box at all.

The way to make a cord like this safe is, therefore, to terminate it with a two pin plug, so you physically can't plug an earthed appliance into it.

(If you want to get really fancy, you can give it an inline fuse too, to prevent it from overstretching the socket it's plugged into. A reader's now also pointed out to me that you could put an RCD/GFI/ALCI/WTF-protected outlet on the end of such a cable. "Safety switches" do not actually need an earth connection - they detect earth leakage, to the proper circuit earth or anything else, by looking for a difference in the current flow through the active and neutral conductors. If there's a difference, the extra current must be going somewhere, that somewhere may be bad, and so the safety switch trips.)

Here in Australia, I don't think there's any such thing as a two pin extension cord. Or a separate two pin in-line socket, for that matter. You could achieve the same result by blocking a three-pin socket's earth hole with glue or something, though.

This safe version of the cable would, of course, be much less useful than the unsafe version. I couldn't have plugged the fridge into a socket with a blocked earth pin. Well, not without busting the earth pin off the fridge plug, anyway.

(Do people ever do that? Of course they do! What better way could there possibly be to get rid of an earth loop?! [PDF])

So I shall leave this cable in its unsafe state. It'll lurk, like the poisonous snake that it is, in our electrical-junk cupboard, waiting for the next time half the house goes dead.

With any luck, no further deaths will result.