Wednesday, August 19, 2009

Ideal Conditions

There's a Canadian Tire right across the street from the hotel, so I walk over there the next morning to buy an extension cord. You'd think this would be pretty easy, right? You'd think I couldn't make an entire blog entry out of buying an extension cord. Actually regular readers know I could make an entire blog entry out of choosing which colour cord to buy. Do I think too much, or do I think the regular amount and just write it all down too much?

I pick up an orange one with a locking connector. That's just like the one I thought I had, probably the same brand. I don't know what wattage the equipment is, but I'm pretty sure I don't need a heavy duty cord for this. The others aren't any heavier. There are also some green ones on sale, but they are only ten metres and I need at least a fifteen metre cord to bridge the gap currently served by the patchwork collection. The orange one has a cardboard sleeve around it for display purposes. It says it is a fifteen metre long, 16 gauge, outdoor locking extension cord. Sounds good. I flip it over and read the back. The warnings include "Do not plug extension cord into another."

A Canadian Tire employee sees me staring at the package like Wonko the Sane looking at the instructions on a package of toothpicks, and asks if he can help me. I indicate the warning. "Isn't that what normal people do with extension cords?" They're for extending other cords!

He manages to say with a straight face, "that would be under ideal conditions." Great phrase. Henceforth that's going to be my standard answer to anyone telling me I'm doing something wrong.

I buy the extension cord and I plug it wantonly into the other extension cord, in order to plug the airplane into ground power. I'm aware of the irony that less than a week ago I was mocking someone who was too important to pay attention to the safety briefing on board an aircraft, and now I dare to plug one extension cord into another. I imagine some extension cord experts read this blog and are ready to castigate me for it, so I'll leave the comments open for them to tell me the dangers posed by the added resistance of the second cord.


Chris Taylor said...

It might be fun to experiment with how many of the shortest extension cords you can daisychain together before it all goes up in a flaming mess.

But uh, not with an airplane plugged in at the other end.

Geekzilla said...

Too funny!

Justin said...

The reason you shouldn't chain extension cords tgoether is because you will increase the resistance of the earth wire. The problem is that the circuit of faulty equipment + your body + 2 extension cords + earth may not draw enough current to blow the fuse, but it will certainly draw enough current to do you some harm.

jinksto said...

chaining cords also causes a voltage drop so if you're using highly sensitive equipment you can damage it, or more likely, cause it to provide false readings. Or, you know... big flaming mass... something like that.

Unknown said...

After reading this post and the comments to it I looked behind my entertainment center at the mass of cords and connectors that bring my 65" Sony Aquos and the various other electronic components into my living room and I pictured a mushroom cloud above my house and what the headlines will say the next da. Ugh, thanks a lot. I will be reviewing my homeowners insurance policy before going to bed.

Anoynmous said...

Do I think too much, or do I think the regular amount and just write it all down too much?

Even if you don't think more than the average Jane (and I'm pretty sure you do), you definitely pay more attention to what you're thinking. But thinking too much becomes a problem only when it leads to "analysis paralysis".

Astroprof said...

On the other hand, as long as you are using equipment drawing far less current than the rated max on the extension cords, you are OK with chaining them together. If you use two cords, limit yourself to half the current. If three cords, then limit yourself to one third of the current. Etc.

Paul B said...

I suspect much of the reason for "don't plug into another extension lead" is to protect the manufacturer's asses!

The obvious difference between 2 x 15mtr cords, and 1 x 30 mtr one is the extra connectors, and connectors (esecialy older ones) can add a very small amount of extra resistance. Oh, and connectors are always a very minor fire risk (high-resistance connection = heat, need I say any more).

I can remember chaining 4 or 5 long extensions together on at least one occasion (I needed a light a LONG way away from the power socket!)

However, I don't think the world will end by chaining them.... I would be dead hundreds of times over if that were the case!

Chad said...

Has anyone ever actually seen an extension cord go up in smoke? The only one I've seen is on mythbusters, and they had a pretty extreme setup with one of those tiny indoor extension cords with a bypassed breaker.

When I worked construction we'd sometimes have to string cords from obscene distances, and we'd be using several high-load tools like skill saws. The worse that would ever happen would be the breaker would pop.

I think they should write warnings closer to reality, then people might not ignore them so much and there'd actually be a point in reading them!

Jim said...

Many years ago, when I worked on the remote unit for a TV station, we had a full mobile unit in a step van - 2" tape unit and everything. We were doing a remote at some snowmobile races, and had stretched many extension cords a long way. The voltage at the origin was 120V, and at the mobile it was 90V -- and this was over 12 guage wire.

We had an auto-transformer to increase the voltage to 120V for the electronics, but the electric heaters received only 90V didn't generate anywhere as much heat. And we had to set up separate cooling for the motor for the vacumn pump - I believe electric motors compensate for reduced voltage by drawing a higher current.

I can imagine a homeowner running 400 feet of cord and then trying to run a 15 amp device at the end. Something is not goign to be happy, and in our litigous society there must be a windfall in there somewhere.

Timo Kiravuo said...

In theory there is no reason not to chain cords, as electricity does not care if it is traveling in wire or over metal connectors touching each other with a nice, large surface area.

In practice the connectors may have the edge or even poin of one piece of metal resting on the other piece, thus reducing the electricity transmitting surface area and reducing the amount of current than can be passed.

So in practice, unless you are sure that you have good quality connectors in the extension cords, power strips etc. you should be careful with them.

This is what I have seen based on the wimpy American standard. Here in Finland we use 220V (twice the power with the same current) and German plug system. We just keep extending the cords. :-)


Anonymous said...

Actually, also many years ago (mid-80s), some dorm mates thankfully in the other hallway from my room had their room, well, go up in a flaming mess.

The outcome was "no mini-fridges plugged into extension cords" for the rest of us 8th floor residents. And actually it may not have been the load on the cord but the fact that using an extension cord allowed them to squirrel the mini-fridge away in the closet that was the problem.

Ah, college days.


Cirrocumulus said...

Chad: I have.
In this very office a couple of years ago, some builders were removing asbestos from the ceiling.
They split the floor in half with an airtight polythene sheet wall and did it half at a time at nights and weekends.
Their power supply came from our half via joined extension cords with the join underneath the poly sheet wall.
On Monday morning we found they'd blown our power. I traced the cause to the join: it couldn't be unjoined because it'd melted together. There'd even been some small flames. How they didn't set fire to the poly sheet I don't know.

Anonymous said...

As an (well, the) extension cord use enforcement agent I feel it is my duty to inform you that, although budget constraints have limited my deployment in your sector (currently my range is limited to my apartment), your flagrant disregard of extension cord safety procedures has not gone unnoticed.

Lord Hutton said...

I have an 8 plug extension under my desk. It keeps my feet warm

dpierce said...

Do I think too much, or do I think the regular amount and just write it all down too much? ...

You observe your own observations. Which is great.

I saw someone run 3 heavy duty extension cords in parallel over a distance to distribute the load. I'm not kidding.

zb said...

I can assure you that most worries about extension cords are urban legends.

Here's the physics:

Justin points in the right way when he says that the reason you shouldn't chain extension cords together is because you will increase the resistance of the earth wire.

A dangerous voltage on the housing of earthed equipment with an insulation fault on the live wire will be (fault current) times (resistance of earth connection): V = I * R

Let's consider voltages greater than 42V as dangerous voltages and let's assume the outlet is fused at a rating of 32A.

The worst case is an extension cord that just has enough resistance to limit the fault current to a value just below the rating of the fuse.

Thus, under unlikely and absolute worst case conditions, in order to keep the potentially dangerous voltage below 42V, the max resistance of the earth wire must remain smaller than 42V/32A = 1.3Ohms. Mains plugs, especially old ones, can likely have resistances of 0.1...0.5Ohms.

However, if your outlet is equipped with an RCD, there's not much to worry about: The RCD will disconnect the outlet at a fault current of something between 0.01...0.05A, depending on it's rating, which is way smaller than what we assumed in the example above. RCDs even work in installations or with equipment where no earth wires are present at all or are completely disconnected.

As long as each cord with its respective plugs and sockets is in a good condition and you don't supply equipment with very excessive power ratings, it will be very, very hard to start a fire. Each plug/socket combination of contacts has a certain resistance R_PS, and a load current I_L will cause a power dissipation of P_D = I_L^2 * R_PS on each joint. The heat (read: Power dissipation) will be generated in more locations, but it will not increase on each contact. Unless you put all extension cord contacts on one big pile and create the heat in a very small spot, you will hardly be able to start a fire. You could even argue that more cords will decrease the current in the whole circuit due to their resistances and make everything safer.

A somewhat theoretical pitfall is present with some equipment that will not act as a resistive load. Resistive loads like heaters or light bulbs draw a current proportional to the voltage you apply. When the voltage gets low because some volts are lost along the long cord, the equipment will draw less current and the dissipated power will become smaller. Equipment with modern switch-mode power supplies like computers will not act resistively but will demand a constant power, P = V*I. If the voltage V gets smaller, the current I has to increase in order to keep the V*I=P product constant. You need a lot of computers, though, and really, really long cords with really bad contacts, if you want this to become a serious problem. Normally, a power supply that is rated at 110 V will still work with 20% less voltage, and I'd say it's safe to assume that the increased current (+20%) won't harm anything.

Electrical! It's fun.

Aviatrix said...

Stay tuned, zb. This is all-electrical week on Cockpit Conversation.

Ed said...

It seems to me that the increased resistance of the earth wire is the only plausible explanation given here for the limitation.

Just curious, but are outside sockets allowed without RCDs (ground-fault thingies - can't remember the full American name) in North America? I'd expect an external socket on an airfield to be updated often enough to be RCD protected though it would be worth checking.

Just to be sure though, it might be worth using an RCD protected extension lead as the first leg of the journey.

This is one of the ways in which wiring regulations in Britain have been tightened up over the last decade or two. For quite a few years all sockets which could plausibly used for outside use (typically all ground floor sockets) have to be RCD protected and the trend is to RCD protect pretty much every circuit except perhaps a spur for fridges and/or freezers.

The wiring in my house is pretty old so I use an RCD extension lead for connecting electric tools used in the garden.

A squared said...


IN the US they are called Ground Fault Circuit Interrupters (GFCI)

In the US electrical Codes vary from location to location, and I have no idea about codes in Canada. Most places in the US have adopted some version of the NEC, which require GFCI's in outdoor locations (as well as in kitchens, bathrooms, and Garages)

But, it's entirely possible in the US at least that an older building will not meet the current code.