Daisy-chaining extension cables is bad - why? Featured

I remember my parents' three-bar electric fire began to emit "a funny smell" when it was on three bars for a while. I investigated with my multimeter.

There was no measurable resistance between the live wire in the plug and one end of the heating element, nor between the neutral wire and the other end of the element.

I noticed that the live pin was getting too hot to touch, and this happened in a variety of sockets, so it *probably* wasn't high resistance contact between socket and pin. It turned out to be poor contact between the live-wire side of the fuse and the fuse itself, between spring clip and circular barrel of fuse - I measured about 5 ohms which is 13^2 * 5 = 845W being dissipated in that bad junction. :-(

Hardly. In the case you are postulating, the volt-drop across the fuse-end would be 65 volts so the load would receive about 165 volts. What sort of 13A-socket-compatible load would draw 13 amps at 165 volts?

Interesting anecdote - shame about the dodgy digits ;}}

Quality granny chargers have a heat sensor in the 13A plug, and can dial down the current or shut it off altogether. They also tend to have B-type RCD protection built into the control unit.

The actual resistance of a bad contact is completely undefined, and will be affected by slight movement, vibration, temperature change etc. So he could be right at the time of measurement, but incorrect while power had been drawn. It's also common for the displayed resistance to depend on the current being drawn e.g. at 0.1mA it could be 5 Ohms, and much less when a higher current has affected some of the corrosion. Ohmmeters for electrical testing (e.g. PAT testers) will usually source many Amps to avoid this issue when checking earth resistance.

I once had a Netgear powerline adapter (a.k.a. mains wifi extender) which wouldn't work properly if one of the two adapters was plugged into a short trailing mains socket.

That's not an electrical power or resistance issue, it's because the signal being transmitted has a very high frequency which does not travel well in power cables, and particularly not through plugs and sockets. I have a pair of powerline devices which work through the particular extension cable I used to use at one end, but the instructions simply say not to use them at all.

Transmission will depend on lots of things about the house wiring, the shape of the cabling, how many spurs, and other things. All the things that need to be done carefully in high frequency transmission lines are completely wrong in electrical wiring. It's a wonder they work at all.

I used those things, Sketchy at best, totally useless at worst

Mostly, they don't IME

But Americans don't do this, of course.

This is one of the reasons the triple output cords had the fitting removed, and an extension has just one hole on the end. Indicating to the user, the intention of powering one load. They used to encourage abuse, by putting a three hole plug on the end of 50 or 100 foot cords.

For example, when I mow the lawn with my electric lawn mower, that is the only load.

I actually concatenate cords on purpose. I *need* the voltage drop, because the mains is typically 122VAC (when it was supposed to be on the order of 113VAC nominal). Adding some voltage drop, helps avoid overheating the motor of the appliances.

For example, one day, I was running the string trimmer on the end of my very nice (low impedance) yellow cord. I could smell something. Holding the motor area of the string trimmer near my nose, I was smelling overheating motor.

By replacing the yellow low-impedance cord, with some old red ones, the string trimmer stopped smelling of overheat.

Electric motors can overheat with too much voltage, and can also overheat on too little voltage ("lugging"). You need to study the conditions you are applying, to get the best life from the items.

My electric lawn mower is now *35 years old*. And one of the two brushes has been replaced (they did not wear at the same rate, the other brush having lots of wear left).

When people here create giant collections of cords, they are for wall adapters, where there is no danger at all of exceeding the current carrying capacity of the equipment. You would be lucky if the load in such cases was an ampere or two.

Only people who work out of pickup trucks, seem to have no sense of ampacity. For example, when I got a new furnace, the installer had a pipe cutter in the back of his truck, and I suspect that was drawing 20 amps from one of my external outlets. It didn't trip anything... but the lights were dimming every time he finished a black-pipe end (cut thread on it). These are the iron (black) pipes legacy installs use for gas connections.

When I run the heaters on the roof, there are two of them. I run separate extension cords for each one (a red chain, and the prized yellow one). Each cord is plugged into a different outlet box. You don't plug both of the chains into the same duplex box. This puts the two cords on separate breakers, and they're running at maybe 40% capacity.

Even when you take care, no matter what country you're in, you check for ohmic connector failures. Check the outlet temperature, for signs there is excessive ohmic loss in the box area. I replace duplex outlets with "severe service" ones, if I detect a thermal issue. That type has a slightly higher spring pressure on the contacts, and not much else of note (costs a buck or two more). All made in Mexico.

Paul

The PLC isn't a simple scheme.

In North America, for instance, HomePlug AV only uses 917 of 1155 sub-carriers.[9] "

The massive number of sub-carriers, similar to how DSL works, is how an impaired frequency is ignored, on a less than ideal transmission structure. Unless there is systematic interference (a broadband noise source), some of the channels should work and give somewhat of the rated bandwidth. For example, if you were welding with a legacy welder, on the same circuit as a PLC, that would suffer a bit from the experience.

Similarly, there is a possibility of a defect on an ATX PC power supply, which blasts right through the ATX entry filter items and you end up with "noise on mains". That can disable other equipment sharing the same mains. I had that happen here, on an Antec (ChannelWell) power supply, where opening the unit showed no visible evidence of a failure in the switching section. Not using that ATX supply, all the other equipment went back to functioning OK.

PLC works best, if there are mains transformers or filters, to isolate sections of line from one another (filtering between home owners sharing a phase).

Paul

Most places don't have fused plugs, so it is not just an American issue.

(Americans do have some additional problems to contend with; like half the voltage, so twice the current and four times the power loss in the cable, plus some fairly shonky standard plugs and sockets)

I tested my multimeter here, the <cough> good one.

On ohms, the 200 ohm range, it measures 5.2 ohms with the leads shorted together and no ohmic gadget in the path. You must measure this value and subtract it from all actual measurements made on that range. That's with standard blunt needle tip probes (a bad way to be doing this measurement). The digital meter, has no zeroing capability.

For low impedance measurements, there are lots of little details that matter. Thermocouple effects from dissimilar metals, might matter, if the amplitude of what you're measuring is small enough.

If you're a smart guy, you could test your craft on a manganin shunt resistor, some of which are calibrated to 0.25% and have four fasteners for doing a four point measurement of the resistance. (Input current on the big nuts, output voltage on the small screws) "This would separate the men from the boys, when checking the ohms" You would use the small screws, if bodging a measurement with a cheap multimeter.

In the old days, with analog meters, there may have been a "zero knob" which could be used to zero out the effects of the leads. As before, with great care, because you're measuring a quantity they weren't really intended for.

You can make a lab bench constant current source fairly easily, if you need a higher probe current to add some drama to your measurement. The LM317 needs a heatsink, the resistor also needs a power rating and free air. I have one of these in the kitchen, running the LED lighting :-) (half an amp)

Since the fusing characteristic is temperature sensitive, if you had actually suffered an ohmic connection, the fuse would have blown, long ago. Fuses must be derated for higher temperature applications. Even whether ambient air can get at the thing and reduce the temperature, that could matter. I suspect this is one reason that Polyfuses aren't used in laptops (no air flow in the area the fuses are placed). Laptops use silicon fuses (an eight pin chip).

Zero the meter and try another measurement. On a digital meter, subtract the test leads value from the measurement. As my poor result shows, the lead value is significant and unrealistic. The leads should NOT be five ohms.

Paul