Chaining multi-socket extention cords, can it be done safely?

Question

I've always read that extention cords should not be chained. Also one should not build a tree of multi-socket cords. My background is physics, and I cannot see a problem with it, as long as one keeps in mind the current that flows. I might be neglecting something, that's why I want to make sure that I did not miss anything.

In Germany the electrical power is at 230 V, and fuses have current ratings of 10 A or 16 A. On many power strips I've seen that they have a maximum rating of 3500 W. The cable of the extension cord should be specified in a way that it can stand the current of around 16 A.

When using multiple extension cords, there are two fallacies that I can see:

1. People just see more sockets and connect more and more devices, going over the 3500 W of allowed current. The fuse might not trigger, but the cable will become hot and unsafe. This is something that I can easily avoid as in my use case would only have two nightstand lights and a phone charger at 100 W maximum.

2. The resistance of the total construction becomes so large that it will become a problem. Especially the DIN VDE 0100-600 seems to say that the resistance of the protective conductor must not be over 0.3 Ω for a cord up to 5 m length. For every additional 7.5 m of length, the resistance may go up by 0.1 Ω. The maximum may be 1 Ω. Assuming that individual cords are according to standard, I could hook up three of them; but that would totally ignore the resistance of the plug-socket-connection.

Where does the problem really come in? Is it that the protective conductor becomes useless if its effective resistance is too large? Or can I indeed chain power strips as long as I keep the total power consumption of that system well below the 3500 W threshold?

Answer 1

The wires themselves are not the problem. Humans are, because it is easy to overload the circuit, and even if it is below the safe margin of the fuse tripping or wires burning, you forget contact resistance and loose connections.

When given enough use, the contacts come loose and they oxidize and the contact area becomes smaller and contact resistance increases. Thus the contacts heat up more and given bad enough connection, the heat starts to melt things and cause fires.

Answer 2

Your analysis seems sound. I would give a little more consideration to "but that would totally ignore the resistance of the plug-socket-connection." That could increase as the items are plugged and unplugged over time. You should also consider that longer runs of cord have a higher probability of being stepped on. That would accelerate wear and deterioration. I believe that the mitigating factors can be legitimately considered as offsetting the risks. Not everyone will understand the risks and mitigations, perhaps not even everyone in your own household.

Answer 3

There is no hard and fast limit. It's more the spirit of the law, than the letter.

One of the problems is the risk of overheating and possibly fire due to bad contacts. The more contacts you have in series, the greater the risk of getting a bad one. Even one extension lead is not a great way to power a heavy load like a kettle or fan heater, but people do it. The risk is accepted, just like driving is dangerous, but people do it, and accept the risk. Putting several in series is a bit like driving faster, or when you're tired. It increases the risk, don't do it if you can avoid it.