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I know this might be a dumb question, as wiring a plug isn't some rocket science, but I am wondering if there is something not so obvious that can go wrong, or any tip at all for me to care when doing a cable assembly for a custom power extender cable?

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As obvious as it gets, but better safe than sorry, and I've re-done enough such DIY plugs to think this is answer-worthy:

  • make sure that the strain relief presses on the outer jacket (the outer jacket should reach 2-3 mm beyond the strain relief "bridge")
  • cut the L and N wires ~1 cm shorter than the PE wire, then bend the excess PE wire so that it will detach last when the cable is forcedly removed from the plug
  • use suitable pointed pliers to arrange all three wires neatly around the threaded studs that will later take the screws from the top part of the plug assembly (often insulation is damaged by squeezing between the plastic parts, or even the screw, which can be really dangerous with cheap models where the case screws are accessible while the plug is in the socket - the plug casing is plastic so no PE protection here).
  • test the cable after assembly, with a load close to its rating (for example a 3000 W radiator)

Note that neither SCHUKO nor EN 60204 (VDE 011) explicitely mandate solder or ferrules on stranded wire - but using ferrules is common practice and should be standard whenever a screw is used to clamp on a stranded wire.

Tin-coating may seem a cheap alternative to ferrules but is difficult to get right and dangerous. For the last 15 years I've seen no professional electrician in Central Europe practising it any more.

A peculiarity of SCHUKO is that "polarity" does not matter - there is no marking for where the L and N should go because plugs are not directional like for example the British ones.

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    \$\begingroup\$ WARNING! Don't do tin-coating! When tightening the screw, it gives you a good feeling of tightness, BUT over time the tin creeps away from the pressure, leaving you with a loose connection that will get hot and maybe burn. \$\endgroup\$ – Ralf Kleberhoff Dec 29 '17 at 11:42
  • \$\begingroup\$ Hey, sorry for the late reply - holidays were here and I needed to check few shops before responding to you. Here is an album I compiled with few questions/comments within it. What was not clear to me is that the ferules seem weird to use in my case, because the connectors are having this screw in connection, should I change them? But I also noticed that your third bulletin point says that indeed you are talking about the same connection as mine, I am not sure how would that be done, should I put the cable with the ferule inside the hole and smash it in? \$\endgroup\$ – appwizcpl Jan 10 '18 at 11:47
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I assume the plug uses screw terminals. In this case, and if you use stranded wires, use wire ferrules to protect the wire strands from breaking. The wire must be rated for 16A usually. Ensure that the strain relief is tightened properly. Make the earth/PE wire little longer than N/L. This is to ensure that the PE wire stays connected longer than N/L in case someone manages to pull the cable out.

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If you don’t have any ferrules handy and, from experience, most ordinary people don’t, then I tend to bare about 12mm of wire, twist it gently and then double it back on itself - then put it in the terminal and tighten the screw.

Only time one has come out on me was when somebody pulled so hard on the cable trying to pull the plug out...

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  • \$\begingroup\$ How about soldering the ends in lieu of ferrules, to keep the strands together? I have done it before with speaker cables, and it has worked somewhat OK. \$\endgroup\$ – Dampmaskin Dec 28 '17 at 15:40
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    \$\begingroup\$ @Dampmaskin that trick has fallen out of favor these days. The wire tends to snap in the short gap just where the solder meets the insulation, because that's now the most flexible part. \$\endgroup\$ – Simon B Dec 28 '17 at 16:53
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    \$\begingroup\$ @Dampmaskin Beware - solder is soft and flows slowly away under the screw pressure. The joint becomes loose and starts to become hot. \$\endgroup\$ – user287001 Dec 28 '17 at 21:23
  • \$\begingroup\$ Aha! That explains all the loose speaker wires I experienced back when I used to do that. :) \$\endgroup\$ – Dampmaskin Dec 29 '17 at 16:16
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I assume that you use flexible (multi core) cable.

When I make an extender, after assembly I make a load test (running a high power resistance, about 2 kW for a few minutes) and be sure about the temperature of contact points are not more than usual.

Also, I shape the peeled tip of the cable spirally (as much as it goes without giving harm to the cable) with help of a pliers. This way, you raise the density and solidity of material which will be squized with the screw and this way you get better contact conductance and better mechanical properties.

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