I wanted to put the pi that controls the robot arm, onto the relays I've set up. But the distance from the relays to the pi is more than allowed by the rules, more than five meters.
But what are rules for, if not to be broken? The problem I had before, was that the voltage drop between the relay and the Pi was too great, so the Pi wouldn't work. And the reason for that, is that USB wires are very fine, they aren't expecting to be used over more than five meters.
So I made my own. I'm using bell wire (a pair of single code copper wires, but a lot thicker than USB wire). I soldered the female to a strip of metal, soldered a few inches of multicore to the positive and to the negative connectors, and put a terminal block on the end of that. Then I did the same for the male, except without the strip of metal. Then I ran a long length of bell wire from one to the other, and I was ready.
I connected this up to the relays at one end, and to the Pi at the other, and it works fine. But the Pi also has a relay controller, and that won't work with the small amount of current it can get from the Pi, so I'll give it a power supply from the relay box. To do that, I've ordered a USB splitter cable, so I don't have to run a second wire from the power source.