Earthing?

[Clive S]

If you have a circuit going through a hole in a metal box, I suggest you make sure that both wires go through the same hole.

Is that because you will get a ground loop

[Robin Hewitt]

I cannot pretend to understand metal boxes and the strange things that happen when you drill holes and run wires through them. Putting the whole circuit through a single hole sounds like a good idea, balance things out, like an earth leakage trip.

OTOH, sounding like a good idea is not always enough. Recently there was a lovely picture doing the rounds of a plated steel nut and bolt holding an earth loop to a copper bus bar. Should have used copper, they used to use copper nuts and bolts in that golden age of understanding when I were a lad. In the picture dissimilar metals had formed a thermocouple. The earth loop probably only gave a fraction of a Volt but the current was enough to heat the nut red hot

[Neale]

Electric current through a wire creates a magnetic field around it. Current flowing in opposite direction creates an equal and opposite field. Keep the wires close, ideally twisted together, and the magnetic fields cancel out, don't transmit interference, and are resistant to outside magnetic fields for the same reason (rather simplified but the principle is true). Pass wires through separate holes and not only do you lose that "cancellation" effect because the wires are further apart, but you now create a magnetic field between the two wires that can induce (= create) a current in the metal box between them, which is exactly what you don't want to do. I can bang on about differential signalling and common-mode rejection, but the simple model is pretty sound to understand what's happening.

[Edward]

I can't see why anyone would want to separate wires of a circuit through different holes or not....

A practical question about wiring the 8 wires of a stepper in parallel:

For short run desk tests I normally join and solder each pair length as supplied to go to the respective A+,A-,B+,B- twisting it together along the way, as Neale mentioned above. But now I have to do it properly using 4 core screened CY cable, 1.5mm2. which I think is rated at a whopping 18A. Should I cut the motor cables as close to the motor source as possible, solder each pair to one of the four cores of the CY cable and use heatshrink on each joint, and then a bigger dia heatshrink to hold together the lot? This bulky point where all the cables join at the motor side, and the screening ends, isn't it a possible source of interference?

If so, is there a recommended best way to do this?

In terms of magnetic fields and interference, is it better to instead use 8 core CY cable all the way to the driver but of a smaller cross sectional area?

Edward

[Robin Hewitt]

But now I have to do it properly using 4 core screened CY cable, 1.5mm2.

Why is that "properly"? I like big fat wires, as big as will fit in the screw terminals on the driver. I run my big fat wires through flexi-plastic conduit to keep it tidy.

Why would you want screened? Armoured would be more understandable.

If you are worried about interfering with your high speed computer connections, you could screen those perhaps.

[Edward]

Robin, now you are confusing me:)

As far as I understand, "armoured" hardly offers any electromagnetic containment. All signal cables will be screened. I will use flexi conduit for the motor wires too. Do you mean you run 8 separate fat wires inside the conduit?

Edward

[Clive S]

Robin, now you are confusing me:)

As far as I understand, "armoured" hardly offers any electromagnetic containment. All signal cables will be screened. I will use flexi conduit for the motor wires too. Do you mean you run 8 separate fat wires inside the conduit?

Edward

Edward just do as you suggested chop the wires off about 250mm from the motor and then join them to the CY cable personally for the motors you are using I would go with 1mm size. If you want to be posh you can get an end cover for the motors and take the cy cable into that.