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Everybody knows various flavors of the rule "don't connect too long electrical lines to an outlet / generator / whatever". They sometimes come in the form of "don't chain your extension cords". As far as I know, the main reason for this is that you might raise the wire resistance to a level that prevents fuses from blowing if you create a short-circuit.

Now, at the German TWH (Federal Agency for Technical Relief), there's an additional rule when working with (non-grounded) generators: You may connect anything up to 100m, measured over any pair of two phases. So, connecting 50m cords to all three phases is fine. Connecting 75m to L1 and 25m to L2 and L3 is fine, too. Connecting 75m to L1 and L2 would not be okay.

I wonder what's the reason for this? We sort of suspect that it's because the generator is not grounded, i.e., your neutral is "not really neutral". It would be great if somebody could elaborate on this in terms I understand. I'm not an electrical engineer (obviously, otherwise I should probably figure this out myself…?).

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  • \$\begingroup\$ Not sure about your rule, but wire has resistance. As you attach extension cords, voltage lost to wires increase decreasing voltage at end. Voltage may be reduced to an extent that heavy loads do not work properly, causing damage to extension cords with possible fire. You need a 100m extension cord, get a 100m extension cord. It will be appropriately sized for a given current. \$\endgroup\$ – StainlessSteelRat Nov 23 '18 at 13:58
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It is about disconnect times. As the loop impedance rises due to the higher resistance of longer wires the time it takes for a fuse or circuit breaker to open the circuit in the event of a fault increases because there is less current available.

The UK standards say for example that if the fault can cause a touch voltage larger then 50V then the protection must disconnect the circuit within 0.5 seconds, which requires a fairly large current to be able to flow.

For this reason a rule of thumb over here is that an extension should not be longer (in meters) then 10 times its CSA (in mm^2), so a 1.5mm^2 cable is good for up to ~15m and a 2.5mm^2 one 25m. Past that, pull in something heavier and break it down near the point of load.

The German generator thing is probably another case of trying to ensure that touch voltages stay sane, but really with a three phase machine you should be driving in some ground stakes (and measuring the ground impedance).

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  • \$\begingroup\$ Thanks! As far as I understand it, your first three paragraphs are a (really good!) explanation for the "don't connect lines that are too long". I didn't realize that the current played a role for the speed in which a fuse works. However, my main concern really was the "why measure over two phases". We can't really always ground our generator (or rather: we can't properly measure the grounding in the scenarios we're training for). What do you mean by "ensure the touch voltages stay sane"? \$\endgroup\$ – Lukas Barth Nov 23 '18 at 20:13
  • \$\begingroup\$ Fuses are thermal devices, they are seldom fast unless the overload is LARGE (2 times overload, you can be looking at half an hour or so)... Consider a fault connecting say L1 to ground out at one end of one of those cables, nothing happens because the generator is not grounded (But it is a nice surprise for the next person to lean on the generator frame if that is bonded to the star point). Now a fault on the other cable will have (best case, nice conductive ground) that >100M of cable forming the loop impedance and even with a dead short between the phases it may take a long time to trip. \$\endgroup\$ – Dan Mills Nov 25 '18 at 18:43

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