How to sense a small current while handling larger ones
10A is 2.5% of 400A, which is more than 'near negligible'. However a shunt with reasonable loss at 400A will have very low output voltage. The answer is to use a low-drift op amp to amplify the voltage enough to drive a switching device.

Let's say you can accept a shunt loss of 4 watts. At 400 A that corresponds to 10 mV, with a shunt resistance of 0.01 / 400 = 0.025 m&ohm;. 10 A would then produce 10 * 0.025 = 0.25 mV or 250 &mu;V. The AD8551 is an example of a so-called 'zero-drift' op amp, with an offset voltage of 1 &mu;V and drift of 0.005 &mu;V/&deg;C. This could theoretically measure that 10 A to an accuracy of 0.4%. 

Another alternative that doesn't require a shunt is a Hall-effect current transformer. 
The HT500M is an example which uses closed loop sensing for greater accuracy. It has an output current of 200 mA at 400 A, and offset current of +-200 &mu;A. At 10 A the output would be 5 mA +-0.2 mA = +-4% - 10 times worse than the current shunt, but with the advantage of non-contact sensing.