Hello all,
Thank you all for your responses.
Russell, I think the method that you speak off is being used at some places instead of the bar-to-bar resistance test that I hope to implement. The Baker tester is used in that case.

Kai, Sorry that my explaination was not clear before, so please allow me to answer your questions...........

1. Only the armature is under test, the armature and stator are seporate at this piont, the stator is tested elsewhere, hence there is no movement of the armature unless it is turned by the technician. The shaft of the armature sits on rollers and the test supply is applied to the commutator via brushes mounted on a semi-circular frame. I plan to use an IGBT to switch this supply. Currently, a simple mechanical footswitch is used to switch this current.

2.I am not sure how to handle these switching spikes yet, But I plan to try the Zener Clamps that Steve suggested. My only concern is if the zeners are fast enough to prevent damage to the rest of my cct. I dont knoe how high these spikes are as there has been no tests in place to measure them. I dont think that it takes very long for the current to settle at its final value.

3. I am intersted in the variance of the readings from each other, i.e. by how many pecent they vary from the reference reading. The reference reading is the reading taken on the forst bar. But, I also have to log each value, so I also have to know each reading.

4. By implement digitaly I mean take actual readings via an ADC.
The cct that takes the reading is the first phase of an automation project. The idea is to automate the entire test, I have to take the volt-drop reading, feed to an ADC, analyse the reading and then save it on pc using a serial interface. The application of the test current etc, will be switched by a control cct that will be designed at a later stage.

5. The test supply. The test supply is a welding machine, I have been told that these are low voltage, high current supplies. at some test bays it is set to 10V/300A at others 60V/300A. As for safty earth, all that I have been told is that the welding machine is a seporate DC supply. As for my cct, I plan to buy a power supply off the shelf that is plug into the wall plug which is 230VAC. The output of the supply is 24VDC.

6. The voltdrops across each bar I hace been told is in the region of millivolts. Hence the cct has to be have a very, very high accuracy. Im thinking accuracy and resolution should be in the odrer of tens or microvolts. What sort of resoultion can an op apm offer, i.e. what is the smallest input change it can detect. does the INA117,have a better resolution and accuracy, and can it be used in the case of the 10V and 60V supply, as its CMR is +- 200V?

7. When a short cct is detected the volt-drop reading at that point is almost zero, and when a open cct is detected, a full scale deflection is seen, i.e. the reading is that point is the supply potentioal (either 10V or 60V). With the 60V supply this will cause a problem as the max input to these apmlifire devices is +- 15V depending on the supply. Here I plan to Use Zener Clamps that Steve suggested (as I mention before). But its the speed that I am worried about.

I hope that this gives you a better idea of what going on here.
At this point it seems that I need a high accuracy , high resoultion input phase, that needs to well protected from high voltages and currents.

Thank you all for your time, effort and ideas.
Sun.