| There are two problems:
First the ignitor may ignite the discharge in the outer when the arc in the arctube fails to ignite. That will happen mainly if the arc get extinguished (e.g. by some mains irregularity) and the thing will be attempting to restart when the lamp is still hot. Unlike the PSMH or HPS, the probe start lamps are notdesigned to handle the high voltage from the ignitor.
So in order to operate a probe start lamp on it, you need to deactivate the ignitor, disconnecting (and capping off) the Neutral wire to the ignitor does the job. You may also add a switch to the fixture, to allow to change the lamp type later without messing with the wiring, you need a 250V AC rated switch, current rating is not that important (there are just some 10's mA max flowing through the wire)
Second problem is related to the exact current spec of the lamp, but I'm not 100% sure it really applieshere: MVs use to be rated for something around 3.25A (I do not remember the exact number now, I know it is somewhere around that), it is a kind of world wide standard. But 400W pulse MH in their European origin used to be made along two specifications: One the same current as 400W MV so the mentioned 3.25A, second type along an European 400W HPS rating, so 4.3A (again, do not crucify mefor an exact number). Some more recent lamps were actually rated for both ballasts, but with different performance rating (color, output,...) for each of the ballast types. At this point I do not know whivh of them was adopted by the US standards for the 400W pulse-MH rating (or if both were; then each under its own ANSI code). If the ballast is for the 4.3A rated lamp, the 400W MV will be overloaded on it.
So you need to check the lamp current rating (or arc voltage, the 3,25A has ~140V arc, the 4.3A one ~100V arc). You may measure the lamp current with the lamp rated for that ballast.
Voltage measurements are trickier, as most meters won't be able to safely handle the ignition pulses, so you can connect the meter only after the lamp has ignited. That means messing up with live wires, so a danger on its own. Plus you would need to make sure the lamp has really parameters corresponding to the standard rated mean - many lamps tend to deviate, mainly when aging or when their maker wants to compensate for its lower efficacy by allowing higher than rated voltage drop so burning more real power, or when it is some energy saver bulb designed to lower the actual power by having lower than standard mean arc voltage.
So better to stick with the current measurement, you may do all the instrument wiring when the thing is not powered and do not need to touch anything when turned on. Or the safest is to use a clamp Ameter, if you have one.
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