Note that rogowski coils are really not that expensive - I think we bought two at ~$1500 each not long ago (unfortunately the company tanked a few short months later). This is really chump change when working at a large corporate company, as I believe you are. I also had to demo one to convince the boss that I needed one to get the job done properly before he'd shell out the cash. I thought that was a reasonable request. It was an easy sell once he saw the side-by-side comparison.

The expensive current probe engineering has is for a pretty expensive Teledyne LeCroy scope. I think the probe is like $3K. Only problem is it is a 20A probe, so I can only compare our A622 using a driver with a small inrush. My thought is just that the A622 probe will capture most of the inrush except maybe the initial peak that occurs in 10uS or faster. The energy under the curve for that brief time is going to be relatively small when compared to the total joules of a driver inrush that has a 10%-10% rise-fall time around 2-3mS. We have seen drivers with lower peak inrush currents, but longer duration cause more relay contact welding than drivers with much higher inrush currents, but shorter durations less than 1mS.

 

We have seen drivers with lower peak inrush currents, but longer duration cause more relay contact welding than drivers with much higher inrush currents, but shorter durations less than 1mS.

Now this is an interesting thought - and something I don't specifically recall considering when I was working at my company. My thought was always that the higher short pulse was probably what was welding the contacts together - but I never evaluated it to get proof other than the fact that we saw more failures at 277Vac than 120Vac which would have a much higher dv/dt. It also makes sense (to me at least) that arcing would be what caused any welding. Welding would be more likely when the resistance was high with a large surge current (i^2*R) while the relay was in the process of actuating and arcing over for initial contact.

As I mentioned though - I don't recall evaluating specifically if it was the high frequency / high amplitude surge currents killing my relays or if it was the low frequency / low amplitude surge currents. I worked to eliminate both by developing a way to switch at the zero-cross. Last I knew my buddy had one in his basement getting close to 1M cycles - the mechanical failure lifetime of the relay.