PCB Failure.

here are just a few possibilities:

1. Large loads are switched off causing a feedback.
2. Changing ground potential due to electrical storms
3. Inconsistent generation
4. Radio frequency interference, noise from electrical devices
5. Solar radiation
6. Meteorites
7. Lack of surge suppressors, noise filters
8. Electromagnetic pulses
9. Poorly shielded conduits
10. Static electricity
11. Not using a static ground strap when handling the PCPs will cause damage before the PCB is even installed.

A good way to start, is to monitor the lines for a long period of time. That way when you have a failure, you may be able to identify what is causing the surges.

You could also monitor the lines and shut down each piece of equipment 1 at a time. When you find the culprit, either add a line filter, clamp, or find a way to shut it down in stages.

It is not just the switching off that is the problem, when you switch a machine on that contains an direct on line transformer, it will spike back at approximately 7 times the going load current for some 50 to 100 ms due to the magnetic field being build up inside the transformer. This building up is perceived as a moving magnetic field and thus generates a reverse current.

If this building or the other equipment has no protection against this the contractors should be found and asked to rectify this.

Harmonics is actually a good possibility. If the capacitors are 3-phase and all 3 poles are connected simultaneously, at least 2 of the 3 will connect at some point on the sine wave other than zero crossing. This causes either a spike or sag on that phase, depending on the charge state and polarity of the capacitor.

One of my manufacturing customers had a similar problem. I used a Dranetz disturbance analyzer to monitor the internal distribution feeder for 24 hours, and found that their own PF correction capacitors were imposing some severe harmonic spikes during switching. Since they needed the caps to avoid a PF penalty fee, I recommended they install a full-isolation UPS (the only connection between the incoming AC and the load AC is through the battery) and connect all of the machine control circuits to it. For eight years, they have had no further problems. They also sized the UPS to supply all of the computers in the offices, receiving & shipping, preventing any loss of data or productivity.

A Dranetz or similar full-spectrum analyzer should be available for rent from a test equipment house for a reasonable price, and it is very easy to connect and program. Even if the problem is not the capacitors, the analyzer will record the time and magnitude of spikes so you can find a pattern and track down the cause.

I agree on the harmonic analysis recommendation because it should help to identify the likely internal source of the voltage spikes.

If any of the loads are large three phase electric motors then dynamic braking voltage spikes during motor braking are a likely source for problems. These should be properly ground shunted but can severely back feed especially if the ground connection itself is insufficient or just old.

Are these not shorted over large capacity resistors?

I work with dc motors and have yet to see a dynamic braking system that shorts to earth when the motor is slowed down. All the generated power is absorbed internally by the dc control.

Maybe it is different with 3 phase motors but I would not like to use my earth connection for that purpose.

It is suggested that:

Following points clarified:
1. Do the PCB(s) itself fails after assembly or
[PCB] = certain cards fail?
2. Are these Cards Professional designs & manufactured? And being used in an OEM Euipment?
3. Are these cards part of some Standard Equipment or p/o some Fabrication / Manufacture Setup
for Cards?
4. Are the failures during normal operation or due to Power Outages?
5. Can we get some summary of his findings on failed PCBs [Cards]?

I have some suggestions in general:

6. The Power Supply for Equipment or Indiginuous Setup be checked for:
a. Output Voltages' OV limits? Are these protected? Passively or Dynamic?
Some Discrete OV protectors be used.
b. The DC Filter capacitor:
A Hi-Capacity [Backup] capacitor in DC output may absorb Spikes in power line.
And Holds POWER to critical circuits for normal SAGs.