What a ground strike does

The arc reaches something earthed. That is one event, and it has consequences on five different parts of the machine, which is why the pieces of it are usually met separately and never add up to a picture.

The event

arc -> earthed object
     secondary loaded by
     a resistance the coil
     did not choose

Everything below follows from that last line. A coil's own loading is designed: the arc you draw is the load you tuned for. A strike substitutes a resistance set by what the arc happened to touch, how wet it was and how far the current has to travel through it — anywhere from a few hundred ohms to megohms, with no say from you.

Consequence one: the secondary stops reporting its own frequency

This is the oldest of the five and it is the one that shaped every driver in use. Steve Ward, in his DRSSTC log book:

When a ground strike occurs, the secondary appears to jump to another harmonic, usually 3X or 5X the Fres of the coil… This causes all sorts of bad switching transitions… I believe this is why I've lost too many IGBTs in the past.

A secondary ringing at three or five times its resonance is not telling a driver anything useful about when the current crosses zero. A driver that believes it switches into a full bus at the wrong instants, and the bill arrives at the bridge rather than at the topload.

The fix is structural rather than protective: take the feedback off the primary current, and the loop stops being able to hear what the secondary does under a strike. That is why the feedback comes off the primary at all, and it is a decision that gets presented as a design preference far more often than as the failure analysis it was.

Consequence two: it pulls the frequency further than an arc does

And this is the one place on the page with a measurement behind it rather than a model. davekni, on his oversized QCW in HVF 2397, reports the upper pole at three points:

unloaded            485 kHz
end of a normal arc 440      -9.3 %
end of a ground     413     -14.8 %
  strike

The strike took it 27 kHz below where an ordinary arc of the same machine ended. Which is the direction you would expect — an earthed object is a larger load than air — but the size of it is not something anybody had put a number to here, and it says the detuning budget a coil is built with is not the budget a strike will use.

The same coil is where the pole equation itself got checked, and the check is on the poles page.

Consequence three: the pole structure can come apart

Load a coupled pair hard enough and its two resonances collapse into one near the primary's. The condition, the source and what it costs you are a property of coupled resonators rather than of this event; what matters here is that a strike only does it across a band of resistances.

Damping peaks where the strike's resistance matches the secondary's reactance, which on this coil is 21.1 kΩ, and the structure comes apart across roughly 11.2 to 39.6 kΩ. Outside that band, in either direction, both poles survive: too high a resistance barely loads it, and too low a one stops being a damper and starts being a short.

Consequence four: the strike that leaves the machine intact is the worse one

Which reads backwards until you see it, and is the useful half of the whole page. **The dangerous strike is the one that does not collapse the structure.**

After a collapse there is a single frequency near the primary's 349.9 kHz, above the sword threshold, and nothing for the driver to fall onto. With both poles still alive, and the upper one ringing for barely more than a cycle at two metres, the driver recaptures from noise — and from noise the lower pole is the stronger. So the mild strike, the one that does not even interrupt the bang, is the one that can move the machine to the wrong mode and leave it there.

That is also why "did it survive the strike" is the wrong acceptance question. The one worth asking is what frequency it came back on.

Consequence five: the current falls first and rises later

The direction is the surprising part, and the timing is the part that matters.

Damping raises the resistance reflected back into the bridge, and a higher reflected resistance at a fixed bus lowers primary current. So a strike sitting in the band that collapses the poles is also a strike near the current minimum — the opposite of what an overcurrent detector is built to catch. On a coil of this size the fall is large rather than marginal: hundreds of amps down to tens.

But that is a place on the path, not a place the machine stops. A real strike travels through it:

streamer approaches, touch
  hundreds of kohm
  current falls, OCD quiet

channel establishing
  tens of kohm
  current at minimum, quiet

channel conducting
  a few kohm
  current climbs, OCD FIRES

So the detector is not blind to the strike. It is late to it. Once the channel conducts, the secondary is closer to shorted than to loaded, the reflected resistance falls away and the current goes up past any threshold worth setting. The exposure is the interval between the touch and that moment, during which the machine runs in a mode nobody chose and the protection has nothing to say.

Which decides an argument on the next page over

The detector fires either way, so the question is what it does when it fires, and the delay above settles it:

latching OCD
  burst over, mode never
  had a chance to change

pulse skip
  drops a cycle, returns —
  into a different buck
  voltage and a different
  pole structure

Pulse skip is the right choice for a long bang and the wrong one for this, and the page that owns that setting should be read with the delay in mind.

And there is indirect evidence that a burst does survive a strike rather than being ended by it: davekni's 413 kHz was read at the end of the ramp that struck earth. His flash ran to completion. The remainder of it ran in whatever mode the strike left behind, which is the entire reason for telling these two apart.

What to do about it

• Feedback off the primary. Not optional, and consequence one is the reason.
• A ground plane or a target, so the strike lands where you chose. A strike you arranged has a resistance you roughly know, which removes the only genuinely uncontrolled term on this page.
• Coupling high enough to keep the margin. At k = 0.15 there is no margin to keep; at 0.4 to 0.5 there is.
• Check the frequency after a strike, not the fuse. Consequence four, and a quiet detector is not evidence that nothing happened.
• Latching rather than pulse-skip overcurrent, if the machine draws to earth at all.

Where next

• The two poles, and which one the driver lands on — what the collapse above takes away.
• Inside the driver — where the primary feedback decision lives.
• Overcurrent is not a setting — what the detector is actually for.

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The strike is the one event on the department diagram that is not drawn, because it happens outside the machine and arrives at every box in it.