Danger, and I don't understand why.

Hi all,

Today something a little scary and even more strange happened
during what I thought was a fairly routine test.

I hooked up a simple static gap across a neon transformer,  with a
view to later examining the RF behaviour of the gap with my
A real simple circuit as show below:

      Power switch

  O-----o  o-----D  C----------------!
240VAC           D  C                O
Supply           D  C--- Ground         2mm air gap (bare wire ends)
                 D  C                O
  O--------------D  C----------------!

I tried the circuit without the scope connected first.  Sure enough a
tiny flame like arc leapt across the gap between the two pieces of bare
HV wire.  No surprises there.

Now here is the weird bit.  After a few seconds of buzzing and fizzing
one of the wires moved a little and the gap closed up (shorted.)  I
turned off the power switch, unplugged the supply cord,  walked over
to the gap, and pushed the wires apart with a plastic rod.  To my
surprise a 4mm white spark snapped across the gap.

This caught me by surprise.  I thought the circuit would be dead once
the power was switched off, as there are NO CAPACITORs etc. in circuit.
I could easily have grabbed hold of the wires with my hands, because I
expected no danger !

I tried to repeat this "occurence" and found that the wires would only
spark when pulled apart if they had first SHORTED WHILE ARCING with
the power on, and it was then turned off before pulling the wires apart.
The spark produced quite a snap but is no where near like a cap
discharge,  more like a spark-plug type click.

I am posting this because there may be a safety implication here,  and
I am keen to know if anyone else has observed this behaviour before.
Maybe I am being a little slow but I certainly did not expect an arc
after the power was turned off.

				- Richie,

				- Re-thinking my safety procedure
				  in Sunny Newcastle.