In article , Don Pearce
wrote:
On Sun, 03 Aug 2014 10:16:40 +0100, "Dave Plowman (News)"
wrote:

We now have the annual PAT test. In my lab once a year a chap comes
round and tests the integrity of the grounding of everything that has a
mains plug, from the 100 grand Network Analyser to the kettle in the
kitchen.

Once he has gone all the oscilloscopes get their grounds removed once
again so they can make floating voltage measurements and not introduce
ground loop mess into normal measurements.

It doesn't actually make anything safer, but the company lawyers like it.

Nice to see that people are maintaining the old traditions. :-)

FWIW When I used to install and use systems on the old UKIRT (Telescope)
they were always 'grounded'. But always gave you a multi-kV static shock if
you were the first to touch them after they'd been left un-touched for
about 10 mins or more. The ground connection was essentially dry rock in a
dry atmosphere. So was little more than a common reference. Certainly not a
way of ensuring no overall charge potential. 8-]

One trick was to send the newest PhD student to adjust the kit and the
watch to see if we could see the spark in the darkened dome. Usually
accompanied by a suitable noise from the student. :-) Rite of passage.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

On Sun, 03 Aug 2014 16:37:01 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sun, 03 Aug 2014 10:16:40 +0100, "Dave Plowman (News)"
wrote:

We now have the annual PAT test. In my lab once a year a chap comes
round and tests the integrity of the grounding of everything that has a
mains plug, from the 100 grand Network Analyser to the kettle in the
kitchen.

Once he has gone all the oscilloscopes get their grounds removed once
again so they can make floating voltage measurements and not introduce
ground loop mess into normal measurements.

It doesn't actually make anything safer, but the company lawyers like it.

Nice to see that people are maintaining the old traditions. :-)

FWIW When I used to install and use systems on the old UKIRT (Telescope)
they were always 'grounded'. But always gave you a multi-kV static shock if
you were the first to touch them after they'd been left un-touched for
about 10 mins or more. The ground connection was essentially dry rock in a
dry atmosphere. So was little more than a common reference. Certainly not a
way of ensuring no overall charge potential. 8-]

One trick was to send the newest PhD student to adjust the kit and the
watch to see if we could see the spark in the darkened dome. Usually
accompanied by a suitable noise from the student. :-) Rite of passage.

Jim

What do you reckon was the capacitance to true ground? More than a
couple of hundred pF and you can start to feel it properly.

d

In article , Don Pearce
wrote:
On Sun, 03 Aug 2014 16:37:01 +0000 (GMT), Jim Lesurf
wrote:


FWIW When I used to install and use systems on the old UKIRT
(Telescope) they were always 'grounded'. But always gave you a multi-kV
static shock if you were the first to touch them after they'd been left
un-touched for about 10 mins or more. The ground connection was
essentially dry rock in a dry atmosphere. So was little more than a
common reference. Certainly not a way of ensuring no overall charge
potential. 8-]

What do you reckon was the capacitance to true ground? More than a
couple of hundred pF and you can start to feel it properly.

No idea of the capacitance. It gave a nice spark so the potential was high.
And the discharge was enough to cause pain. The telescope was essentially a
metal cage about 2-3 metres in diameter and maybe 8 metres long. With a lot
of kit hanging from the back. I'm not sure where the charge was leaking
from. But the prime suspect was either the 2kV dc PSU when using a klystron
or the 8kV one when using a carcinotron.

In practice I suspect what controlled the discharge was the electrostatic
capacitance of the human as the charge densities on the telescope and the
person came to equilibrium. The floor was usually also dry. So someone else
being 'first' didn't guarantee that the next person wouldn't get a tingle.

Once experienced you either used something resistive to make 'first
contact' or used the ploy that firemen are taught. First touch with the
*back* of your hand so the stimulated reflex is to jerk away.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html