Hi John,
Thanks for the reply and explanation. Yes, I was aware of the
capacitors (although I didn't know their name) and the neon tester
(already own one) but unaware of the lurking dangers for instruments.
One is therefore obligated to use ground rods when operating off of
portable generators - correct ?
Regards,
-Benoit
--- In hp_agilent_equipment@..., "jbarnes" <jbarnes@a...>
wrote:
The problem arises because some, (most) instruments contain
capacitors from both line and neutral to the chassis. These
capacitors are generally refered to as "Y caps".
If an instrument is operated without its chassis connected to line
ground the chassis will float to half the supply voltage. The current
is limited by the impedance of the "Y capacitors".
Y capacitors generally have a value of less than 4700pF in order to
meet earth leakage requirements.
Damage can occur is the centre conductor of the spectrum analyzer
input is connected to a circuit floating at half the line voltage
before connecting the outer ground connections.
Damage can occur if either the spectrum analyzer or the other
instruments are not grounded.
You can obtain a cheap device from Home Depot with three neon
lights that will indicate if the ground connection is broken or if
the line and neutral are transposed.
I damaged a 3325A function generator while troubleshooting by
connecting to an oscilloscope that had become ungrounded by a poor
connection in a power bar.
You can always attach an external permanent ground to the equipment
if you are concerned about this.
I hope that this helps,
John Barnes
---------- Original Message [but some HTML junk removed]-----
From: "Benoit Robert" <robertb3@t...>
Date: Tue, 31 Aug 2004 20:59:36 -0000
Hello to all,<BR>
Got a newbie question about <BR>
a label found on top of my<BR>
analog CRT main frame (housing <BR>
my 8558B Spectrum Analyzer).<BR>
The label said that a grounded <BR>
power source is required for <BR>
operator safety (no surprise there) <BR>
but then it went on about<BR>
consequences and how the frame will <BR>
float to half the supply value<BR>
possibly *damaging* the instrument.<BR>
<BR>
I've seen similar labels before but nothing about damage to<BR>
the instrument. In what scenario can it damage the equipment ? <BR>
Is this specific to Spectrum Analyzers ?<BR>
If this is critical, I am considering installing a lack-of-
ground<BR>
warning system in my rack (or main frame). Also, I suppose that<BR>
a ground rod would be required if I ever operate on a portable<BR>
generator (mountain top repeater servicing for example) ?<BR>
<BR>
Thanks for the wisdom!<BR>
Benoit Robert<BR>
VE7EBR<BR>
