Jim,
You do in fact have a potential ground connection - at the phone line. In
the static (on-hook) state you will have -48 Vdc, referenced to earth. If
you look at your home phone service you will see a wire running to a cold
water pipe, with a tag that says something like "Telephone Company Ground -
Do Not Remove". PBX systems at work will be different. I have little
knowledge of whether they ground or not. I have learned the hard way to look
for these non-obvious ground connections.
Best wishes,
Scott Lacey
-----Original Message-----
From: Jim Hulbert [SMTP:[email protected]]
Sent: Thursday, March 02, 2000 8:52 AM
To: Lacey,Scott; [email protected]
Subject: RE: Surge Testing per EN 55024/EN61000-4-5
Scott,
The product I'm currently looking at uses a 2 -prong AC mains plug.
Actually
the supply is small direct plug-in type with DC leads that connect
to the
product. The product itself is encased in plastic and the only
other connection
is through an internal modem board to a standard analog telephone
line. The
connection to the phone line is a simple 2-wire (tip and ring)
configuration.
As I see it, surge tests with reference to earth ground are
pointless.
Others have pointed out that an "ungrounded" product may in the real
world
actually have a reference to ground depending on how it is installed
or what
other equipment it is connected to. That is a good point which I
hadn't
thought about. However, I don't think that in my particular
situation that's
that case.
Thanks to all who responded with their opinions.
Jim
"Lacey,Scott" <[email protected]> on 03/01/2000 01:31:46 PM
Please respond to "Lacey,Scott" <[email protected]>
To: "'Jim Hulbert'" <[email protected]>
cc: "'[email protected]'" <[email protected]> (bcc: Jim
Hulbert/MSD/US/PBI)
Subject: RE: Surge Testing per EN 55024/EN61000-4-5
Jim,
I'm not quite sure from the description what your product looks
like, but,
here are a few things to keep in mind. If you use a 3-prong ac mains
plug
you absolutely should do the line-to-ground test. If you use a
2-prong ac
mains plug you still might want to perform the test anyway,
depending on how
your product is used. For example, if your plastic housing may be
DIN rail
mounted or screwed to a wall, you may have a leakage or arc path to
earth.
Your customer may have deliberately grounded the DIN rail to satisfy
the
requirements of other vendor's equipment already mounted. Even if it
only
gets screwed to a plasterboard wall, it still may get grounded by
accident.
I have seen two such scenarios over the years. In the first instance
one
mounting screw went through the wallboard and touched the grounded
sheath of
a BX cable in an older building. In the second instance a screw
penetrated a
metal stud used to frame the wall. An outlet box was attached to
another
stud. Bingo - instant ground.
I'm sure there are many who will disagree with me, but we test
products not
only to conform to a standard, but also to ensure a more robust
product. I
want to find any vulnerability first, before the product gets to a
customer.
Customers tend to get VERY angry when their new product "hiccups",
and the
nasty stuff tends to flow downhill real fast when their CEO calls
yours.
Scott Lacey
-----Original Message-----
From: Jim Hulbert [SMTP:[email protected]]
Sent: Wednesday, March 01, 2000 10:32 AM
To: [email protected]
Subject: Surge Testing per EN 55024/EN61000-4-5
Colleagues,
EN 55024 calls for surge pulses to be applied line-to-line and
line-to-earth on
the AC mains port and line-to-ground on signal and
telecommunications ports that
connect directly to outdoor cables. However, if my EUT is
encased
in plastic
covers and has no direct earth ground connection (class 2 power
supply), is the
line-to-line test on the AC mains the only surge test that I
need to
apply? It
seems to me that performing a line-to-earth test on either the
AC
mains port or
on signal/telecommunications ports is not warranted since the
basic
standard EN
61000-4-5 does not specify placing the EUT over a reference
ground
plane. With
no reference ground plane and no direct ground connection how
can a
test be
applied with respect to ground?
Jim Hulbert
Pitney Bowes
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