Somewhat related to this discussion is the opportunity I had last week to
perform surge testing on an off-the-shelf, rack mounted PC. The power supply
in question was from a reputable manufacturer and had safety, EMC and CE
markings. The system (power supply) passed the "Heavy Industrial" limits
when tested line to neutral but the computer would reset when tested L/N to
ground. My rationale for this anomaly was the lack of a suppression device
(MOV?) to ground.
Rich's argument makes sense, and apparently most PC supply manufacturers do
not provide this protection to ground. Why then would I see this problem and
what could I do to correct it?
Rick Busche
Evans & Sutherland
.
-----Original Message-----
From: Rich Nute [mailto:ri...@sdd.hp.com]
Sent: Tuesday, June 13, 2000 1:16 PM
To: dan.kin...@heapg.com
Cc: c...@dolby.co.uk; pmerguer...@itl.co.il;
emc-p...@majordomo.ieee.org
Subject: Re: Line/Neutral to Earth MOVS
> Why do you need the MOVs? Why not just take the L & N
to ground via a gas
> tube each?
Indeed!
Why, even, use any suppressors?
Supposedly, there is only insulation from L and N
to ground (earth). As such, it is easy to achieve
5 kV worth of insulation. Without much effort, this
can go to 10 kV.
5 kV will easily withstand most mains transients.
If there is concern for transient suppression above
this value, simply use an air gap between a pair of
terminals in the circuit. Use the minimum spacing
allowed by the safety standard. Make sure the
terminals are robust and coated with solder. This
way, there will be little or no degradation when an
arc occurs. (The solder may melt, but it will re-
form when it cools.)
We always test mains circuits to breakdown. Most of
our mains circuits go to >5 kV simply by meeting the
standards' requirements for spacings and Y-caps.
Our typical >5 kV breakdowns occur between the terminals
of the Y-caps (on the back side of the PWB, between the
Y-cap leads protruding on the back side).
I am often amused by the commercial "surge suppressors"
offered by computer stores and similar stores. If a
product meets the various "surge" and safety standards,
there is no need for such devices.
Best regards,
Rich
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