> What I and others have seen is that the Common Mode Choke and Y caps will
> act
> as a voltage doubler during a common mode surge.
> I would see a 2X voltage doubling during the surge event.
> I believe the surge builds up a voltage on the Y caps and a big field on
the
> CMC.
> When the surge tops off the CMC field collapses and springs off the Y caps
> => doubling of the exposure.
> You can simulate this on spice easily.
> This puts a very big stress on the isolation barriers with possible arcing
> across optos and miss-operation.
> It would be so much easier to use MOVs line to ground and limit the surge
in
> the first place.
> The best I have done with out MOVs is about 3KV CM exposure.
> Since the new EN61000-6-2 CM limit is 2KV I haven't worried about pushing
> this limit.
>
> Problem with a Gas tube in series with a MOV would be turn on time.
> The gas tube is very slow.
> My understanding of a gas tube - hot to earth is that once it fires it
will
> not clear untill zero crossing if at all.
> One would have to blow a fuse on the exposure?
> If anyone has something better please let me know!
>
>
> QUESTION FOR THE GROUP PLEASE
> I understand the issue with line powered user devices and MOVs
> leaking/shorting.
> What about panel mounted automation equipment for industrial applications.
> Does the MOV to ground issue still hold?
>
> Thanks
>
> Chris Wells
> Senior Design Engineer
> Cutler-Hammer
> [email protected]
>
>
> ----- Original Message -----
> From: <[email protected]>
> To: <[email protected]>
> Sent: Tuesday, June 13, 2000 4:01 PM
> Subject: RE: Line/Neutral to Earth MOVS
>
>
> >
> > 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:[email protected]]
> > Sent: Tuesday, June 13, 2000 1:16 PM
> > To: [email protected]
> > Cc: [email protected]; [email protected];
> > [email protected]
> > 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
> >
> >
> >
> >
> > -------------------------------------------
> > This message is from the IEEE EMC Society Product Safety
> > Technical Committee emc-pstc discussion list.
> >
> > To cancel your subscription, send mail to:
> > [email protected]
> > with the single line:
> > unsubscribe emc-pstc
> >
> > For help, send mail to the list administrators:
> > Jim Bacher: [email protected]
> > Michael Garretson: [email protected]
> >
> > For policy questions, send mail to:
> > Richard Nute: [email protected]
> >
> >
> >
>
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
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For help, send mail to the list administrators:
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