I think IEC TC108 needs to review this urgently. If no MOV can meet
G.8, then the standard should not ask for the unattainable.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2017-11-13 16:40, Joe Randolph wrote:
Hi John:
I understand that it might seem preferable to just use 62368-1 instead
of 60950-1. In this case, though, there are two reasons why 60950-1
appears preferable:
1)In a few weeks, the product will be entering the testing process for
compliance in Australia. As far as I can tell, Australia has not yet
transposed 62368-1 into a national standard.
2)The latest version of 62368-1 has its own set of problems with MOVs
on the AC mains input. The first problem is that for ordinary
consumer products that use a pluggable Type A mains connection,
62368-1 requires a GDT in series with the MOV. The second problem is
that the MOV itself must meet the requirements of Annex G.8. I don’t
think that any MOV, by itself, can meet the requirements of G.8. It
appears to me that the best chance of meeting the requirements of G.8
is to use an MOV that is physically coupled to a thermal fuse. Such
products are available, but considerable testing would likely be
required to determine whether the thermally-protected MOV will meet
all the requirements of G.8.
Eventually, I hope to fully understand the MOV requirements in both
60950-1 and 62368-1, so that I know how to implement solutions that
comply with either standard. For the short-term, though, it appears
that 60950-1 is the best option for this particular project.
Thanks,
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com
*From:*John Woodgate [mailto:[email protected]]
*Sent:* Sunday, November 12, 2017 3:43 AM
*To:* Joe Randolph <[email protected]>; [email protected]
*Subject:* Re: [PSES] Regulatory requirements for MOVs placed
line-to-ground on AC mains ports?
It would be better to concentrate on 62368-1, because 60950-1 is on
the way out.
Regarding 'adequate breaking capacity', this is in the relevant Part
of IEC 60127, probably Part 3.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk <http://www.woodjohn.uk>
Rayleigh, Essex UK
On 2017-11-11 23:33, Joe Randolph wrote:
Hi Rich:
Thanks for your feedback on this topic.
My impression is that the intent of the requirement for a fuse in series
with an MOV is not related to concerns about protecting users from surge
transients. I believe the intent of the requirement is to protect users from
the effects of an MOV that has become leaky and is allowing excess current to
flow into the ground conductor.
While the cause of the MOV becoming leaky is typically considered to be
degradation caused by surges, the surges themselves are not the focus of the
requirement for a fuse. Rather, the focus is on hazards created by leaky MOVs.
Regarding the interpretation of 60950-1, I have two questions:
1) For equipment that has two MOVs, each connected line-to-ground, does
60950-1 require fusing both of the line leads? It appears to me that the
answer is yes, but I'm not an expert on this topic.
2) Clause 1.5.9.2 in 60950-1 requires that the fuse must have "adequate breaking
capacity." Where in 60950-1 do we find a definition and/or test for "adequate breaking
capacity"?
I agree with your observation that if the purpose of the fuse is to keep
the leakage current under 0.5 mA, there is no practical way to achieve this.
The same fuse must also be able to survive the anticipated surge currents for
which the MOV was installed.
So, the authors of clause 1.5.9.2 must have had some other definition in mind for the
term, "adequate surge protection."
Thanks,
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected] <mailto:[email protected]>
http://www.randolph-telecom.com
-----Original Message-----
From: Richard Nute [mailto:[email protected]]
Sent: Thursday, November 09, 2017 6:34 PM
To: 'Joe Randolph'<[email protected]>
<mailto:[email protected]>;[email protected]
<mailto:[email protected]>
Subject: RE: [PSES] Regulatory requirements for MOVs placed line-to-ground
on AC mains ports?
Hi Joe:
"I still have a question about whether clause 1.5.9.2 in 60950-1 means that
there must be fuses in each of the Line conductors when two MOVs are installed
line-to-ground on the AC mains input. It sure looks that way when I read the
requirement."
I have some doubts as to whether a fuse will open the circuit when an MOV
conducts. Or, when the MOV leakage is excessive.
Continuous current in the earth conductor must not exceed 0.5 mA. I
couldn't find any fuses rated less than 32 mA. So the fuse cannot protect
against MOV deterioration.
Supposedly, the MOV operates when the transient voltage exceeds, say, 2100
volts peak. When the MOV operates, the line conductor is almost shorted to
ground. So, 2100 volts peak across the fuse exceeds its voltage rating by 500%
(assuming the fuse is rated for the mains voltage). If you exceed the fuse
voltage rating, the fuse is not likely reliable, and may become a conductor.
Another interesting point. While the standard severely limits the use of an SPD for
"Type A" plug-connected equipment, the SPD cannot operate if the ground is
open. For an injury to occur, three conditions must be met:
1) Transient voltage exceeding 2100 volts peak (maybe only a few times per
year).
2) Person touching an otherwise grounded part of the equipment.
3) The transient voltage must have enough energy to cause an injury.
The probability of an injury is the multiplication of the individual
probabilities. Very small.
If the transient waveform is the standard transient waveform, 1.2 usec x 50
usec, then the duration is much shorter than the curves shown in IEC 60479-5.
So, there is insufficient energy in the transient waveform to cause an injury.
Best regards,
Rich
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