I agree with this step-wise route to failure. But I wonder about the
intended meaning of 'adequate breaking capacity'. In the context of the
fuse standard IEC 60027 (multi-part), this means that the fuse must not
shatter or arc-over with the largest fault current that can be applied
to it. It's all about the fuse, not about what it is supposed to protect.
I believe TVSs (BIG diodes with integral heat sink) are much nicer
devices to use than MOVs. They don't suffer from energetic disruption.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2017-11-17 21:39, Joe Randolph wrote:
I have not performed any actual testing, but I believe that the basic problem
is that MOV leakage current increases slightly each time the MOV experiences a
big surge. So, in the early stages of this mechanism, the leakage current will
increase step-wise each time the MOV experiences a big surge.
In practice, these successive "big surges" could be spaced months or years
apart. Each successive surge will increase the leakage current, but if the current
remains well below the level that causes significant self-heating of the MOV, the
situation will remain stable.
After some number of big surges (which could take years to accumulate), the
leakage current will have increased enough that the self-heating process itself
leads to higher leakage current. This sets up a positive feedback loop that
causes the MOV to go into thermal runaway and self-destruct.
I'm not sure the thermal runaway could be described as an "avalanche," since
the process might take considerable time to destroy the MOV, but the general direction of
the failure gets establishes as soon as the positive feedback mechanism gets started.
If it could be shown that any dissipation level less than 240 W cannot cause ignition of
nearby materials, then a 1A fuse would appear to provide the "adequate breaking
capacity" called for in 60950-1.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
-----Original Message-----
From: Ralph McDiarmid [mailto:[email protected]]
Sent: Friday, November 17, 2017 1:05 PM
To: [email protected]
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
I wonder if a 1A fuse would protect against MOV flameout just as well as a
100mA, if these MOV fail as an avalanche. (get hotter = more leakage) I've
seen some standards use 240VA (assume 240W) as a power limit for protection
against fire. However, I wonder if a MOV could burn nicely at say 0.5A without
further increase in current.
Ralph McDiarmid
Product Compliance
Engineering
Solar Business
Schneider Electric
-----Original Message-----
From: Joe Randolph [mailto:[email protected]]
Sent: Monday, November 13, 2017 7:56 PM
To: [email protected]
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
Hi Mick:
Annex Q in 60950-1 shows that the authors were concerned about increased
leakage current in MOVs that are subjected to multiple surges. Perhaps the
authors assumed that if the equipment passed the earth leakage test with new
MOVs, allowing the MOVs to deteriorate by up to 10% after the Annex Q surges
would be acceptable.
Unfortunately, Annex Q does not help to define the term "adequate breaking
capacity" as called for in clause 1.5.9.2 for fuses required in series with the MOV.
As Rich Nute has pointed out, a fuse that will open when the leakage current exceeds 0.5
mA cannot be expected to survive a 3 kA surge.
So, the purpose of the fuse does not appear to be directed at the 0.5 mA
leakage current requirement. Rather, I think the authors were trying to use
the fuse to protect against the three conditions itemized in clause 1.5.9.2:
* Temporary overvoltages
* Thermal overload due to increased leakage current
* Burning and bursting of the MOV in the event of a short-circuit fault
The problem with this is that it takes very little leakage current to cause
thermal overload of the MOV. At 230 VRMS, and RMS leakage current of 100 mA
would dissipate 23 Watts in the MOV. A fuse that reliably opens at 100 mA will
not survive a 3 kA surge.
In general, MOVs subjected to thermal overload allow more leakage current as
they heat up. With a fixed voltage supply (such as 230 VRMS), this creates a
positive-feedback condition of ever-increasing power dissipation that
eventually drives the MOV to destruction.
So, perhaps the requirement for "adequate breaking capacity" was directed at trying to prevent burning and
busting of the MOV, rather than excessive earth leakage current. So far, it appears that the term "adequate
breaking capacity" is not actually defined in 60950-1. I suppose that the authors intended "adequate
breaking capacity" to mean "adequate to prevent burning and bursting" of the MOV.
I'm not sure that this is even possible if that same fuse has to survive a 3 kA
combination wave surge. However, it is possible that 60950-1 does not actually
require the circuit to survive such surges. Perhaps all that 60950-1 requires
is that the MOV itself tolerates such surges with no more than 10% degradation.
It would then be left to the product designer to select a suitable fuse.
Between the conflicting goals of preventing nuisance tripping of the fuse and
preventing overheating of the MOV, I'm not sure that an ordinary fuse can be
found that meets both goals.
This may be why 62368-1 seems to be steering us to using a thermal fuse that
senses overheating of the MOV, rather than a fuse that responds only to current.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
-----Original Message-----
From: Mick Maytum [mailto:[email protected]]
Sent: Sunday, November 12, 2017 3:55 AM
To: [email protected]
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
For my sins I am currently editing a 40 page MOV application guide. As the
guide was written by a Chinese expert I have the additional task of translating
the Chinlish to English. However this forces understanding and I found the
document contains many gems I did not appreciate. On degradation three areas
are mentioned:
1. Varistor voltage at 1mA has decreased by more than 10 % of the
initial value.
2. Surge limiting voltage at a specified impulse current has increased
by more than 10 % of the initial value.
3. Leakage current or watt-loss shows a steady increase.
Then the comment is made that generally if the Varistor voltage has decreased
by 10 %, the limiting voltage will have only increased by 3 %.
Thus the Annex Q limiting voltage change limit of 10 % would be better replaced
by a Varistor voltage change.
On fuses I did some calculations and found an anti-surge fuse in the 4 A region
was required to withstand the Annex Q MOV requirement of a 3 kA
8/20 surge. It should be remembered that most fuses will not interrupt an 8/20
surge as the fuse link plasma carries on conducing the 8/20 impulse current.
Ted makes the excellent point that in the equipment additional series
impedances will exist that reduce the peak the peak surge current from an
1.2/50-8/20 generator.
Tests are now appearing for thermally protected MOVs, which disconnect the MOV
in the event of exceeding a pre-set body temperature limit.
Thermal disconnect, rather than current disconnect, gets to the heart of the
matter. But, as far as I'm aware, none of these tests apply a surge voltage to
check for open thermal switch arc over.
Regards,
Mick Maytum
Safety and Telecom
Standards
[email protected]
https://ictsp-essays.info
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