Ralph, et al,
My long term understanding is that the fault current is a curve. High
values initially draining to lower values as the stored charge in the adjacent
wiring is pulled thru the short circuit and maintained by the supply impedance
at the longer term value.
According to what UL has told us for years, the peak SS current from a
usual commercial or residential 120V outlet on a 15A or 20A circuit is 7000 A
peak (probably for microseconds).
I have seen SS tests done on equipment used on 600V industrial circuits
where a 10,000 A clearing capacity was tested.
The failure in misapplication results in ionization of the gas in the
fuse along with the metallic residue from the interrupted fuse which continues
to conduct until some other component open (hopefully before the conflagration
is fully underway).
Europeans claim a lower SS current from their systems (higher
impedance, it seems); so the fuses used there have lower clearing capacity.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Affairs Consultant
PO Box 23427
Tigard, ORe 97281-3427
503/452-1201
p.perk...@ieee.org
-----Original Message-----
From: Ralph McDiarmid [mailto:ralph.mcdiar...@schneider-electric.com]
Sent: Monday, November 20, 2017 10:42 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
According to IEC/TR 60725, 90% of customers in the UK and in Australia, will
have electricity supply impedance less than or equal to 0.25 + j0.23 ohms.
That according to a survey or residential service for 230V/50Hz
That tells me that short circuit current would be at least 680A, but could be
twice that value or higher for some customers
Ralph McDiarmid
Product Compliance
Engineering
Solar Business
Schneider Electric
-----Original Message-----
From: Brian O'Connell [mailto:oconne...@tamuracorp.com]
Sent: Monday, November 20, 2017 10:26 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
Mr. Woodgate,
Is there a recently published spec for a 'typical' 230V mains impedance for the
EU? Have also noted that the source Zs in 61000-4-5 for the instrument seem
rather high. So what is the basis for 1500A interrupt rating?
For U.S., even for an artificially low-Z electronic AC source, seldom see fault
currents exceed 200A peak for 120V mains. Conversely, have noted that at over
100A fault current, any fuse not rated for the 'high' interrupt value will
probably explode.
So either our ratings are suspect, or perhaps the physics behind the standards
are not complete?
Brian
From: John Woodgate [mailto:j...@woodjohn.uk]
Sent: Friday, November 17, 2017 11:16 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
I'm afraid that's not so. The short-circuit current of a 20 A circuit is
normally at least 500 A, maybe 1500 A.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk Rayleigh, Essex UK On 2017-11-18
03:49, Joe Randolph wrote:
Yes, that is a very good point. I may have misinterpreted what the term
“adequate breaking capacity” means. If all it means is that the fuse must be
able to safely break the short-circuit current, then a 20 Amp, 250 VRMS fuse
would be fine for use on a 20 Amp, 240 VRMS circuit.
Of course, this would not provide much protection against overheating of the
MOV unless the MOV failed at an effective resistance of less than 12 ohms.
Anything higher than that would just keep producing heat.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
From: John Woodgate [mailto:j...@woodjohn.uk]
Sent: Friday, November 17, 2017 4:47 PM
To: Joe Randolph <j...@randolph-telecom.com>; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on
AC mains ports?
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)
j...@randolph-telecom.com
http://www.randolph-telecom.com
-----Original Message-----
From: Ralph McDiarmid [mailto:ralph.mcdiar...@schneider-electric.com]
Sent: Friday, November 17, 2017 1:05 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
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:j...@randolph-telecom.com]
Sent: Monday, November 13, 2017 7:56 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
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)
j...@randolph-telecom.com
http://www.randolph-telecom.com
-----Original Message-----
From: Mick Maytum [mailto:mjmay...@gmail.com]
Sent: Sunday, November 12, 2017 3:55 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
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
mjmay...@gmail.com
https://ictsp-essays.info
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