I would like to attempt a response to question 2 regarding adequate breaking 
capacity.



Fuses should have an ampere interrupt capacity (AIC) rating. A more extensive 
discussion can be found on an application note from Bussmann Fuse at this 
link<http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Interrupting_Rating.pdf>.
 Finding this rating for a fuse is easy. Figuring out what the potential fault 
current in your circuit would be is not easy. It will depend on the impedances 
within your product, your products power connection, building wiring and the 
step-down transformer providing power to your circuit. The rating of the branch 
circuit breaker usually doesn’t matter as much as the circuit impedances. If 
there is a fault leading to a dead short, the circuit breaker will take a cycle 
or two before it opens. The current surge during that period can be 
significant. I’ve seen available fault currents in the kiloampere range in 
residential applications and 50 kA in some commercial application, all on 20 A 
circuits.



This is a problem in power supply design and the fuse is often placed after the 
common mode choke. The choke provides enough impedance to limit the rise time 
of the current under fault conditions. This give the fuse the chance to open 
before the current exceeds the fuse’s AIC rating.



Long ago, I was working on a power supply that failed testing. Under fault 
conditions, the fuse would open catastrophically. The fuse had an AIC rating of 
50 A and the current was hitting over 100 A before the fuse opened. The metal 
filament would vaporize and then condense across the circuit board leaving a 
thin metalized plating. The solution was to use a higher impedance choke. The 
designer had used the same common mode choke that was used in other designs. 
However, this was a smaller power supply. It turned out that a higher impedance 
but lower current rated choke was available in the same footprint. The current 
rating was still sufficient for the design and the new common mode choke solved 
the problem. Its impedance was high enough to limit the rise time of the 
current. Under fault conditions, the rise time was slow enough to allow the 
fuse to open before the current exceed the 50 A limit.



It’s hard to make accurate assumptions about the impedance of the circuits into 
which your product will be connected. Assume the worst-case based on electrical 
construction methods, transformers and other factors for your customer’s 
installations. Then you can calculate the breaking capacity your fuse will 
need. If you fuse is after impedance in your own product’s design, the 
calculations are simplified.



You can run testing on your product, but that can be more difficult to do. If 
you happen to insert a fault at the current’s zero crossing, you may not see 
the same instantaneous fault current you would see if the short is applied at 
the peak of the waveform. If you are using a laboratory AC supply, you may find 
that it has a lower available fault current than a wall outlet. If you have a 
good high-frequency current meter, you can watch the fault current to see if it 
is getting close to the AIC rating of your fuse. As with any measurement at 
hazardous voltages and with these currents, care should be taken to ensure that 
the setup does not create a hazard for the laboratory staff.


Ted Eckert
Microsoft Corporation

The opinions expressed are my own and do not necessarily reflect those of my 
employer.



-----Original Message-----
From: Joe Randolph [mailto:j...@randolph-telecom.com]
Sent: Saturday, November 11, 2017 3:34 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Regulatory requirements for MOVs placed line-to-ground on 
AC mains ports?



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)

j...@randolph-telecom.com<mailto:j...@randolph-telecom.com>

https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.randolph-telecom.com&data=02%7C01%7Cted.eckert%40microsoft.com%7C43f4ff993f9c436f3b3008d5295cbb38%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636460400616955651&sdata=OTdtDsMtW3G7Dn2uW01tho8n02aHopWNbEUJ%2Beq8Xi0%3D&reserved=0



-----Original Message-----

From: Richard Nute [mailto:ri...@ieee.org]

Sent: Thursday, November 09, 2017 6:34 PM

To: 'Joe Randolph' 
<j...@randolph-telecom.com<mailto:j...@randolph-telecom.com>>; 
EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG>

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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