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From: emc-pstc@ieee.org on behalf of Piotr Galka [piotr.galka@micromade.pl]
Sent: Thursday, April 26, 2007 7:48 AM
To: EMC-PSTC
Subject: EN 60950-1:2006 Cl.1.5.9.2
All 60950-1 Experts,
I have in my device RS232 input line.
I need to protect it against surge (because of EN 50130-4).
Is it true that if I use the 33V varistor I will need to add a fuse in
series with it (Cl. 1.5.9.2), but if I use the 33V TVS I need not to add
the fuse.
Cl. 1.5.9.1 note 1 tells me that TVS is not VDR (in 60950-1), so 1.5.9.2
not apply to it.
Reading Cl. 2.4 I understand that 12V/10mA (RS232 driver capability) is
not the LIMITED CURRENT CIRCUIT so VDR needs fuse.
If the fuse is needed do the PTC resistor (for example Raychem
miniSMDC020 - 0.2A/30VDC) can be considered as "an interrupting means
having an adequate breaking capacity" ?
If not the fuse with what approval should I look for ?
In my opinion the same what can happen to the varistor during
"-temporary overvoltages above the maximum continuous voltage." can
happen to TVS.
It is the reason I use 33V devices and not 12V ones.
Thanks in advance for your answers.
Best Regards
Piotr Galka
-----
From: emc-pstc@ieee.org on behalf of Tarver, Peter [peter.tarver@sanmina-sci.com]
Sent: Thursday, April 26, 2007 11:31 AM
To: EMC-PSTC
Subject: RE: EN 60950-1:2006 Cl.1.5.9.2
> Is it true that if I use the 33V varistor I will need to add
> a fuse in series with it (Cl. 1.5.9.2), but if I use the 33V
> TVS I need not to add the fuse.
Hi, Piotr.
That appears to be true, if the text is taken literally), but it doesn't
make sense, considering §1.5.9.2, Note 2, so I'm not certain that was
the intent. VDRs could be a concern for increased leakage current in
TNV-2 and TNV-3 circuits (remember that ringing voltages are applied
during some touch current tests), but I don't think the intent was to
apply this to all data bus circuits that typically do not contributing
much to touch currents and for which transients are not considered in
touch current testing.
> Cl. 1.5.9.1 note 1 tells me that TVS is not VDR (in 60950-1),
> so 1.5.9.2 not apply to it.
A VDR is still a TVS. §1.5.9.1, Note 1, only intends to differentiate
which are considered VDRs and which are not.
> Reading Cl. 2.4 I understand that 12V/10mA (RS232 driver
> capability) is not the LIMITED CURRENT CIRCUIT so VDR needs fuse.
Again, if literally taken, you'd be correct.
> If the fuse is needed do the PTC resistor (for example
> Raychem miniSMDC020 - 0.2A/30VDC) can be considered as "an
> interrupting means having an adequate breaking capacity" ?
Probably, but it may depend on how the RS-232 circuit is used, the
anticipated transients and the power supply capacity.
> If not the fuse with what approval should I look for ?
I don't understand this question. Can you be more specific?
> In my opinion the same what can happen to the varistor during
> "-temporary overvoltages above the maximum continuous
> voltage." can happen to TVS.
True, but other TVS devices have generally proven themselves more immune
to the kinds of problems VDRs exhibit, provided they are rated
appropriately for the application.
-----
From: emc-pstc@ieee.org on behalf of Piotr Galka [piotr.galka@micromade.pl]
Sent: Friday, April 27, 2007 8:11 AM
To: EMC-PSTC; Tarver, Peter
Subject: Re: EN 60950-1:2006 Cl.1.5.9.2
Hi Peter,
I see I have wrongly used the TVS abbreviation.
Writing TVS I had in mind only semiconductor non-linear devices (TRANSILs).
Comparing I/V curves of VDR and TRANSIL (both comparable sizes and for
the
same working voltage) I would expect smaller (DC) overvoltage needed to
burn
TRANSIL than VDR, and TRANSIL also makes shortage like VDR.
I know, other reasons (1.5.9.2 Note 2) can damage VDR and not TRANSIL.
Having no experience I have one way of understanding the standards -
take
the text literally. I also expect that if any problems arise the text
will
be taken literally. I know, that even I ask any experienced agency the
whole
responsibility for the product is only mine. So when they tell me the
intention was other should I follow it ? The intention isn't written in
standard.
I have asked about PTC, because I prefer them over fuses but recently I
was
told, that PTC can't be understand as interrupting device, as it makes
no
permanent interruption.
>> If not the fuse with what approval should I look for ?
>
> I don't understand this question. Can you be more specific?
Asking that I had in mind:
If EN 60950-1 needs me to use here (or anywhere else) an adequate
interrupting device, and I decide to use fuse, can it be any fuse which
in
my opinion is good, or I have to check the fuse manufacturer for some
agency
reports that the fuse comply with any particular component standard, and
what standard ?
May be it is the question everyone knows the answer, but I don't.
Best Regards
Piotr Galka
-----
From: emc-pstc@ieee.org on behalf of John Woodgate [jmw@jmwa.demon.co.uk]
Sent: Friday, April 27, 2007 8:29 AM
To: emc-pstc@IEEE.ORG
Subject: Re: EN 60950-1:2006 Cl.1.5.9.2
In message <004801c788c5$1df37da0$154d4d0a@MmPc21>, dated Fri, 27 Apr
2007, Piotr Galka <piotr.galka@micromade.pl> writes:
>If EN 60950-1 needs me to use here (or anywhere else) an adequate
>interrupting device, and I decide to use fuse, can it be any fuse which
>in my opinion is good, or I have to check the fuse manufacturer for
>some agency reports that the fuse comply with any particular component
>standard, and what standard ?
>May be it is the question everyone knows the answer, but I don't.
Your fuse need to comply with the relevant part of IEC 60127, and be of
adequate breaking capacity.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of
2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-----
From: emc-pstc@ieee.org on behalf of Tarver, Peter [peter.tarver@sanmina-sci.com]
Sent: Friday, April 27, 2007 10:26 AM
To: EMC-PSTC
Subject: RE: EN 60950-1:2006 Cl.1.5.9.2
> From: Piotr Galka
> Sent: Friday, April 27, 2007 5:11 AM
>
> Hi Peter,
>
> Having no experience I have one way of understanding the
> standards - take the text literally. I also expect that if
> any problems arise the text will be taken literally. I know,
> that even I ask any experienced agency the whole
> responsibility for the product is only mine. So when they
> tell me the intention was other should I follow it ? The
> intention isn't written in standard.
Hi, Piotr.
Your approach is a good one. The problem is with the standard. Even
before IEC60950-1:2005 was published, there were proposed corrections to
the Creepage Distance tables we discussed a few months ago. I suspect it
was intended that §1.5.9.2 was to apply only to those circuits where
transients are known safety concerns: ac mains and exposed tnv. I have
no hard evidence for this assertion, but it's what makes the most sense
to me.
> I have asked about PTC, because I prefer them over fuses but
> recently I was told, that PTC can't be understand as
> interrupting device, as it makes no permanent interruption.
Whether or not a PTC is suitable depends entirely on its application
(see §2.5, ¶ b. The response you received may have been based on how you
asked teh question and the assumptions made by the responder. If you
asked about using a PTC to comply with §1.5.9.2, the responder may have
assumed you intended teh VDR to be in a primary circuit, in which case,
a PTC might not be suitable.
PTCs are often used in conjunction with VDRs to meet CSA an UL power
cross requirements for on TNV-1 and TNV-3 circuits.
> >> If not the fuse with what approval should I look for ?
> >
> > I don't understand this question. Can you be more specific?
>
> Asking that I had in mind:
> If EN 60950-1 needs me to use here (or anywhere else) an
> adequate interrupting device, and I decide to use fuse, can
> it be any fuse which in my opinion is good, or I have to
> check the fuse manufacturer for some agency reports that the
> fuse comply with any particular component standard, and what
> standard ?
As the standard is written, it's left open. You only need to chose a
device that provides 'adequate' protection. Considering your application
is in a secondary circuit and if you can demonstrate what levels of
current are involved during the transient events your protecting your
equipment from, it may be that a PTC is 'adequate.'
Another approach you might think about relates to §1.5.9.2, Note 2. This
explanatory note relates to increases in leakage current over time.
Contrary to my earlier reading, I'm now leaning towards this note
referring to leakage current through the device, rather than protective
earth currents (it would be most useful, if someone who sat in on this
decision can post a response). Consider demonstrating (by test) that the
board you mount the device on can withstand currents up to LPS limits
with the VDR shunted.
Regards,
Peter L. Tarver, PE
ptarver@ieee.org
-----
From: emc-pstc@ieee.org on behalf of Piotr Galka [piotr.galka@micromade.pl]
Sent: Friday, April 27, 2007 1:19 PM
To: EMC-PSTC; Tarver, Peter
Subject: Re: EN 60950-1:2006 Cl.1.5.9.2
From: "Tarver, Peter" <peter.tarver@sanmina-sci.com>
Sent: Friday, April 27, 2007 4:25 PM
>
> Your approach is a good one. The problem is with the standard. Even
> before IEC60950-1:2005 was published, there were proposed corrections
to
> the Creepage Distance tables we discussed a few months ago. I suspect
it
> was intended that §1.5.9.2 was to apply only to those circuits where
> transients are known safety concerns: ac mains and exposed tnv. I have
no
> hard evidence for this assertion, but it's what makes the most sense
to
> me.
Hi Peter,
Thanks for your answers.
May be based on 1.3.1 - "The requirements detailed in this standard
shall be
applied only if safety is involved." I should decide that in described
by me
situation safety is not involved ?
Really I have about 25 low voltage, low current inputs and outputs in my
device, but because of EN 50130-4 I need surge protect all of them. Then
how
to be sure for each of them that safety is no a concern here.
My device is powered from AC 230V by external DC 12V power supply, and
in
most cases devices connected to my inputs are powered from the same 12V
source, so really no possibility to overvoltage the 33V TVS, but I can't
exclude connecting some other devices powered from other sources to my
inputs.
I suppose to use TRANSILs and no VDR to avoid my own question ;-)
Best Regards
Piotr Galka
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