Re: Hipot requirements for TNV circuits

[JPR3]

In a message dated 7/1/99, Peter Merguerian writes:

>  By the way, a transformer is not always needed on a modem. The 
>  new design of modems have capacitors (not required to be 
>  Approved) between the TNV-3 and SELV.


Hi Peter:

It is certainly true that some of the new modem DAA devices use capacitors 
instead of a transformer (Silicon Labs, Krypton, Conexant, Siemens, Analog 
Devices, etc).  

However, I do not agree that the capacitors somehow do not have to be 
approved.  On what basis have you reached this conclusion?

There certainly does seem to a debate about what requirements might apply to 
capacitors used in this application.  For basic insulation, some people have 
argued that as long as the physical size of the cap meets the creepage and 
clearance, and the cap passes the hipot test, you are all set.  My impression 
is that some safety labs accept this interpretation, while others do not.

Sometimes the design goal is to provide supplementary insulation, to comply 
with requirements in Norway and Sweden.  In this case, the isolation 
requirement includes a minimum distance throgh solid insulation of 0.4 mm.  I 
believe that most multilayer ceramic caps violate this requirement.  

It is still possible to use such a capacitor by using an approved Y2 cap (per 
IEC 384-14), but these caps tend to be physically large and somewhat 
expensive in surface mount.  I am aware of a couple of vendors who are trying 
to get  smaller caps approved for bridging supplementary insulation, but I 
have not heard yet that these efforts have been successful.

Do you have more information on this subject?


Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (voice)
781-721-0582 (fax)

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Re: Hipot requirements for TNV circuits

[Peter Merguerian]

George,

Do you have transient voltage surge supressors. Maybe this is why 
you failed the hipot. The standard allows you to pass the Impulse 
Test in lieu of hipot. Try this method.

Regarding insulation between TNV-3 and SELV/Earth the standard 
allows you the following alternatives routes to compliance:

Min. Basic insulation (clearance/creepage) and the 6.4.1 tests 
(hipot or impulse).

Installation by qualified personnel, permanent connection to earth 
instructions and the tests of 6.2.1.3 (surge test). However, please 
note that prior to the surge test, anything less than basic insulation 
on the board is shorted.

By the way, a transformer is not always needed on a modem. The 
new design of modems have capacitors (not required to be 
Approved) between the TNV-3 and SELV.

Hope this helps a little. Read your standard. You will find a 
statement which says "other solutions are possible". Therefore, 
you must discuss your problem with your friendly test lab.


Best Regards,

> 
> Hello Once Again .. Everyone,
> 
>   I have a modem product that failed the hipot requirement defined in
> cls 6 of 950.  My understanding is that an isolation xfmr between Telco &
> SELV is needed and clearance from Telco & gnd is to be 3mm minimum. Does
> anyone have any other general design guidelines that they can share on this
> topic ?
> 
>   Also, Can anyone point me to any manufacturers app notes for
> components used for telco isolation ?  
> 
>   And if anyone can recommend any reading material,  I'd like to read
> up on these and other issues relating to compliance design aspects of Telco
> circuits.
> 
> 
>   Thanks,
>   George
> 
> 
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> 


PETER S. MERGUERIAN
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PRODUCT TESTING DIVISION
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