European Safety Approval for POTS

Jon D Curtis Mon, 17 Jun 1996 19:12:22 -0400

From: [email protected]
Date: Mon, 17 Jun 1996 08:44:18 -0500 (EST)
Subject: Re: European Safety Approval for POTS
To: [email protected]
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Dear TREGgers,

On Tue, 11 June 1996 22:03:55 -0400 (EDT)
Martin Garwood wrote :

    As your product generates TNV, it will require basic
    insulation between the ring/POTS circuitry, and other
    SELV circuits within the device. It may be best to try
    keeping ring voltage within SELV limits, thus 
    simplifying testing.
    
    Martin Garwood
    [email protected]
    
A number of suggestions have been posed on this subject.  A 
recent review took place by leading compliance experts in 
Europe, on the very subject of ISDN safety design.  The 
output of this group has been presented to the telecoms 
safety WG of CENELEC TC 74, as an interpretation of the 
precise applicability of test and approvals requirements.  
This is due to the currently varied interpretations taken by 
test laboratories in this area. 

One point that designers implementing POTS ports should not 
over look, is that connection of analogue apparatus to a 
POTS port is not as straight forward as might be first 
envisaged.

Assumptions made of a POTS port :
1.  It doesn't connect to the PSTN.
2.  Safety classification of the POTS port is TNV, unless 
    a SELV ringer is implemented.

Unfortunately it isn't always obviously clear that the TE 
connecting to the POTS port might actually have more than 
one PSTN interface.  It is not uncommon for modems (or 
telephones for that matter) to provide more than one 
connection to the PSTN.  Given that the requirements for 
separation between two PSTN ports have been removed, due to 
the correct interpretation that pins on a PTN connector are 
largely inaccessible from a user safety perspective.  The 
situation where a TE connecting to your POTS port might 
actually have another, non-insulated, connection to the PSTN 
is very real.  

To cater for this scenario, the philosophy of a SELV ring 
generator actually carries a restriction of apparatus 
connecting to the POTS port.  The agreed interpretation of 
this is that a SELV ringer will only reduce the safety 
requirements, if a statement is made clear to the user (& 
test lab) that 'ONLY a SINGLE LINE apparatus may be 
connected', classifying the POTS port as SELV.  No other 
restriction will reduce the port classification from TNV. 

I apologise to those of you who are finding my words 
difficult to visualise, email is not the best media where a 
drawing would say one hundred words.

I hope that my longer than anticipated response will provoke 
yet more interesting debate on this subject.

Best regards,

Edward Fitzgerald
International Approvals Consultant

GSM      : +44-4685-33-100
Internet : [email protected]
X.400    : G=Edward; S=Fitzgerald; A=400net; P=itu; C=ch
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European Safety Approval for POTS

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