I agree that *if* the Ethernet cable extends outside the building for a *considerable* distance, it's safety classification enters a no-man's land in terms of safety classification. UL attempted to address such situations with their PAG on POE, which states that POE lines that are routed outside should be treated as TNV-1, but ordinary POE that is routed inside can be treated as SELV. This leaves some ambiguity because often the manufacturer does not know whether the user's POE network uses outside lines.
A few months ago, those of us on the IEEE Telecom TAC had a very robust discussion about this problem and possible ways to resolve it in a clear manner. Unfortunately, we were unable to come up with a simple solution.
The fact remains that under IEC 69050 and it's national derivations, Ethernet is classified as SELV and for the vast majority of applications, this classification is entirely appropriate. Now that *some* users are stringing Ethernet cables outside, the question can be asked, "at what cable length does my Ethernet cable become subject to the same safety threats as a TNV-1 circuit?" The only official attempt to answer this question is the UL PAG for UL 60950, and even this attempt has some ambiguities.
Given that the maximum length of an Ethernet cable is specified in 802.3 to be 100 meters, it can be argued that this is insufficient to create the same level of exposure hazard as a conventional phone line that can easily exceed 5000 meters.
Getting back to Ian's original question, though, one could ask what the effect would be of classifying his Ethernet port as a TNV-1 circuit. I'm speaking from memory here, but as I recall the TNV-1 classification does not invoke explicit creepage and clearance distances (these are only called out for TNV-2 and TNV-3 circuits). I think the only isolation requirement for TNV-1 is a dielectric test of 1000 VRMS to user-contractible points and/or 1500 VRMS to hand-held items. In other words, if his design meets the 1500 VRMS isolation requirement called out in 802.3, it will also meet the TNV-1 isolation requirement in IEC 60950.
Now I know that the TNV-1 requirements in IEC 60950 are expanded in the UL/CSA version to cover power cross testing, and the Australian version uses a higher isolation test. However, I'm not going to get caught up in these nuances unless Ian tells us these countries are important for his particular product, *and* that he thinks he has to classify his Ethernet interface as TNV-1.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
Be careful Joe,-
There is a UL PAG that says that PoE must be treated as a TNV1, but overvoltage tests can be waived from Annex NAC. It is a botched PAG that needs to be fixed for other reasons. Also if it crosses the boundary between buildings it needs to be treated as TNV-1 even for international safety standards which may impose creepage and Clearance distances.
Also people forget there can be a lot of current coming from a shared PoE DC supply and eventhough safety requirement creepage and clearances do not apply between the high and low voltage rails it is good to impose separation so you dont have a fire.
Jim
Jim Wiese
Senior Compliance Engineer
ADTRAN, Inc.
901 Explorer Blvd.
Huntsville, AL 35806
256-963-8431
256-714-5882 (cell)
256-963-6218 (fax)
[email protected]
From: Joe Randolph [mailto:[email protected]]
Sent: Monday, November 26, 2012 1:43 PM
To: [email protected]
Subject: Re: [PSES] POE creepage & clearances
Hi Ian:
I think the short answer to your question is that under 60950 there are no requirements for creepage and clearance distance between the Ethernet circuit and ordinary SELV circuits that are user-contactable. This is because under 60950, Ethernet is classified as an SELV circuit. There are no requirements to separate one SELV circuit from another.
Where this issue becomes confusing is that under the Ethernet 802.3 standard, an Ethernet port must provide a 1500 VRMS isolation barrier between the Ethernet conductors and earth. This is a legacy requirement whose origins seem to have been lost in the sands of time. However, I believe the consensus view is that the original intent of this requirement was to protect against ground loops and was not related to user safety. Nevertheless, the 802.3 isolation requirement has sometimes been treated as though it is a safety requirement. Note that the 802.3 requirement is for a simple dielectric test, with no reference to creepage, clearance, or distance through solid insulation.
So, in principle, there are no safety isolation requirements for your application, since both of the proposed power supplies have a SELV output. This makes the entire device a SELV circuit (unless there are other ports on the device that are not SELV).
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
Dear colleagues
We are developing a hand held product that is powered from +48V DC over Ethernet or from an external +12V DC wall wart PSU. The maximum internal voltage will be the +48V POE.
Can anyone let me know what the creepage and clearance requirements between the ?primary? Ethernet circuit and the ?secondary? circuit.
The product will be tested to IEC 60950 or possibly IEC 60065 so I am assuming a safety isolation creepage & clearance is required.
Many thanks in advance;
Ian McBurney
Design Engineer
Allen & Heath Ltd
Kernick Industrial Estate
Penryn, Cornwall
TR10 9LU
United Kingdom
+44 (0)1326 370121
[email protected]
www.allen-heath.com
A DMH Pro Company.
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