User protection for non-isolated PoE PD - Page 1
Forum section: electronics / projects

author: foxtrot
author_rank: Contributor
timestamp: October 21, 2023, 07:21:53 pm
content: Hey all! Pleased to be here. I'm hoping that someone can provide me with a little bit of guidance with regard to protecting end users of a device from high-pot. I'm going to do my best to give a structured explanation of the situation, but I'm not too familiar with the terminology/ontology of user protection, so please bear with me.The equipment I'm designing is an 802.3bt PoE PD (Powered Device) drawing around 70W. Typically, PoE PDs require isolated power supplies due to the fact that electrical distribution topology, building electrical faults, natural conditions, and transients (for example, lightning-induced voltages) can result in drastically different potentials between the PSE and the PD. My understanding is that PoE PDs are required to provide electrical isolation that withstands either ±1500V RMS steady-state at 50-60 Hz (60s) or 10 x 1500V impulse (10/700µS, 60s interval). The risk associated with the hazard of exposure to these conditions is typically mitigated by the use of an isolated converter at the powered device, which is bonded to any user-contactable metallic surfaces (electrical contacts, device chassis, etc.) so that they are floating with respect to the PSE – thus, if a user at some local earth potential makes contact with the device or contact of one of its connectors, they are not shocked. This isn't an option for this design, however, due to the fact that the best isolated supply I can come up with is only ~92% efficient, and I can't afford to lose ~5.6W on that supply's inefficiencies.Our enclosure is entirely plastic, not accessible to a user during normal operation (by virtue of its location) and we include no connectors that are intended to be used by the end user to connect their own equipment; therefore, I am developing the device around a non-isolated power supply. There is, however, a connector for a module (developed/supplied by me, also entirely enclosed in plastic) that the user may insert into the parent device. This connector supplies unisolated power to the module. My question is as follows: what are the drivers of the requirements for the insulation/physical protection of such a connector? Is it sufficient to think about this from the perspective of flashover, clearance, and creepage? Do I need to worry about users being dumb with anything other than their appendages (for example, inserting a paperclip?) or can I just make the aperture to the connector deep enough and small enough that it prevents contact by fingers?Thanks in advance for everybody's input!

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 21, 2023, 11:00:06 pm
content: Quote from: foxtrot on October 21, 2023, 07:21:53 pmA lot of commercial mains operated products will fail that test, therefore it would be silly to suggest such an unreasonable test would be required. PoE is also low voltage and wouldn't be subject to anywhere as strict requirements as mains. And don't forget that the pins of an Ethernet connector are very easy to touch.

author: wraper
author_rank: Unknown
timestamp: October 21, 2023, 11:24:02 pm
content: Likes like you want to make something not adhering to standards.QuoteSo how efficient will be your non isolated converter version? Efficiency of buck converters with large input-output voltage difference generally sucks.PoE requires negotiation before applying voltage, so no power must be present on empty connectors. Looking on some mass produces PoE switches should make it obvious how things are done.

author: wraper
author_rank: Unknown
timestamp: October 21, 2023, 11:26:57 pm
content: Also <60V DC is considered safety extra low voltage (SELV).

author: coppice
author_rank: Super Contributor
timestamp: October 21, 2023, 11:27:52 pm
content: Quote from: wraper on October 21, 2023, 11:24:02 pmSome voltage is initially present to allow the target device to consume enough to perform the negotiation. Only then will the main power be applied.

author: wraper
author_rank: Unknown
timestamp: October 21, 2023, 11:35:09 pm
content: Quote from: coppice on October 21, 2023, 11:27:52 pmSignature resistance is discovered before applying any real power.https://www.skyworksinc.com/-/media/SkyWorks/SL/documents/public/white-papers/understanding-the-ieee-8023bt-poe-standard.pdf

author: asis
author_rank: Frequent Contributor
timestamp: October 22, 2023, 02:13:43 am
content: Hi,Extensive topic.....https://datasheetspdf.com/datasheet/search.php?sWord=TI+PoEhttps://www.ti.com/lit/an/slva233a/slva233a.pdf?ts=1697939818792&ref_url=https%253A%252F%252Fwww.google.com%252F

author: foxtrot
author_rank: Contributor
timestamp: October 22, 2023, 03:16:12 am
content: Quote from: wraper on October 21, 2023, 11:24:02 pmYeah; this actually does adhere to standards. PoE-based lighting, for example, is often non-isolated – both for cost and efficiency reasons. The IEEE802.3af specification § 33.4.1 "Isolation" says the following:QuoteI think I've answered my own question. It looks like the insulation requirements are only for "accessible external conductors" as defined by IEC 60950-1 § 2.1.1 ("Access to energized parts") and IEC 62368-1 § 5.4.10.1. I can ensure that the design and accessibility of the connector prevents it from being categorized as an accessible external conductor, and I can also apply creepage/clearance requirements for additional peace-of-mind.

author: foxtrot
author_rank: Contributor
timestamp: October 22, 2023, 03:18:28 am
content: Quote from: asis on October 22, 2023, 02:13:43 amI'm already familiar with the actual transient-immunity requirements, but that's not what the question is about. Either way, these are good documents to share for the group, overall

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2023, 08:04:43 am
content: at 70W you need a 56VDC PoE supply about 1.5A.Misapplied thus can damage the end equipment, or use, wiring.In USA this 56V PoE + DOES NOT fall to SELV, 42.2V peak.See SELV def and PoE classes.Every PoE injector/hub/supply I have seen is designed to meet safety requirements.Jon