Lennart –
Thanks for your thoughts on this, and I generally agree with your sentiment on interoperability.
However, while what you say is true about designing PDs, it ignores the impact of using a higher voltage on the installation issues.
I, too, have pause at keeping 3 voltages, but think that 2 can be managed, and that 2 voltages actually fits well with the market requirements.
I suspect that we cannot go with fewer than 2 voltages without limiting our market potential. This is because single pair applications go beyond Information Technology environments
and have to deal with regulatory issues in environments which are not as benign as enterprise LAN applications.
Generally, PoE was designed in a world where we considered SELV limits, similar to US NEC Class II inherently limited power sources. Those set up a 100VA per circuit limit and
60 Volts as the threshold.
However, if you are in damp, dusty or other environments – and 10BASE-T1L is designed to go into these – you may have other limits. These are best allowed by a 24 volt system.
There are also (as I know you are aware) a host of regulations which say “50 volts” (sometimes without either AC or DC) and a 50 volt system would avoid arguing about installations because of this limit.
A long time ago we asked the industrial crowd whether they could live with the SELV 50-60 V systems that normal PoE would give – the answer was that they generally worked with 24
volts. The reason why was buried in various codes and regulations.
Looking through the NEC, below are a summary of a number of the limits:
30V limits:
For example, low voltage lighting where wet contact is likely to occur has a limit of 30 volts DC in the NEC (Article 411). This includes landscape lighting in damp areas and lighting
around pools (Table 300.5).
Class I power limited circuits are rated at 30 volts max (725.41A).
(Hazardous locations - fire or explosion hazards) called Class I, II or III locations (article 501) require covers for protection at greater than 30 volts, and in wet locations,
greater than 15 volts.
Swimming pools require less than 30 volts for safe contact on continuous dc circuits (article 680)
42 volt limits:
Elevators allow flexible cords and other rules which make installation easier when voltages are less than 42 volts dc. (article 620)
50 volt limits:
Electric vehicle charging systems (article 625) have more stringent rules for connections supplying ventilation equipment when the voltage supplying the ventilation equipment is
more than 50 volts dc.
Circuits under 50 volts get all sorts of special treatment like exceptions from requirements and special requirements making things easier: (articles: 200.7 (wiring), 250 (various
grounding & marking), 409 (disconnects), 422 (listing of appliances), 430 (motors), 522 (control circuits in amusement environments), 625 (see above), 665 (heating equipment), 668 (battery power supplies), 708 (Wiring of HVAC, Fire Alarm, Security, Emergency
Communications, and Signaling Systems.), and then there are articles 720 (circuits under 50 volts).
While in many cases, the NEC says a voltage limit (30 or 50 volts) based on AC, the lists above do not include those which say AC. They are either for DC or they don’t specify
AC or DC, so they apply to both. While some of the 50 volts might be extended to 60 volts dc in the future, or better yet, class II, they are not currently. SO, what this means is that permitting, licensing, labeling, listing, and various regulatory rules
governing installations get harder at these voltage limits, making it harder to address the market.
In short, we need to be cognizant of more than just what it takes to build a PD.
The easiest path, I think, from a market acceptance standpoint is to specify 2 voltage limits. The 24 volt limit seems an obvious choice.
The question in my mind is for the higher limit:
-
whether to go for a 50 volt limit and maximize ease-of-use, or go for a 60 volt limit and allow a little more power transfer. (what do we enable by that?)
-george
The goal of 10BASE-T1L + PoDL should be to maximize
interoperability while offering the performance needed for as many applications as possible.
We need to answer two questions to come to a conclusion on your proposal.
1. Do we need a multi-voltage power standard ?
2. What is/are the right voltage(s) ?
With regard to (1), we can learn from other standards that multi-voltage power schemes tend to be far more complicated and fail to achieve wide interoperability.
Take for example USB-PD. This standard supports 3 voltage levels. Despite being released already years ago, the market traction is low.
In fact, this USB standard is eclipsed by a proprietary fast charging standard (you can all guess which one), that offers a simpler mechanism to achieve high charging speeds.
For a PD design, supporting multiple voltage levels creates quite a bit of extra complication and testing. The PD can choose to only support one of the voltage, but this then goes
at the cost
of not working with all PSEs.
Similarly on the PSE side, a multi-voltage PSE is much more complicated than a single-volt one. Also power budgeting becomes more complicated.
I believe that interoperability and product simplicity is best served by choosing a single voltage to be supported.
That takes us to question (2)... which one ?
Higher voltages are (far) more efficient at transferring power.
For example, we need to deliver 8W over 100m of single-pair cable with AWG24.
One supply is 24V, the other is 53V.
The 24V supply will source 13.1W, with 5W dissipated in the cable.
The 53V supply will source 8.4W with 0.4W dissipated in the cable.
Both deliver 8W to the respective PD.
Additionally, 8W is about the limit that a 24V supply can deliver in a stable manner (for this cable), the 53V supply can go above 40W.
Very few electronic designs operate from 50V directly. We know this problem from PoE.
But converting from 50-60V down to 24, 12, or 5V is not a complicated or costly issue, many efficient switch mode designs exist that can do this.
- Operating from a single voltage will greatly simplify PD and PSE design and support interoperability
- Picking the highest voltage range allows us to deliver the most power and deliver power FAR more efficiently than a lower supply voltage can
- It is not complicated to build PDs that need 24V internally to support a PoDL voltage of 50+V
On Wed, 2019-05-15 at 23:45 +0000, Stewart, Heath wrote:
All,
I wanted to open a discussion on the power class proposal referenced in i-321.
http://www.ieee802.org/3/cg/comments/Comment_i-321_Stewart_3cg_clause_104_modifications_v1.pdf
The proposal assigns class power voltage, current and power values to make required corrections to cable resistance and to accommodate the following use cases:
-
24V nom
-
50V max
-
SELV max
All other changes derive from the above introduced values. The proposal attempts to cover all suggestions offered during a large number of discussion during and between cg sessions
in the Vancouver meeting.
In the time since the proposal was submitted a couple of weeks ago I have collected a number of additional inputs from the cg task force. My goal in sending this to the reflector
is to kickstart public and private discussions with the goal of streamlining our efforts in cg next week.
It is worth pointing out the there are a number of possible end markets: eg building automation and industrial automation. We have gotten really good input from these end markets
and strive to enable relevant cabling resistances while remaining aware of economic and technical feasibility.
While process automation is an exciting market as well, efforts to marry PoDL to process automation Intrinsic Safety (IS) requirements stalled early on. Therefore process automation
requirements are not directly considered in this PoDL contribution.
The vast majority of the feedback from colleagues has regarded the 3 proposed voltage ranges. Mainly the comments address the appropriateness of supporting 1 vs 2 vs 3 voltage ranges.
Please feel free to reply on this thread or offline. I will attempt to come prepared to modify proposed text to reflect the will of the group.
Cheers,
Heath Stewart
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