Re: power delivery question from Liaison report
In terms of what the "connector" sees as far as voltage, the series
voltage is the bulk resistance of the conector, <.2 Ohms typically and
the current through the contact, so we're looking at less than .1 V. In
IEC 603-7 section 6.2.4 20 m Ohms is specified for initial contact
resistance (measured after subtracting out bulk resistance of the
connector) It is allowed to rise to 40 mOhms after 750 mate/unmate
As far as voltage proof between contacts, IEC 603-7 section 6.2.2
specifies 1000 Vac P-P contact to contact (or DC) and 1500 VAC P-P
contact to test panel. Section 6.2.3 specifies current carrying capacity
at about 1.8 A per contact at 20 C derated to zero at about 72 C. The
same spec section 6.2.5 specifies 500 M ohm insulation resistance
tested at 100 V d.c.
The cable must meet higher requirements for all of the above voltage
proof and insulation resistance measurements.
I know there are some safety requirements in UL as to exposure to
voltage and current contacts, I'm not sure which specification covers it
"Donald S. Stewart" wrote:
> Thanks for raising the issue of the "48 VDC max" statement in the liaison
> report. I assume it is in error (i.e., is not the MAXIMUM but is intended
> as a nominal) but we need someone to confirm this. I took a quick look to
> see if I could find an ISO/IEC JTC 1 standard that addresses this voltage.
> Scanning the ISO/IEC 11801 "Information Technology - Generic Cabling for
> Customer Premises", I found no mention of anything but breakdown voltage,
> i.e., no safety/fire related voltages and no mention of 48 volts or
> anything close to that.
> The 10 watt per pair limit and the 175 mA per conductor current limit the
> liaison report states, would be compatible with 57.14 volts, which seems a
> reasonable upper limit. My view is the upper limit should be about 56
> Don Stewart
> > Rick Brooks wrote:
> > I was reading in the Draft Liaison report from ISO/IEC JTC 1/SC 25/WG 3
> > to IEEE802.3 on power feeding that was
> > handed out at the July Plenary.
> > IEEE802.3af had question 4: Info on parameter limits (voltage, current,
> > power, source impedance, ...) for world wide standards.
> > i.e. what are the restriction beyond SELV.
> > The response back was 48 VDC max, 175 ma max per pin.
> > My question is:
> > Is the 48VDC output from the port really 48VDC max as the response to the
> > question indicates?
> > If so, my thoughts are the following:
> > We would have to spec our power output at the PSE as 48 VDC + 0%, - 8%,
> > or something like that,
> > so that it never exceeds 48 VDC continuously.
> > This will further limit the available load power;
> > it would be less than the load power that was discussed at the last
> > meeting namely 14.6 watts.
> > So, in that case the PD must be designed to draw at most 350 ma, as we
> > discussed.
> > And the power delivered at 100 meter cable would then be:
> > Pwr = [44.2 - (12.5 x 0.35)] 0.35 = 13.9 Watts. (where 44.2 VDC is the
> > lowest output voltage to still be in spec)
> > For long cable lengths, the current per pin will be balanced, and we
> > don't exceed the 175 ma per pin.
> > For short cable lengths, we probably need an additional power spec, so
> > that neither RJ-45 pin exceeds 175 ma.
> > Say that due to connector imbalance, one pin is 175 ma, and the other is
> > 20% below that, or 140 ma, which is a total of 315 ma.
> > Then the power for a short cable would be (at least) 13.9 watts (44.2 *
> > 0.315).
> > This would say that the PD device should be designed not to draw more
> > than 350 ma,
> > and at the same time not to draw more than 13.9 watts.
> > That way we never exceed 48 VDC nor 175 ma per pin on a continuous basis.
> > This puts the burden on the PD end to meet these current and power
> > requirements.
> > The PSE end would have a max voltage of 48 VDC, but it's current limit
> > would be set slightly higher than 350 ma
> > by some appropriate margin.
> > If, on the other hand, we put the burden at the PSE end, then the
> > available power goes down even more, but that may be OK also.
> > comments?
> Donald (Don) S. Stewart Phone: 732-817-5495, FAX x4666
> Soon to be: Avaya Communication e-mail: dsstewart@xxxxxxxxx
> (former Enterprise Networks Group of Lucent Technologies)
> Cross-Product Architecture
> 101 Crawfords Corner Road
> Holmdel, NJ 07733