I'm wondering if something was missed here. In the telephone world +48 is
grounded... NOT -48V. Mixing telephone systems with other systems where there
are negative grounds like POE could certainly lead to smoke. The fact you have
a resistor to ground is meaningless. What is the value of the resistor? Isn't
one side of all shunt loads connected to ground?
Fred Townsend
Joe Randolph wrote:
On 11/10/2008, Mark Gandler wrote:
here is the case:
Switch powered by 100-230VAC/48DC-1A adapter with earthed plug.
At 48V side,
"Vcc-" is connected through resistor directly to Earth ground.
This switch has PoE port, which is used to power external
device (PD,
powered device), when there is a laptop connected through RS232 port to this
PD. Laptop has his own power adapter, I assume grounded as well.
In certain scenario, powered device in the middle, will get
smoked. One
claims it is due to the ground loop, based on Vcc- of power adapter being
directly tied to chassis ground.
Problem goes away after the connection is cut.
Few questions:
1. are where any 60950 issues which could prevent tying
DC(-) output to
earth ground? (adapter has UL, TUV/CB certificates)
2. assuming the answer to #1 is "no", would this scenario
require switch
adapter and laptop adapter to be connected to different grounds? in order to
create different potentials? and thus will make powered device in the middle
the weakest point in the system?
Hi Mark:
>From your description, I'm not sure I quite understand the
configuration you
describe, but I can offer a few comments that may help you sort this out:
1) The basic IEEE 802.3 Ethernet specification requires 1500 VRMS
isolation
between the Ethernet cable lines and earth ground. This is primarily a
functional requirement, not a safety requirement. It prevents ground loops
that could otherwise occur if the equipment at the two ends of the cable run
(up 100 meters) has two different ground references.
2) When power over Ethernet (POE) was added in IEEE 802.3af, the
requirement
for 1500 VRMS isolation was retained. So, POE interfaces (both the
power-sourcing PSE and the powered-device PD) are each still supposed to be
isolated from ground. This is typically done with isolated DC/DC converters.
3) The power supply for a laptop computer may or may not pass earth
ground
>from the AC mains outlet up to the laptop. That choice is up to the
manufacturer.
So, there should not be any path to ground from any Ethernet port,
whether it
be standard Ethernet, POE PSE, or POE PD. For an RS-232 port on a laptop,
there may or may not be a connection to ground through the power supply.
The fact that the 48 VDC supply to the switch is grounded does not
necessarily mean that the power placed on the PSE Ethernet ports is also
grounded. There might be (and should be) an isolated DC/DC converter inside
the equipment for powering the PSE Ethernet ports. Even if the manufacturer
of the switch violated the 802.3af isolation requirement, you still couldn't
get a ground loop via the Ethernet connection unless the manufacturer of the
PD also violated the 802.3af isolation requirement on the PD. This seems
unlikely, unless the same errant manufacturer made both devices.
Regarding your question about 60950 issues, there are none that I am
aware
of. I believe that POE is typically treated as a SELV circuit under 60950.
The only place where you find an isolation requirement for POE is in IEEE
802.3af, which is only an industry standard as opposed to the regulatory
standard 60950.
I hope the above comments are helpful. The behavior you describe does
sound
like a possible ground loop problem, but properly designed POE equipment
should prevent ground loops. It would be interesting to check the internal
designs of the POE equipment to see if it is, in fact, isolated. It would
also be interesting to determine what specific components are getting "smoked."
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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