Fred, where was no telephone or -48DC system invloved, just a simple case of 100-240/48V AC/DC power adapter. We are actually got to the bottom of this after customer sent us his Ethernet schematics.
PD actually has 5 RJ-45s, only one of which is an EthernetPoE port. The others are non-standard ports that simple use RJ45 connectors. The damaged PDs were a result of the installer connecting PoE port to one of other non-EthernetPoE ports. These ports are not transformer coupled and thereby provide a DC path directly from attached device to other signals, Digital_Gnd being one of them. This in itself is not enough to cause the damage. They then needed to connect an RS232 (diagnostic port) to their PD which closes the ground loop. A voltage potential was then created across the PD ground damaging some of its components. Thanks to Fred and Joe for your comments. Mark ________________________________ List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Wed, 19 Nov 2008 12:11:03 -0800 From: [email protected] To: [email protected] CC: [email protected]; [email protected] Subject: Re: Connecting DC output to earth ground 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-s ourcing 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 Ether net 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/> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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