[SI-LIST] Re: How to connect Chassis ground to DGND
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Steve Weir" <weirsp@xxxxxxxxxx>,"Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>,"Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>, cchalmers@xxxxxxxxxxx,si-list@xxxxxxxxxxxxx
- Date: Sun, 29 Feb 2004 13:54:22 -0800
I've watched this one from a distance. I'm not sure what the objective of
tying logic ground to "chassis" ground. There are at least tow possible
reasons.
1. To meet the UL safety specification.
2. To help control EMI.
In the first case, logic ground doesn't need to be attached to the chassis
ground to do this. The chassis needs to have a good connection to the
green wire that goes back to earth in the AC system,
In the second case, using the word chassis when discussing EMI confuses
things. In this case, the product is surrounded by a Faraday cage, which
likely has parts of the chassis as sides. If this is the situation, it is
important to make sure currents that belong in the logic ground system
don't have an opportunity to flow in the Faraday cage. This can happen if
logic ground is tied to the Faraday cage in more than one place. Symptoms
of this happening are the complaint that EMI is "leaking" at the cracks. A
common error is to tie the backplane logic ground to the Faraday cage
around its periphery (Good Idea) to form one side of the Faraday cage.
Then, the faceplates of the PCBs or their edges are also tied to the
Faraday cage. (Not a good idea)
As to the 48 VDC supplies that many of us use now, neither rail of this
supply should tie to logic ground or to the chassis.
Hope this helps.
Lee
> [Original Message]
> From: steve weir <weirsp@xxxxxxxxxx>
> To: Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>; Chris Cheng
<Chris.Cheng@xxxxxxxxxxxx>; <cchalmers@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 2/27/2004 5:23:57 PM
> Subject: [SI-LIST] Re: How to connect Chassis ground to DGND
>
> Chris, no shorting doesn't dictate a fire. Shorting improperly does
> particularly if there is any decent source of potential anywhere. In the
> example I cited, the people who designed that system got so wigged out
> about DC returns that they didn't return anything to earth properly. By
> one means or another one of the shelves became hot, probably due to a
> component failure and when they closed the loop with a PC, the cables
> became the safety ground and that is what caused the fire.
>
> The big shield currents can still be an issue. This is one reason why
> industrial controls and instruments are isolated. Now, within a chassis
or
> rack, isolating the power converters and tying the battery return to
earth
> at one point will prevent power return current from flowing through the
> chassis and/or the signal cable shields. However, if we have a big
> installation, and we use non-isolated connections, like RS-485 shields
> between rooms, we can be right back to getting really big currents
flowing
> in the shields.
>
> If you want to do an eye opening experiment with equipment that shorts
the
> DC return to chassis, isolate the chassis from the power plant return,
and
> then measure the return current in return of one shelf, or drive or what
> have you, and then the other. Then change the length of one of those
> return cables by a modest amount and remeasure. The whole mess makes a
> dandy wheatstone bridge. Now, if you connect the chassis back to the
power
> ground / earth through a cable and measure the current through that, you
> will see it carrying a lot of the device current.
>
> Steve.
> At 03:12 PM 2/27/2004 -0800, Chris Cheng wrote:
> >Steve,
> >I have a complete open mind on this one. However, if what you say is
> >true, there will be a lot of FCAL disk subsystem on fire now since
> >they are mandatory DC shorted together.
> >
> >As for the minimal differences between AC short vs. dead short, my
> >suspicion is the disk drivers and add on card themselves have already
> >done the dead short for you already. Whether you want to use cap
> >or zero ohm short on system board does not matter anymore.
> >
> >-----Original Message-----
> >From: steve weir [mailto:weirsp@xxxxxxxxxx]
> >Sent: Friday, February 27, 2004 2:44 PM
> >To: Chris Cheng; cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: RE: [SI-LIST] Re: How to connect Chassis ground to DGND
> >
> >
> >Chris, ground seems to confuse a lot of people. DC isolation is usually
> >motivated by limiting loop currents in shelf to shelf or equipment to
> >equipment cables. It can be important. Shields in computer room
equipment
> >cables have been measured carrying many 10's of amperes. A company that
I
> >know of decided to "protect" themselves by only using optical connects
> >between shelves. Then they forgot to isolate the RS-232 craft
maintenance
> >ports. They managed to set fire in one customer's installation when the
> >customer connected one PC to two shelves.
> >
> >I had another case where due to powering requirements, we needed to DC
> >isolate. Given the opportunity in new equipment that didn't have the
same
> >powering issues to get rid of the DC isolation, I jumped at the
> >chance. This completely baffled my colleagues who were and remain smart
> >people.
> >
> >I can't explain the random goat sacrifices and other rites that some seem
> >to use with add on cards other than the observation that mythology
doesn't
> >die very easily. PCs have some issues because the power return and logic
> >ground are not isolated. So, the power return does run through the
chassis
> >metal.
> >
> >If your EMI scan showed no difference you are lucky. Usually the caps
> >don't do as well as bonding. At least that has been my experience.
> >
> >Steve.
> >
> >At 01:56 PM 2/27/2004 -0800, Chris Cheng wrote:
> > >Steve,
> > >
> > >This is an interesting area I am always curious. If you look at the
GBIC
> > >spec, it requires DC isolation at the connector point between chassis
> > >and logic gnd. FCAL on the other hand is exactly opposite and requires
> > >dead short between chassis and logic gnd. I never quite figure out
> > >why two huge groups of very smart people can come out with specs that
> > >are completely opposite to each other. It certainly makes my grounding
> > >scheme looks funny.
> > >
> > >To make things worst, a lot of add on cards like PCI cards seems to
> > >have a grounding scheme based on the phases of moon. And most of
> > >the disk drivers vendors like to tie their chassis to logic gnd
> > >internally just to make your integration a little more complicated.
> > >
> > >EMI engineers always told us to DC isolate the entire chassis with
> > >a single shorting point between chassis and logic ground. The rest
> > >of the system have decoupling caps to AC connect the chassis to
> > >logic ground. I never quite understand the logic and I have
> > >experimented with DC shorting the entire chassis with logic ground
> > >at every point. The EMI scan shows no difference.
> > >
>
>
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