John and all,
> Can somebody explain to me why an isolated power supply is 'safer' than an
> earthed one?
Consider electric installation with zeroing and not grounding. I'm not sure
how it is in another countries so to be precise grounding is the 3 wire
installation, and zeroing is the 2 wire installation, but with 3 contact
sockets (the grounding pin in each socket is connected with neutral power
cable).
All house buildings older than about 10 years in Poland have zeroing and not
grounding.
When it happens that the neutral power cable is broken than if any one
device is on (nowadays plenty devices are on in standby mode) than this
device connects live 230V power to your grounding system. All devices metal
cases becomes "grounded" to 230V.
This is specially a problem in old buildings with aluminium wires ("Big
Brother" who decided about all in Poland, in hopefully past times, decided
aluminium installations are cheaper and you had no chance to do another
way).
If it happens it is danger, but as all metal cases are at the same 230V
potential you have a chance to survive.
Now consider the SELV circuit which is not only isolated but also grounded
in one of your devices and the SELV cable goes to another place in the same
building, where there is everything OK with power (it may be your another
room). At this SELV circuit there is 230V against all metal cases someone
can tough at the same time he touches something connected with this SELV.
I'm not sure if it is any sense to install differential breakers in such
installation, but even one is there it will not see that current as
differential.
In my opinion until 100% of electric installations all over the world will
be 3 wire the isolated SELV is more safely then isolated and grounded (if
grounded is understand as connected to socket grounding pin).
But I'm certainly not an expert in safety issues.
Best Regards
Piotr Galka
----- Original Message -----
From: "John Woodgate" <j...@jmwa.demon.co.uk>
To: <emc-p...@ieee.org>
Sent: Friday, March 27, 2009 9:39 AM
Subject: Re: PoE injectors
> In message <6.1.0.6.2.20090327004517.030f4...@pop.randolph-telecom.com>,
> dated Fri, 27 Mar 2009, Joe Randolph <j...@randolph-telecom.com> writes:
>
>>A Class 2 supply is the most common type of supply that I have seen for
>>POE injection. And yes, the output should be isolated from earth ground to
>>comply with IEEE 802.3af.
>
> Can somebody explain to me why an isolated power supply is 'safer' than an
> earthed one? I am particularly concerned that this applies to IT networks
> and anything else that may run through, for example, roof voids and
> similar spaces. Within one room, as for Class II consumer products, the
> techniques is probably acceptable.
>
> I know there are 'segregation' requirements, but cable runs can be
> disturbed by non-electrical people during maintenance. The point is that
> cable damage (by humans, rodents or building movements) can create a
> connection between live mains and the 'SELV' circuit. Since the latter is
> not earthed, the fault can persist indefinitely, until someone contacts
> the SELV circuit. The hazard is great, the probability is low, and to me
> that results in a tick in the 'unacceptable risk' box.
>
> We now have the concept of 'PELV', protected extra-low voltage, which has
> reinforced insulation from mains voltage AND is earthed (preferably at one
> point only, to avoid currents due to earth potential differences. I think
> that all 'SELV' requirements must be re-examined to see if they should be
> changed to 'PELV'. In hazard-based terms, there is no contest. SELV has
> two safeguards, PELV has three.
>>
>>Personally, I do not see any safety compliance problems with this
>>construction when used as intended (inside lines).
>>
>>By the way, one problem that I *have* seen with this construction is
>>performance related. Most of these POE injection power supplies are
>>switching converters, and the Y-caps used on the input for EMI suppression
>>create a voltage divider that places a very high 50/60 Hz common mode
>>noise signal (typically one half of the AC mains voltage) on the Ethernet
>>outputs.
>
> How can Y-caps do that? Y caps go from both mains poles to earth.
>
>>This means that an IP telephone powered by the POE injector will be riding
>>on the 50/60 Hz common mode noise. The analog audio circuits in the IP
>>phone, such as the microphone for a hands-free mode, can be sensitive to
>>this common mode noise and produce audible hum.
>>Grounding the chassis of the POE injector fixes the problem, but most of
>>these devices are ungrounded.
>
> Then they cannot possibly have Y-caps.
>
>>So, the IP telephone has to be designed with an extraordinary level of
>>50/60 Hz common mode immunity in order to avoid hum in the analog audio
>>path.
>
> It sounds as though people are making some fundamental error that shows up
> as this problem.
> --
> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
> Things can always get better. But that's not the only option.
> John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
>
> -
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This message is from the IEEE Product Safety Engineering Society emc-pstc
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<emc-p...@ieee.org>
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