Ground Fault Protection Trigerring in Server Room
Will
protected by ground fault protection. As I understand it, a GFP will
trigger when it senses current on the ground, and when it does then the GFP
shuts off the circuit. We tried today to rewire some things in our
server room, and the GFP kept triggering, and I want help understanding
possible causes.
Each server has two power supplies. In an effort to minimize cabling, we
purchased from Dell some cables that plug into the PDU using Nema 5-15P, and
split to two different IEC C14 plugs that go to the server power supplies.
It appears that use of this power cord to two power supplies is what is
causing the ground faults. We can cure the GFP fault simply by leaving
just one of the two plugs in each server. Does someone have an explanation
as to why the GFP would fault when both power supplies are served from the
same plug?
Perhaps the power supplies start up at different times, and there is some
kind of reverse current associated with the startup?
--
Will
Andrew Gabriel
"Will" <weste...@noemail.nospam> writes:
> Our computer server room outlets for our power distribution units (PDUs) are
> protected by ground fault protection. As I understand it, a GFP will
> trigger when it senses current on the ground, and when it does then the GFP
No, it triggers when the difference in current flow in the hot
and neutral exceeds a permitted value. This could be because
the current is leaking to the ground, or anywhere else.
> shuts off the circuit. We tried today to rewire some things in our
> server room, and the GFP kept triggering, and I want help understanding
> possible causes.
>
> Each server has two power supplies. In an effort to minimize cabling, we
> purchased from Dell some cables that plug into the PDU using Nema 5-15P, and
> split to two different IEC C14 plugs that go to the server power supplies.
> It appears that use of this power cord to two power supplies is what is
> causing the ground faults. We can cure the GFP fault simply by leaving
> just one of the two plugs in each server. Does someone have an explanation
> as to why the GFP would fault when both power supplies are served from the
> same plug?
It might be that the leakage in the PSU's is exceeding the trip
current.
In UK (and I think all the EU), RCDs (GFIs) to protect against
electrocution are 30mA trip (which means they must trip at
somewhere between 15mA and 30mA). General recommendation is that
max design leakage is 25% of the trip rating, i.e. 7.5mA. Each
piece of Class I (earthed) IT equipment is allowed to leak up
to 0.75mA through its RFI components. That gives a maximum of
10 pieces of Class I (earthed) IT equipment per 30mA RCD. In
terms of desktop PCs, that's about 5 (each one consisting of
a base unit and monitor, which are Class I). (However, we would
not normally use RCD protection in server rooms at all.)
Now, all these figures are going to be different in the US
and you need to perform a similar calculation. In particular,
your GFI's are normally 5mA, which I would imagine is going to
severely limit the number of pieces of Class I (earthed) IT
equipment per GFI.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
Charles Perry
"Andrew Gabriel" <and...@cucumber.demon.co.uk> wrote in message
news:48706fe0$0$78076$5a6a...@news.aaisp.net.uk...
What he said. Some (most now) power supplies are power factor corrected.
They have filters that cause a small amount of leakage current to flow on
the ground. Sum enough of these together and you trip a GFI (or even an RCD
for really large gear). It appears your two machines each have power
supplies that are close to tripping the GFI with one power supply connected,
and will trip when both are connected.
Charles Perry P.E.
phil-new...@ipal.net
There are 2 levels of GFI protection used in the US, 6ma and 30ma. For the
6ma level, the allowed range is 2ma (most not trip below that) and 6ma (must
trip any higher than this) with the nominal supposedly at 5ma (where most
will trip). We do have the 30ma level for industrial (not home) use, which
might be suitable for a server room where GFI is somehow required. I do not
know the range for that.
--
|WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
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| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
phil-new...@ipal.net
| Our computer server room outlets for our power distribution units (PDUs) are
| protected by ground fault protection. As I understand it, a GFP will
| trigger when it senses current on the ground, and when it does then the GFP
| shuts off the circuit. We tried today to rewire some things in our
| server room, and the GFP kept triggering, and I want help understanding
| possible causes.
Do your local requirements actually specify that you must have GFI protection?
If so, do you know if the 6ma level or 30ma level is specified? What is the
level installed?
If you don't need to have this kind of protection, then maybe it's better to
just avoid it.
| Each server has two power supplies. In an effort to minimize cabling, we
| purchased from Dell some cables that plug into the PDU using Nema 5-15P, and
| split to two different IEC C14 plugs that go to the server power supplies.
| It appears that use of this power cord to two power supplies is what is
| causing the ground faults. We can cure the GFP fault simply by leaving
| just one of the two plugs in each server. Does someone have an explanation
| as to why the GFP would fault when both power supplies are served from the
| same plug?
You are using "double cords" plugging two computers into one outlet. Does
each outlet have its own separate GFI protection (such as being fed by a
separate GFI type breaker)? If so, the other explanations about marginal
leakage would cover it. And those filters could be leaking extra to ground
if your power quality is bad, and there's lots of noise on the line.
FYI, GFI can be tripped with high frequency energy. I've done so with 146 MHz
that was apparently resonating in common mode on the power wires the GFI was
on. Common mode is what GFI would trip on.
| Perhaps the power supplies start up at different times, and there is some
| kind of reverse current associated with the startup?
That by itself would not trigger the GFI protection.
RFI-EMI-GUY
I wondered about this, is there actually a leakage current or is th
current phase shifted to the extent the GFCI "thinks" there is an imbalance?
--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."
"Follow The Money" ;-P
Charles Perry
"RFI-EMI-GUY" <Rhyo...@NETTALLY.COM> wrote in message
news:uy6ck.18765$LL4....@bignews7.bellsouth.net...
Actual current.
Charles Perry P.E.
phil-new...@ipal.net
| I wondered about this, is there actually a leakage current or is th
| current phase shifted to the extent the GFCI "thinks" there is an imbalance?
If the shifted phase is carried opposite on each of the 2 conductors and none
of it is on the ground or leaks to an external ground, the GFCI will see it as
a net zero common mode current, and do nothing.
The GFCI design is basically a single current transformer around the group of
2, 3, or 4 current carrying conductors (the number depending on the system
type). The ground conductor(s) are not involved with the current transformer.
What it is measuring is the sum current in common mode on all those conductors
together. There cannot be such a current in common unless it forms a complete
circuit somewhere else. The whole idea is that if it is forming a complete
circuit somewhere else, that is a hazard (you might be part of that circuit).
RFI-EMI-GUY
> On Sun, 06 Jul 2008 12:42:01 -0400 RFI-EMI-GUY <Rhyo...@nettally.com> wrote:
>
> | I wondered about this, is there actually a leakage current or is th
> | current phase shifted to the extent the GFCI "thinks" there is an imbalance?
>
> If the shifted phase is carried opposite on each of the 2 conductors and none
> of it is on the ground or leaks to an external ground, the GFCI will see it as
> a net zero common mode current, and do nothing.
>
> The GFCI design is basically a single current transformer around the group of
> 2, 3, or 4 current carrying conductors (the number depending on the system
> type). The ground conductor(s) are not involved with the current transformer.
> What it is measuring is the sum current in common mode on all those conductors
> together. There cannot be such a current in common unless it forms a complete
> circuit somewhere else. The whole idea is that if it is forming a complete
> circuit somewhere else, that is a hazard (you might be part of that circuit).
>
Which begs the question why there would be leakage current in the design
of a PF corrected UPS?
ARLOWE
Possibly through the EMI filters.
phil-new...@ipal.net
Which is even more likely with dirty (lots of noise and harmonics) power.
Maybe we need a ground fault detector design that avoids issues like this by
having a high frequency bypass around the current transformer? OTOH, if 6ma
is the danger level at 60 Hz, I'd think it would also be at higher frequencies
as well. So such a design might not be safe.
Maybe the solution is to clean up the power _before_ it reaches the GFCI.
Andrew Gabriel
phil-new...@ipal.net writes:
> On Mon, 07 Jul 2008 20:29:47 +1000 ARLOWE <ARL...@nose.picker.com> wrote:
>| RFI-EMI-GUY submitted this idea :
>|
>| Possibly through the EMI filters.
>
> Which is even more likely with dirty (lots of noise and harmonics) power.
Not aware that PF correction generates more RF noise.
The EMI filters are aiming at RF noise, not harmonics of
the line frequency (that's what the PF correction is doing).
> Maybe we need a ground fault detector design that avoids issues like this by
> having a high frequency bypass around the current transformer? OTOH, if 6ma
> is the danger level at 60 Hz, I'd think it would also be at higher frequencies
> as well. So such a design might not be safe.
Actually, skin effect comes into play at high frequency.
You can get a skin burn at 20kHz, but it's pretty difficult
to actually get the current deep enough to get near vital
organs. High frequency neon display tube power supplies are
consequently much safer than the old line frequency step up
transformers which used to be used.
> Maybe the solution is to clean up the power _before_ it reaches the GFCI.
--
phil-new...@ipal.net
| In article <g4tf7...@news5.newsguy.com>,
| phil-new...@ipal.net writes:
|> On Mon, 07 Jul 2008 20:29:47 +1000 ARLOWE <ARL...@nose.picker.com> wrote:
|>| RFI-EMI-GUY submitted this idea :
|>|> PF corrected UPS?
|>|
|>| Possibly through the EMI filters.
|>
|> Which is even more likely with dirty (lots of noise and harmonics) power.
|
| Not aware that PF correction generates more RF noise.
| The EMI filters are aiming at RF noise, not harmonics of
| the line frequency (that's what the PF correction is doing).
Not generate noise, but rather, divert it away, and some of that to ground.
This would be noise in the voltage of the supply possibly due to current
harmonics of other loads elsewhere.
|> Maybe we need a ground fault detector design that avoids issues like this by
|> having a high frequency bypass around the current transformer? OTOH, if 6ma
|> is the danger level at 60 Hz, I'd think it would also be at higher frequencies
|> as well. So such a design might not be safe.
|
| Actually, skin effect comes into play at high frequency.
| You can get a skin burn at 20kHz, but it's pretty difficult
| to actually get the current deep enough to get near vital
| organs. High frequency neon display tube power supplies are
| consequently much safer than the old line frequency step up
| transformers which used to be used.
I've been hit with 146 MHz before. That is definitely a burn feeling.
daestrom
> Our computer server room outlets for our power distribution units
> (PDUs) are protected by ground fault protection. As I understand
> it, a GFP will trigger when it senses current on the ground, and when
> it does then the GFP shuts off the circuit. We tried today to
> rewire some things in our server room, and the GFP kept triggering,
> and I want help understanding possible causes.
>
> Each server has two power supplies. In an effort to minimize
> cabling, we purchased from Dell some cables that plug into the PDU
> using Nema 5-15P, and split to two different IEC C14 plugs that go to
> the server power supplies. It appears that use of this power cord to
> two power supplies is what is causing the ground faults. We can
> cure the GFP fault simply by leaving just one of the two plugs in
> each server. Does someone have an explanation as to why the GFP
> would fault when both power supplies are served from the same plug?
>
If you just 'rewired' some things, there's a good chance you've crossed up
neutrals somewhere. For example if there are two circuits feeding a string
of outlets (sometimes called a mulit-wire branch circuit??), and you somehow
tied the neutral wires from two different circuits together, you won't have
a *short circuit*, but the small signal that is injected into the neutral by
the GFCI will find a different return path and that will be enough to trip
the unit.
FYI, the modern GFCI will inject a small common-mode signal onto both the
hot and neutral that pass through it. If the neutral conductor is connected
somehow to ground downstream, this signal will return to the service panel
via the ground wire (formally known as the 'Equipment Grounding Conductor')
and loop back to the supply side of the GFCI and create a trip signal. The
same thing can happen if the neutral wire downstream is connected to another
neutral wire on another circuit.
daestrom
phil-new...@ipal.net
| Will wrote:
|> Our computer server room outlets for our power distribution units
|> (PDUs) are protected by ground fault protection. As I understand
|> it, a GFP will trigger when it senses current on the ground, and when
|> it does then the GFP shuts off the circuit. We tried today to
|> rewire some things in our server room, and the GFP kept triggering,
|> and I want help understanding possible causes.
|>
|> Each server has two power supplies. In an effort to minimize
|> cabling, we purchased from Dell some cables that plug into the PDU
|> using Nema 5-15P, and split to two different IEC C14 plugs that go to
|> the server power supplies. It appears that use of this power cord to
|> two power supplies is what is causing the ground faults. We can
|> cure the GFP fault simply by leaving just one of the two plugs in
|> each server. Does someone have an explanation as to why the GFP
|> would fault when both power supplies are served from the same plug?
|>
|
| If you just 'rewired' some things, there's a good chance you've crossed up
| neutrals somewhere. For example if there are two circuits feeding a string
| of outlets (sometimes called a mulit-wire branch circuit??), and you somehow
| tied the neutral wires from two different circuits together, you won't have
| a *short circuit*, but the small signal that is injected into the neutral by
| the GFCI will find a different return path and that will be enough to trip
| the unit.
If he crossed neutrals to each other, you don't need a signal injecting
GFCI to be tripped by that. You just need enough of a load for the current
to distribute 6ma through the wrong neutral.
| FYI, the modern GFCI will inject a small common-mode signal onto both the
| hot and neutral that pass through it. If the neutral conductor is connected
| somehow to ground downstream, this signal will return to the service panel
| via the ground wire (formally known as the 'Equipment Grounding Conductor')
| and loop back to the supply side of the GFCI and create a trip signal. The
| same thing can happen if the neutral wire downstream is connected to another
| neutral wire on another circuit.
I thought this was for detecting reversed hot/neutral or neutral/ground at
the source. If neutral is crossed to ground downstream of the GFCI, it will
be detected as a current imbalance in the hot and neutral pair since some of
the current that should be on the neutral is going back over ground.
bud--
> On Tue, 8 Jul 2008 21:00:54 -0400 daestrom <daestrom@no_spam_heretwcny.rr.com> wrote:
> | FYI, the modern GFCI will inject a small common-mode signal onto both the
> | hot and neutral that pass through it. If the neutral conductor is connected
> | somehow to ground downstream, this signal will return to the service panel
> | via the ground wire (formally known as the 'Equipment Grounding Conductor')
> | and loop back to the supply side of the GFCI and create a trip signal. The
> | same thing can happen if the neutral wire downstream is connected to another
> | neutral wire on another circuit.
>
> I thought this was for detecting reversed hot/neutral or neutral/ground at
> the source.
How it detect either.
.
> If neutral is crossed to ground downstream of the GFCI, it will
> be detected as a current imbalance in the hot and neutral pair since some of
> the current that should be on the neutral is going back over ground.
It detects a cross with no load.
--
bud--
phil-new...@ipal.net
Ah, OK. That would be a plus.
Roy
On Wed, 09 Jul 2008 13:20:31 -0500 bud wrote about about what Phil
wrote:
On Tue, 8 Jul 2008 21:00:54-0400 daestrom wrote: | FYI, the modern GFCI
will inject a small common-mode signal onto both the | hot and neutral
that pass through it. If the neutral conductor is connected | somehow to
ground downstream, this signal will return to the service panel | via
the ground wire (formally known as the 'Equipment Grounding Conductor')
| and loop back to the supply side of the GFCI and create a trip signal.
The | same thing can happen if the neutral wire downstream is connected
to another | neutral wire on another circuit.
I thought this was for detecting reversed hot/neutral or neutral/ground
at the source.
| .
| How it detect either.
| .
If neutral is crossed to ground downstream of the GFCI, it will be
detected as a current imbalance in the hot and neutral pair since some
of the current that should be on the neutral is going back over ground.
Ah, OK. That would be a plus.
It's possible that the gfi they have is just old or defective - maybe
they shouldn't have messed with the wiring scheme to begin with -
everyone thinks it's easy until they do it themselves - I've wired jobs
that worked to specs so well, up until someone unqualified went in to
see just how I did it., figuring they could do it better, in effect
ruined functions then callled me asking wtf I had done that they
couldn't reproduce it [the funny part is they claimed it was all
wrong...after a year & ½ of service] - those maligning ingrates.
Think he'll get back to us to tell us how he fixed the problem?
Roy Q.T.
[have tools, will travel]
John Gilmer
>
> Which begs the question why there would be leakage current in the design
> of a PF corrected UPS?
>
It has little or nothing to do with "PF correction."
Because switching power supplies (and, BTW, the computer itself) produce a
LOT of high frequency "noise," there is usually some kind of low pass filter
"network" at the power input.
The network almost always has a DC pathway between each of the power
conductors and "ground"/chassis.
When one of the power conductors is at are near ground potential, the
network will pass a "common mode" net current. If you have several of
these supplies is parallel, you will exceed whatever threathold is set.
In my own home, I am "marginal" is this regard. I have two GFCI's "in
series" (I know, I know, I will get 'a round tuit') and the upstream one
trips occasionally during externally caused power surges. The potential
"leakage" loads include an air conditioned, a PC on a UPS, a monitor, and a
printer. The load includes an X-10 light dimmer and an outside light
fixture with motion detector.
Please don't lecture me on this: it's just a case of the shoemaker's kids
having no shoes! My point is that the minor "commond mode" contributions
of all this crap adds up.
> --
> Joe Leikhim K4SAT
> "The RFI-EMI-GUY"©
>
> "Treason doth never prosper: what's the reason?
> For if it prosper, none dare call it treason."
>
> "Follow The Money" ;-P
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