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'D.C. GFI for Deep Sea Diver Heating During Decompr'
1998\06\19@172003 by Montaigne, Mike

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We are doing a project using a PIC for temperature control at the diver
location and another PIC at the surface for status.  I would like to add
a GFI circuit to the 48VDC / 10 Amps we are sending down to the diver,
such that any leakage to the sea on either line (1ma or so) would give a
fault.  The only way I can think to do it, is amplify the voltage across
a .01 to .1 ohm sense resistor in each line.  Each amplifier would have
its own isolated power.  Take the two voltages, isolated by a optical
coupler, subtract them from each other and amplify the difference to
produce an error signal proportional to the difference in currents
between the two lines.  At first I was just going to use two sensitive
reed magnetic switches with a coil in each line, but my brief
investigation is that I won't be able to get it sensitive enough.  Does
anyone else have any ideas?  Tks in advance.

Mike Montaigne
Atomic Energy of Canada Ltd.
Station 18, Chalk River, Ontario
K0J 1J0, Canada

Phone (613) 584-3311 Ex. 4005
Fax:     (613) 584-4040
email:   spam_OUTmontaignemTakeThisOuTspamaecl.ca


{Quote hidden}

1998\06\19@195914 by Dwayne Reid

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>We are doing a project using a PIC for temperature control at the diver
>location and another PIC at the surface for status.  I would like to add
>a GFI circuit to the 48VDC / 10 Amps we are sending down to the diver,
>such that any leakage to the sea on either line (1ma or so) would give a
>fault.

<SNIP>

Early GFI circuits used to use TWO toroid coils with the current carrying
conductors run thru the cores.  One coil had a high frequency oscillator
driving it, the other was the pickup coil.  An inbalance in the current
carrying conductors coupled energy from one toroid to the other.  As far as
I know, that system worked down to DC.

I may still have the data books for those chips - I will look.  I *may* even
have an old GFI hased upon that circuit.

dwayne


Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1998\06\19@201351 by Mike Keitz

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On Fri, 19 Jun 1998 16:29:16 -0400 "Montaigne, Mike" <@spam@montaignemKILLspamspamAECL.CA>
writes:

>such that any leakage to the sea on either line (1ma or so) would give
>a
>fault.

I would highly recommend isolating all parts of the 48V system (including
the source) from ground.  Then any single fault to ground is not
hazardous.  Fault detection could be as simple as a DC bridge circuit
that compares the midpoint voltage of the 48V lines to sea ground.  If
any DC current flows in either direction, it's a fault (This circuit
won't detect a fault such as one from the midpoint of a heater to
ground).  Using DC around sea water is asking for corrosion problems, so
a more advanced fault detector would use an AC signal between the 48V
lines and round and measure the impedance.  If a big DC voltage shows up
that's also a fault of course.

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1998\06\19@214507 by Reginald Neale

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Mike Montaigne said:

>We are doing a project using a PIC for temperature control at the diver
>location and another PIC at the surface for status.  I would like to add
>a GFI circuit to the 48VDC / 10 Amps we are sending down to the diver,
>such that any leakage to the sea on either line (1ma or so) would give a
>fault.

I'm interested - when you identify a fault, what are you going to DO about
it? Is there a parallel backup cable to take over the load? You can't just
cut the guy off, can you?

Sounds like a pretty tough task...

Reg

1998\06\20@012212 by Chris Eddy

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Cool ap, Mike.

I suggest looking at the Hall effect app notes from Allegro.  I think you
should make a hall sensor to do the GFI.  Picture a split toroid core, with
the hall sensor inbetween (probably the analog out version).  Wind the 48VDC
source and return around the core in OPPOSITE directions.  The flux will
then cancel itself out at any current.  You will also achieve galvanic
isolation.  Trim up the analog output, add a zero offset (it will not be
zeroed on initial assembly).  Make sure you add some sort of time delay to
the device, I am sure you might get false trips if you look for any
instantaneous output value.

Montaigne, Mike wrote:

> We are doing a project using a PIC for temperature control at the diver
> location and another PIC at the surface for status.  I would like to add

1998\06\20@231130 by paulb

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Dwayne Reid wrote:

> Early GFI circuits used to use TWO toroid coils with the current
> carrying conductors run thru the cores.  One coil had a high frequency
> oscillator driving it, the other was the pickup coil.  An inbalance in
> the current carrying conductors coupled energy from one toroid to the
> other.

 You omitted to mention (correct me if I«m wrong, but then I know you
will!) that the "send" toroid had the supply lines passing through it in
opposite directions, while the "sense" one had the lines passing through
in the same direction.  Vice versa will work too.

 Because it is externally excited and it is this current being sensed,
it does not matter at *all* whether the supply is DC or AC or none.  It
is sensing shorts at the excitation frequency after all.

 You can either isolate (float) the source or sense imbalance in the
load.  It doesn«t make sense to do both, except by the method mentioned
of a potentiometer (plus trimcaps) across the floating source, trimmed
to null and detecting a change in the known load balance.

 Cheers,
       Paul B.

1998\06\21@210653 by M Walter

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>
>Early GFI circuits used to use TWO toroid coils with the current carrying
>conductors run thru the cores.  One coil had a high frequency oscillator
>driving it, the other was the pickup coil.  An inbalance in the current
>carrying conductors coupled energy from one toroid to the other.  As far as
>I know, that system worked down to DC.
>
>From what I remember the GFI circuits utilize AC only. One of the toroids
is double wound: one winding is the hot lead, the other is the neutral. The
windings are set to cancel out each other: if the same current is flowing
in each line then there is no magnetic field. A third winding on the toroid
senses the inbalance, and generates a signal only if there is a net
maganetic field. So if there is voltage on the sense winding it means that
there is a magnetic field in the toroid produced by an inbalance in the
currents. This current inbalance is due to differances in the hot and
neutral line currents caused by a ground fault.

The second toroid is there to detect a I believe a neutral to ground short.

Look into a old Raytheon data book; they make a number of GFCI chips and
have a nice write up. I believe that a different method will have to be
used in the diver suit GFCI since it is DC.

1998\06\21@210653 by M Walter

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>
>Early GFI circuits used to use TWO toroid coils with the current carrying
>conductors run thru the cores.  One coil had a high frequency oscillator
>driving it, the other was the pickup coil.  An inbalance in the current
>carrying conductors coupled energy from one toroid to the other.  As far as
>I know, that system worked down to DC.
>
>From what I remember the GFI circuits utilize AC only. One of the toroids
is double wound: one winding is the hot lead, the other is the neutral. The
windings are set to cancel out each other: if the same current is flowing
in each line then there is no magnetic field. A third winding on the toroid
senses the inbalance, and generates a signal only if there is a net
maganetic field. So if there is voltage on the sense winding it means that
there is a magnetic field in the toroid produced by an inbalance in the
currents. This current inbalance is due to differances in the hot and
neutral line currents caused by a ground fault.

The second toroid is there to detect a I believe a neutral to ground short.

Look into a old Raytheon data book; they make a number of GFCI chips and
have a nice write up. I believe that a different method will have to be
used in the diver suit GFCI since it is DC.

1998\06\21@233831 by Mark Willis

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M Walter wrote:
{Quote hidden}

 Possibility:  Does anything stop you from using 48VDC through a couple
chokes & then addding in an AC Bias, so you can detect the GFI leakage
of the AC component only?  Obviously, if you have a 1 VAC Bias voltage,
you'll only have 1/48 of the leakage be AC.  Still possible, though.

 Or could you PWM the 48 VDC, possibly?

 Mark

1998\06\22@012914 by Mike Keitz

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>> >From what I remember the GFI circuits utilize AC only. One of the
>toroids
>> is double wound: one winding is the hot lead, the other is the
>neutral. The
>> windings are set to cancel out each other: if the same current is
>flowing
>> in each line then there is no magnetic field. A third winding on the
>toroid
>> senses the inbalance,

>> The second toroid is there to detect a I believe a neutral to ground
>short.

I think that is how it works.  Simple GFI's just have one coil, which
detects any line-frequency common-mode current on the line and neutral
wires.  They can't tell if the neutral is shorted to ground unless a
portion of the load current ends up flowing through the short (which it
generally will, unless the load current is zero).

A more robust version would apply a test signal to the neutral wire and
ensure that all of the test current (if any) returns via the line wire.
If it applied the test signal to the line wire as well, then the detector
coil could detect leakage from either wire as a test-frequency signal.
These would work for AC or DC lines (as long as the transformers don't
saturate from DC), in fact they would work with no line voltage at all.
Both line wires need to have a low impedance to ground for the test
frequency.  Loads containing capacitors from the lines to ground could
cause a false fault detection if the test frequency were high enough.


>  Possibility:  Does anything stop you from using 48VDC through a
>couple
>chokes & then addding in an AC Bias, so you can detect the GFI leakage
>of the AC component only?

Unlike utility current, the 48V source doesn't need to be connected to
ground.  It should be loosely connected so that it can't rise high above
ground due to static, etc. and so ground faults could be detected.  If it
is isolated then the first ground fault isn't hazardous, it merely
defines what will be 48V "ground" today.



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1998\06\22@234937 by Dwayne Reid

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>
>>
>>Early GFI circuits used to use TWO toroid coils with the current carrying
>>conductors run thru the cores.  One coil had a high frequency oscillator
>>driving it, the other was the pickup coil.  An inbalance in the current
>>carrying conductors coupled energy from one toroid to the other.  As far as
>>I know, that system worked down to DC.
>>
>>From what I remember the GFI circuits utilize AC only.

Yep - you are correct.  I looked in my old nat semi data books last night -
they show 2 toroids with each having both conductors run thru the core: one
toroid is at 1000 turns and used to sense differential current, the other
toroid is at 200 turns and is used as part of an oscillator circuit to
detect a grounded neutral.  Since the differential current sensor is by
nature AC coupled, it won't work at DC.  My appologies for my inaccuracy in
the original message.

My best guess is to use something suggested by someone else on this list:
use a large hi-mu toroid core with a gap and install an analog output hall
effect sensor in the gap.  Wind as many turns of your load wires as will fit
on the core.  Subtract the offset on the hall efect sensor with no
differential current flowing.

You could also try a compass module as a high sensitivity magnetic detector.
You would have to mount it in a small heavy-wall steel box to isolate it
from the earch's magnetic feild.

dwayne


Dwayne Reid   <KILLspamdwaynerKILLspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1998\06\23@031639 by Barry Cooper

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At 09:46 PM 22.6.98 -0600, you wrote:
{Quote hidden}

 This seems a little complex to me. I would sample the voltage off both
pos and neg to earth and if the voltage is not equal (or roughly so) you
have an earth or partial earth. You would have to determine how much
leakage is acceptable. If you are in saltwater any leaky insulation will
show up real quick!

       Barry

1998\06\23@222355 by M Walter

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At 03:16 AM 06/23/98 -0400, you wrote:

>  This seems a little complex to me. I would sample the voltage off both
>pos and neg to earth and if the voltage is not equal (or roughly so) you
>have an earth or partial earth. You would have to determine how much
>leakage is acceptable. If you are in saltwater any leaky insulation will
>show up real quick!
>
>        Barry
Barry: I think you have a much more fundimental(?) problem, and that is the
idea of leakage to ground period. The reason the AC line voltage GFCI's
were developed is because one of the the current carrying path is the
ground. The AC system ties the return lead to ground. As a result, if a
grounded person should touch a conductor at line potential he/she will
receive a shock shince their body is across the line potential.  Some
(small) amount of current flows out of the line thru the person and then to
ground. This is refered to as a ground fault, and is the whole reason for
GFCI circuits. The GFCI measures the current goint from one conductor
(line) to the load and compares it to the current comming back from the
load into the neutral line.

If a system did not have a grounded line then there would be no problem
with a person (grounded) souching the line. This is one technique used to
prevent shock hazards: isolate the system from ground by using a
transformer to remove the ground connection. Of course a person still
cannot grab one conductor in one hand and touch the other. Your DC system
seems to fit into this category: there is no connection to ground (or water
in this case) so a GFCI won't work.


Mark Walter

1998\06\23@222355 by M Walter

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At 03:16 AM 06/23/98 -0400, you wrote:

>  This seems a little complex to me. I would sample the voltage off both
>pos and neg to earth and if the voltage is not equal (or roughly so) you
>have an earth or partial earth. You would have to determine how much
>leakage is acceptable. If you are in saltwater any leaky insulation will
>show up real quick!
>
>        Barry
Barry: I think you have a much more fundimental(?) problem, and that is the
idea of leakage to ground period. The reason the AC line voltage GFCI's
were developed is because one of the the current carrying path is the
ground. The AC system ties the return lead to ground. As a result, if a
grounded person should touch a conductor at line potential he/she will
receive a shock shince their body is across the line potential.  Some
(small) amount of current flows out of the line thru the person and then to
ground. This is refered to as a ground fault, and is the whole reason for
GFCI circuits. The GFCI measures the current goint from one conductor
(line) to the load and compares it to the current comming back from the
load into the neutral line.

If a system did not have a grounded line then there would be no problem
with a person (grounded) souching the line. This is one technique used to
prevent shock hazards: isolate the system from ground by using a
transformer to remove the ground connection. Of course a person still
cannot grab one conductor in one hand and touch the other. Your DC system
seems to fit into this category: there is no connection to ground (or water
in this case) so a GFCI won't work.


Mark Walter

1998\06\24@012144 by Barry Cooper

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At 06:43 PM 23.6.95 -0400, you wrote:
{Quote hidden}

 In the systems I work with (navy shipboard electrical) we use what are
called above ground systems. The frames of all the metal electrical
equipment are bonded to a common ground. NONE of the current conducters are
connected to ground. The power system isolation is tested by connecting
lights to ground. If one of the lights goes out or is dimmer to the 2 on
the other phases you have a ground fault. (in the Canadian navy called an
earth) You then have to track it down before another phases goes to ground.
(usually accompanied by a loud bang and the smell of smoke.(g)

>If a system did not have a grounded line then there would be no problem
>with a person (grounded) souching the line. This is one technique used to
>prevent shock hazards: isolate the system from ground by using a
>transformer to remove the ground connection. Of course a person still
>cannot grab one conductor in one hand and touch the other. Your DC system
>seems to fit into this category: there is no connection to ground (or water
>in this case) so a GFCI won't work.
>

 If you measure the voltage on both legs to ground at the same time you
will find they each drop half the line voltage. If one side goes to ground
the potential between it and ground disappears. Ergo you have a ground
fault. (works on 24 volt should work on 48 volt).

 BTW, most heavy industrial electrical systems, at least in Canada,
operate with an above ground system. This allows you to keep operating the
equipment even with a fault. You can track the fault and repair it without
most of the accompanying smoke and fire. (I love the smell of electrical
varnish in the morning). It does leave you with a potential problem. Your
entire plant is now at line potential to the other 2 phases, which is why
residential systems have one grounded leg. You KNOW which on is at ground
potential and which one is live. It won't booby trap you.

       Barry

1998\06\24@175205 by Reginald Neale

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Barry Cooper said: (much skipped)

which is why
>residential systems have one grounded leg. You KNOW which on is at ground
>potential and which one is live. It won't booby trap you.
>

Good point. People are talking here about systems that are "referenced to
earth ground" or "isolated from earth ground" as if those were absolutes.
Given that it takes only a few milliamperes of current to cause a fatal
shock, it should be recognized that power sources are ALWAYS referenced to
earth ground, even if it's not intentional and not significant from the
standpoint of power usage. The best you could do, if the impedance of the
system was balanced, would be to have each conductor at half the system
potential with respect to earth ground. But that says nothing about what
the impedance to earth ground from each of those points might be, and for
almost any imaginable real-world hardware, it's probably quite low. Let's
say your 48VDC system is balanced about earth ground; the max voltage will
be 24VDC. Is the impedance from either conductor to earth low enough to
pump several milliamperes into a diver? A diver wearing a wet-suit already
has a full-body connection to earth ground. So the current will be limited
mostly by voltage and the afore-mentioned impedance.

Maybe one of the new 8-pin PICs with A/D could be the basis of an
inexpensive GFI unit?

Reg Neale

1998\06\24@204526 by Sean Breheny

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On Fri, 23 Jun 1995, M Walter wrote:

{Quote hidden}

I have another one of my "maybe there is something that I don't see here,
BUT" questions <G>

If a person standing on land (forget the diver's situation for a minute)
with rubber (or synthetic) soled shoes on touches a wire which is at 120V
relative to ground AND TOUCHES NOTHING  ELSE, shouldn't the shoes
insulate them so that only a negligible amount of current would flow?
(unless there was some sweat or other solution bridging the sole of the shoe)

The real problem seems to me to be the situation where someone is also
touching the case or some other grounded object at the same time as they
touch the hot wire. I realize that this situation is quite prevalent.

Am I wrong so far?

Another quick question: what is the resistivity of typical soil? I never
bothered to really figure it out or ask anyone before, but how well do
ground rods really allow a ground based current path? For example, If I
put a 6foot rod into the ground at one spot and another some distance
away and measure the resistance between them, what range could I expect?
Where I live, the ground is very rocky, so I have had trouble
actually trying anything like this :)

Thanks in advance for anyone's answer and putting up with this basic
question!

Sean


>
>
> Mark Walter
>

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