Ground Fault

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Ground Fault

Electric Vehicle Discussion List mailing list
I seem to have a ground fault in my truck.
I was working on the BMS, and when my finger touched one of the busbars, I got a shock. (forearm was leaning on the truck frame.)
To find where it was coming from, I put a 1K resistor in series with my meter lead, tied the other end to ground, and set the meter to read ma.
Touched the other lead to the pack, about 1/3 from one end - about 1.3 ma.
Turned off the main breaker, which splits the pack into 4 (odd sized) sections.
No leakage from the middle 2 sections, but about 0.4ma leakage amounts from either of the 2 "end" sections (about 32v ea). (both still have one end connected to the truck systems.)
So I start disconnecting truck systems.
Found that the leakage seems to be coming from the 2 small DC-DC converters (actually, power supplies) that I use to keep the Zilla powered up, and maintain the 12v battery.  Both of these are UL approved isolated power supplies.

Now what?  I need the DC-DC's, but I really do NOT want a ground fault.

--
Worlds only All Electric F-250 truck! http://john.casadelgato.com/Electric-Vehicles/1995-Ford-F-250
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Re: Ground Fault

Electric Vehicle Discussion List mailing list
On my opinion John,

     If you were able to properly isolate that fault to ground right to
your DC-DC converters, you should fix that (independently how critical
those DCDC are for your Zilla or anything else). On a HV system, safety is
the most important thing to be taken into consideration over anything else.

  With that said, I would recommend you to:
- First: identify if the fault is just in one DC-DC or both, if its one,
probably you could have a defective component inside or just some 'dirt'
terminals, connector, trapped moisture, etc. creating that small current
leak. If both DC-DCs got the fail, don't even attempt to replace them by
same PN. throw them away and get a completely new brand/version with
similar capabilities.
- Usually all OEM EVs have two main contactors, one for +, one for -. So
does BMS; also have the ability to check either the + and the - sides for
isolation faults. This is in order to minimize the risk of an electrocution
to human bodies over all. Check for your BMS for the ability to do both + &
-, if not, You should perform the missing side manually to ensure no more
faults.
- Once you have fixed/replaced your DC-DCs/isolation problem, 'test' your
BMS to confirm it is properly detecting new ground isolation faults and
that has the ability to 'disable' your main contactor(s). Or at least,
notify you there is an isolation problem so you don't attempt to ever touch
any + or - terminal at any time.
- Your BMS must be also able to track isolation issues from your AC onboard
charger while charging, so if should also 'disable' your charger in case
there is a ground fault on it as well.

Marco Gaxiola




On Sun, Nov 10, 2019 at 11:45 AM John Lussmyer via EV <[hidden email]>
wrote:

> I seem to have a ground fault in my truck.
> I was working on the BMS, and when my finger touched one of the busbars, I
> got a shock. (forearm was leaning on the truck frame.)
> To find where it was coming from, I put a 1K resistor in series with my
> meter lead, tied the other end to ground, and set the meter to read ma.
> Touched the other lead to the pack, about 1/3 from one end - about 1.3 ma.
> Turned off the main breaker, which splits the pack into 4 (odd sized)
> sections.
> No leakage from the middle 2 sections, but about 0.4ma leakage amounts
> from either of the 2 "end" sections (about 32v ea). (both still have one
> end connected to the truck systems.)
> So I start disconnecting truck systems.
> Found that the leakage seems to be coming from the 2 small DC-DC
> converters (actually, power supplies) that I use to keep the Zilla powered
> up, and maintain the 12v battery.  Both of these are UL approved isolated
> power supplies.
>
> Now what?  I need the DC-DC's, but I really do NOT want a ground fault.
>
> --
> Worlds only All Electric F-250 truck!
> http://john.casadelgato.com/Electric-Vehicles/1995-Ford-F-250
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
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> INFO: http://lists.evdl.org/listinfo.cgi/ev-evdl.org
> Please discuss EV drag racing at NEDRA (
> http://groups.yahoo.com/group/NEDRA)
>
>
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Re: Ground Fault

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In reply to this post by Electric Vehicle Discussion List mailing list
John,
You do not really have a ground fault, it seems your power supplies have an
input circuit that causes a small current to ground, possibly they have a
MOV or other protective device on their input or a discharge resistor that
takes care of the required removal of voltage from input capacitor when the
power supply is unplugged, so you can't be shocked by the prongs of the
cord after unplugging?

You may need to open on supply to inspect if you can modify it if you need
to remove the leakage.
I have been surprised by the ~380v on the Leaf pack when the BMS is still
plugged in, if you remove BMS first then the cells are isolated and no
electrical stinging happens when working on the cells themselves.
Cor.

On Sun, Nov 10, 2019, 11:45 AM John Lussmyer via EV <[hidden email]>
wrote:

> I seem to have a ground fault in my truck.
> I was working on the BMS, and when my finger touched one of the busbars, I
> got a shock. (forearm was leaning on the truck frame.)
> To find where it was coming from, I put a 1K resistor in series with my
> meter lead, tied the other end to ground, and set the meter to read ma.
> Touched the other lead to the pack, about 1/3 from one end - about 1.3 ma.
> Turned off the main breaker, which splits the pack into 4 (odd sized)
> sections.
> No leakage from the middle 2 sections, but about 0.4ma leakage amounts
> from either of the 2 "end" sections (about 32v ea). (both still have one
> end connected to the truck systems.)
> So I start disconnecting truck systems.
> Found that the leakage seems to be coming from the 2 small DC-DC
> converters (actually, power supplies) that I use to keep the Zilla powered
> up, and maintain the 12v battery.  Both of these are UL approved isolated
> power supplies.
>
> Now what?  I need the DC-DC's, but I really do NOT want a ground fault.
>
> --
> Worlds only All Electric F-250 truck!
> http://john.casadelgato.com/Electric-Vehicles/1995-Ford-F-250
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> ARCHIVE: http://www.evdl.org/archive/index.html
> INFO: http://lists.evdl.org/listinfo.cgi/ev-evdl.org
> Please discuss EV drag racing at NEDRA (
> http://groups.yahoo.com/group/NEDRA)
>
>
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Re: Ground Fault

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On Sun Nov 10 15:05:24 PST 2019 [hidden email] said:
>You do not really have a ground fault, it seems your power supplies have an
>input circuit that causes a small current to ground, possibly they have a
>MOV or other protective device on their input or a discharge resistor that
>takes care of the required removal of voltage from input capacitor when the
>power supply is unplugged, so you can't be shocked by the prongs of the
>cord after unplugging?

In this case, the leakage is from the Pack +/- to the output -12v connection.
The pack circuit is open mid-pack, so there is NO voltage on the supply HV inputs.
When the ma meter is going through a 1K resistor between either side of the pack, and 12v Gnd, I get a solid 0.34 ma that holds steady.  This is from about the 32v point on either the + or - side of the pack.  Current polarity is the opposite between the sides.
So, the only conduction path is through the power supply from either HV input to the -12v side of the output.
If I do NOT open the pack, so the DC-DC are powered, AND check at higher voltage point in the pack, the leakage current goes up significantly.
There is no "ground", as one of these power supplies is a potted plastic box, with no input ground lead, and the other is a metal box that is sitting on a chunk of plastic with no connection from it's case to the metal of the truck.


--

Bobcats and Cougars, oh my!  http://john.casadelgato.com/Pets
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Re: Ground Fault

Electric Vehicle Discussion List mailing list
[hidden email] said:
>> You do not really have a ground fault, it seems your power supplies have an
>> input circuit that causes a small current to ground

John, are these built as DC/DC converters? Or are the AC-input power
supplies that you are using with a DC input?

AC power supplies usually have noise filter caps, and resistors to
discharge these capacitors when unplugged to satisfy UL requirements.
It's common to find capacitors from hot and neutral to ground, and a
high-value resistor or two to ground to discharge them.

If the supply has no ground pin (2-wire cord or wall-wart), then there
will be a capacitor and resistor from the AC input side to the output
side, in the hopes that the output negative side "might" be grounded.
If this is the case, you could remove these capacitors or resistors to
cure your ground fault.

UL requires a considerable amount of insulation between AC and DC sides
(like 3750vac for one minute with less than 1ma leakage). But there are
tons of cheap supplies that simply don't meet the standard. They either
ignore UL, or lie and put a fake UL label on it.

Some small AC switchmode supplies I've taken apart have gone so far as
to wind the primary and secondary of their transformers with ordinary
magnet wire, one directly on top of the other, with no supplemental
insulation at all! That gives them a breakdown voltage barely over AC
line voltage -- no safety margin at all. A voltage spike, age, or
moisture can puncture this insulation, and the supply will probably work
anyway -- but with a path from input to output.

Similarly, some DC/DC converters have only a small amount of isolation
between input and output. It's not required for many applications, where
input and output both share a common ground anyway. Such supplies can be
UL listed with only 500v isolation between input and output (and it's a
1-time, 1-second test; not 1-minute with repeated tests allowed as for AC).

Lee
--
There is a computer disease that anybody who works with computers knows
about. It's very serious, and interferes completely with your work. The
trouble with computers is that you 'play' with them! (Richard Feynman)
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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Re: Ground Fault

Electric Vehicle Discussion List mailing list
On Mon Nov 11 09:47:56 PST 2019 [hidden email] said:
>[hidden email] said:
>>> You do not really have a ground fault, it seems your power supplies have an
>>> input circuit that causes a small current to ground
>
>John, are these built as DC/DC converters? Or are the AC-input power
>supplies that you are using with a DC input?

AC Power supplies

>AC power supplies usually have noise filter caps, and resistors to
>discharge these capacitors when unplugged to satisfy UL requirements.
>It's common to find capacitors from hot and neutral to ground, and a
>high-value resistor or two to ground to discharge them.

One has a ground, which isn't connected, and the case is on a plastic board.

>If the supply has no ground pin (2-wire cord or wall-wart), then there
>will be a capacitor and resistor from the AC input side to the output
>side, in the hopes that the output negative side "might" be grounded.

The other is 2 wire only - but it a potted unit, no way to get inside.

So, it sounds like I need to find a real DC-DC converter that can take 330vdc and make 13.8vdc.
Pretty much the opposite of every unit I've ever seen.


--

Bobcats and Cougars, oh my!  http://john.casadelgato.com/Pets
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Re: Ground Fault

Electric Vehicle Discussion List mailing list
In reply to this post by Electric Vehicle Discussion List mailing list
On Mon Nov 11 09:47:56 PST 2019 [hidden email] said:
>John, are these built as DC/DC converters? Or are the AC-input power
>supplies that you are using with a DC input?

The fun part is that I also have a couple of 600W supplies that put out 50A @13.2 V (a bit low), and these do NOT have any noticeable leakage current.
I just didn't really want to leave one of these running all the time, as their "idle" current draw is significant.


--

Worlds only All Electric F-250 truck! http://john.casadelgato.com/Electric-Vehicles/1995-Ford-F-250

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Re: Ground Fault

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In reply to this post by Electric Vehicle Discussion List mailing list
John Lussmyer via EV wrote:

> On Mon Nov 11 09:47:56 PST 2019 [hidden email] said:
>> [hidden email] said:
>>>> You do not really have a ground fault, it seems your power supplies have an
>>>> input circuit that causes a small current to ground
>>
>> John, are these built as DC/DC converters? Or are the AC-input power
>> supplies that you are using with a DC input?
>
> AC Power supplies
>
>> AC power supplies usually have noise filter caps, and resistors to
>> discharge these capacitors when unplugged to satisfy UL requirements.
>> It's common to find capacitors from hot and neutral to ground, and a
>> high-value resistor or two to ground to discharge them.
>
> One has a ground, which isn't connected, and the case is on a plastic board.
>
>> If the supply has no ground pin (2-wire cord or wall-wart), then there
>> will be a capacitor and resistor from the AC input side to the output
>> side, in the hopes that the output negative side "might" be grounded.
>
> The other is 2 wire only - but it a potted unit, no way to get inside.
>
> So, it sounds like I need to find a real DC-DC converter that can take 330vdc and make 13.8vdc.
> Pretty much the opposite of every unit I've ever seen.

How about a Vicor module? They are US-made, and UL-listed (and actually
tested to comply). The noise filters and discharge resistors are not
inside the unit (which is a potted module). You are expected to add them
externally, if needed for your application.

Vicor modules are expensive new; but widely available at good prices on
the surplus market. I have a bunch; so let me know what you need. :-)

Lee

--
There is a computer disease that anybody who works with computers knows
about. It's very serious, and interferes completely with your work. The
trouble with computers is that you 'play' with them! (Richard Feynman)
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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Re: Ground Fault

Electric Vehicle Discussion List mailing list
In reply to this post by Electric Vehicle Discussion List mailing list
On 11 Nov 2019 at 10:07, John Lussmyer via EV wrote:

> So, it sounds like I need to find a real DC-DC converter that can take 330vdc
> and make 13.8vdc.

Could you use (say) three 120v converters, each connected to part of the
battery, with their outputs in parallel through large diodes?

David Roden - Akron, Ohio, USA
EVDL Administrator

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Re: Ground Fault

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In reply to this post by Electric Vehicle Discussion List mailing list
MeanWell specifies almost all their AC supplies also for DC powering,
typically 130-370vdc for a 90-264vac unit.

I have many supplies for rack mount servers, several are wide input, 12v
output AND have a trim input pin that allows the voltage to be trimmed up
to the overvoltage threshold.
Besides trim input, they also have a standby/on input and a 12v standby
output that delivers a small standby current from a separate little flyback
converter.
Cor.

On Mon, Nov 11, 2019, 10:07 AM John Lussmyer via EV <[hidden email]>
wrote:

> On Mon Nov 11 09:47:56 PST 2019 [hidden email] said:
> >[hidden email] said:
> >>> You do not really have a ground fault, it seems your power supplies
> have an
> >>> input circuit that causes a small current to ground
> >
> >John, are these built as DC/DC converters? Or are the AC-input power
> >supplies that you are using with a DC input?
>
> AC Power supplies
>
> >AC power supplies usually have noise filter caps, and resistors to
> >discharge these capacitors when unplugged to satisfy UL requirements.
> >It's common to find capacitors from hot and neutral to ground, and a
> >high-value resistor or two to ground to discharge them.
>
> One has a ground, which isn't connected, and the case is on a plastic
> board.
>
> >If the supply has no ground pin (2-wire cord or wall-wart), then there
> >will be a capacitor and resistor from the AC input side to the output
> >side, in the hopes that the output negative side "might" be grounded.
>
> The other is 2 wire only - but it a potted unit, no way to get inside.
>
> So, it sounds like I need to find a real DC-DC converter that can take
> 330vdc and make 13.8vdc.
> Pretty much the opposite of every unit I've ever seen.
>
>
> --
>
> Bobcats and Cougars, oh my!  http://john.casadelgato.com/Pets
> _______________________________________________
> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> ARCHIVE: http://www.evdl.org/archive/index.html
> INFO: http://lists.evdl.org/listinfo.cgi/ev-evdl.org
> Please discuss EV drag racing at NEDRA (
> http://groups.yahoo.com/group/NEDRA)
>
>
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