Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-02 Thread Ben Potter 
> From: Gene Heskett [mailto:ghesk...@wdtv.com] 
>>
>> Going really OT here (and I should probably start a new thread for
>> this) - what beyond a 1:1 ratio makes a transformer an 'isolation'
>> transformer.
>>
>No ohmic connection between primary and secondary.  If there is, and its
not just a carbon track from a lightning strike, its an autoformer, and
potentially a killer because it doesn't isolate.  Two pieces of wire are
exactly equal >to a 1/1 autoformer IOW..

Tested with my multimeter - can't measure a resistance between primary and
secondary.
Can't test further without borrowing a Megger.

Probably suitable for what I'm using it for.

Ta
Ben


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Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-02 Thread Gene Heskett 
On Saturday 02 January 2016 05:52:55 Ben Potter wrote:

> > From: Gene Heskett [mailto:ghesk...@wdtv.com]
> >
> >> Going really OT here (and I should probably start a new thread for
> >> this) - what beyond a 1:1 ratio makes a transformer an 'isolation'
> >> transformer.
> >
> >No ohmic connection between primary and secondary.  If there is, and
> > its
>
> not just a carbon track from a lightning strike, its an autoformer,
> and potentially a killer because it doesn't isolate.  Two pieces of
> wire are exactly equal >to a 1/1 autoformer IOW..
>
> Tested with my multimeter - can't measure a resistance between primary
> and secondary.
> Can't test further without borrowing a Megger.
>
> Probably suitable for what I'm using it for.
>
> Ta
> Ben
>
As long as its capacity is up to the job, yes.

Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page 

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Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-01 Thread Bertho Stultiens 
On 01/01/2016 10:28 PM, Bertho Stultiens wrote:
> Short explanation:
> You must ensure that a grounding connection does not carry current and
> is located at such a point where a symmetric coupling can be achieved.
> 
> A bit longer explanation:
[snip]

Just one comment on the explanation...
It is still quite a simplification of the actual electrical AC analysis
of the circuit as a whole. However, such analysis would be far beyond
the scope of this mailing list IMO. The intricacies can be subtle and
complex and sometimes slightly counter-intuitive.

Not to cut of any detailed discussion, but to prevent to go completely
off topic. Otherwise we should start a new thread for that purpose.

-- 
Greetings Bertho

(disclaimers are disclaimed)

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Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-01 Thread Gene Heskett 
On Friday 01 January 2016 18:04:34 Ben Potter wrote:

> > From: Bertho Stultiens [mailto:ber...@vagrearg.org]
> > A bit longer explanation:
> > See attached image. Outputs of the step-down transformer A and B are
>
> capacitively coupled to L1 and L2 through four parasitic capacitors.
>
> > Connecting A to ground when C1A not equal C2A or connecting B to
> > ground
>
> when C1B not equal C2B will create a current in the ground connection.
> 
>
> Thank you for this image - I think I've finally got my head around the
> utter basics of the American electricity supply system. Some of that
> may have been is it single/two phase?
> Having 3 phases and a neutral coming into your premises (aka UK
> 3-phase supply) somehow seems less... subject to complexity
>
> Going really OT here (and I should probably start a new thread for
> this) - what beyond a 1:1 ratio makes a transformer an 'isolation'
> transformer.
>
No ohmic connection between primary and secondary.  If there is, and its 
not just a carbon track from a lightning strike, its an autoformer, and 
potentially a killer because it doesn't isolate.  Two pieces of wire are 
exactly equal to a 1/1 autoformer IOW..

> I have used the below transformer or similar as the primary power
> transformer for machines (in the case of the 2kva one, a ram edm). And
> have considered it to provide sufficient isolation
> http://www.jmsonline.co.uk/product_info.php?cPath=22_id=54
>
> If I'm wrong, please let me know (and ideally how to fix it)
>
> Thanks
> Ben
>
>
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Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page 

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Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-01 Thread Bertho Stultiens 
On 01/01/2016 08:24 PM, Rafael wrote:
>> There are a lot of good reasons to tie one leg the transformer to ground
>> besides to establish the safety ground and neutral as is common on the US.
>> Intermittent faults to ground, with an ungrounded system, can cause the
>> secondary of the transformer to fly way above absolute ground causing
>> connected devices, or the transformer to suffer from insulation
>> breakdowns.   That's the extreme, but it can happen.
> 
> Strongly agree! Do not float transformer secondary and have a PC 
> connected to it! This is not about an isolation transformer, it's about 
> stepdown transformer which is way different. Primary side of this step 
> down transformer needs to float in this case. In ideal situation, CT on 
> the primary side would be neutral and connected to local ground but it 
> is better to not use it; transformer is not ideal.

For good measure; I did not say that you should not ground the PC.
Actually, I noted that it already is, but it is grounded at a
_different_ point in the chain of connections.

There is a reason why the PC /should/ be grounded and that has been
underlined in the replies here. The question that seems to be popping up
is _where_ to have the ground connected.

Short explanation:
You must ensure that a grounding connection does not carry current and
is located at such a point where a symmetric coupling can be achieved.


A bit longer explanation:
See attached image. Outputs of the step-down transformer A and B are
capacitively coupled to L1 and L2 through four parasitic capacitors.
Connecting A to ground when C1A not equal C2A or connecting B to ground
when C1B not equal C2B will create a current in the ground connection.

CxA and CxB form capacitive voltage dividers, which will create a
virtual zero point equal to neutral only if both capacitors are equal.
Otherwise, you will have a potential differential and therefore a
current through the ground connection from the secondary(*).

An RCD measures the currents IL1 and IL2 where it must be ensured that
IL1 + IL2 < threshold, where threshold depends on the particular RCD.
When you cannot guarantee that the ground-current is zero, because the
capacitive couplings are not equal, then you risk tripping the RCD. If
you have no RCD, then you are creating an unbalanced circuit that will
come to haunt you at another time (any imbalance in L1 and L2 will
propagate through the system wreaking havoc).

If you look at the DC side of the system, then you have a symmetric
coupling from 0V (DC) to both A and B through Cf. The coupling stems
from the rectifier system, which couples both A and B through the same
circuit (which is why Cf is drawn twice).

Therefore, connecting the output of the PSU's 0V (DC) line to ground
will carry nearly(**) no current. This will protect the whole circuit
from inadvertent potentials and calm the noise in the system.


The /problem/ with having 0V (DC) at ground are again the rise of the
dreaded ground-loops. If you have multiple PSUs all connected to ground
and connecting the 0V (DC) lines through the signal return connections,
then you have two distinct paths (a loop). That is where you need the
galvanic isolator(s) to break the ground-loop.


(*) JT has done a measurement that show a different voltage from
A-to-ground and B-to-ground. Thereby establishing that the parasitic
capacitances are not equal.

(**) the parasitic capacitances are in series and the differentials are
guaranteed less than 90 degrees shifted. The effective leakage is
normally in the order of micro-amps and a few orders of magnitude away
from tripping an RCD.


>> One of way too many references on the web.
>> http://ecmweb.com/bonding-amp-grounding/basics-bonding-and-grounding-transformers
> I hope that nobody gets zapped before this highly charged and perhaps 
> the long(est) thread is over.

Always have a bleeder resistor over your caps. Running a bleeder
resistor over an email-thread helps too ;-)

And then, coffee will help too.

-- 
Greetings Bertho

(disclaimers are disclaimed)
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Re: [Emc-users] Rewiring the BP - where to connect ground

2016-01-01 Thread Ben Potter 
> From: Bertho Stultiens [mailto:ber...@vagrearg.org] 
> A bit longer explanation:
> See attached image. Outputs of the step-down transformer A and B are
capacitively coupled to L1 and L2 through four parasitic capacitors.
> Connecting A to ground when C1A not equal C2A or connecting B to ground
when C1B not equal C2B will create a current in the ground connection.


Thank you for this image - I think I've finally got my head around the utter
basics of the American electricity supply system. Some of that may have been
is it single/two phase?
Having 3 phases and a neutral coming into your premises (aka UK 3-phase
supply) somehow seems less... subject to complexity

Going really OT here (and I should probably start a new thread for this) -
what beyond a 1:1 ratio makes a transformer an 'isolation' transformer.

I have used the below transformer or similar as the primary power
transformer for machines (in the case of the 2kva one, a ram edm). And have
considered it to provide sufficient isolation
http://www.jmsonline.co.uk/product_info.php?cPath=22_id=54

If I'm wrong, please let me know (and ideally how to fix it)

Thanks
Ben


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