Brother industrial sewing machine repair / reverse engineering (repair success)

[max.wwwang]

Not with an isolation transformer, but I think you can do the same (nearly) with a differential probe. I think Dave sells one, for example, if they are still in stock. With a differential probe each probe is equivalent and neither is grounded. Therefore you can use them to probe two active points in a circuit without risk of upsetting the circuit under test.

Differential or isolated, I think they are the same thing, aren't they? 

But differential does not sound very expensive (maybe it is?), whereas according to tautech, isolated scope is difficult to make and expensive? Why is this?   

[max.wwwang]

Common mode voltage is an offset relative to something, usually ground, that is present on both conductors. For example if a 240V line had a hypothetical 1kV common mode fault, that would mean that neutral was sitting at 1,000V and live at 1,240V.

Thanks for explaining; that's very kind. Now I know what it means.

As I mentioned earlier, neutral is a current carrying conductor. Due to the current passing through it, the resistance of the wire will result in some parts of it being pulled up above ground, possibly by several volts or more. The other issue is that a neutral fault (open circuit) would then result in the cabinet of the appliance floating at mains voltage when no other fault exists. The idea of a protective earth is it is a fallback, no matter what else is going on, any exposed metal should be at earth potential. You're standing on the ground too so you'll be at the same potential and won't get a shock. There is no possible fault condition that is likely to result in protective earth being live.

That makes sense. I need to take time to think about this a bit more. It looks like simple but is actually quite tricky (at least to me). It is probably related to the 'principle' of single fault assumption.

[max.wwwang]

Although a real "neutral" is connected to "ground/protective earth" at some point in the building, due to substantial current flowing in the neutral (white in US) wire between your outlet and the breaker/fuse box where that connection is made, you can have a few volts of AC between the relevant contacts of your outlet when measured with a normal AC voltmeter.

Common-mode definition:  if you have any two voltages A and B, both measured with respect to the same point (for example, the PE contact in your outlet), then by elementary algebra you can define

"D" = difference = differential mode = A - B,   and
"C" = common-mode = (A+B)/2

Thereafter, you can compute A and B from the measured D and C:  A = 2C + D  and  B = 2C - D

Thanks. That's clear and helpful. But I think you mean:
A = C + D/2
B = C - D/2

[IanB]

But differential does not sound very expensive (maybe it is?), whereas according to tautech, isolated scope is difficult to make and expensive? Why is this?   

You will have to decide what "expensive" means for you:

https://www.eevblog.com/product/hvp70/

[max.wwwang]

But differential does not sound very expensive (maybe it is?), whereas according to tautech, isolated scope is difficult to make and expensive? Why is this?   

You will have to decide what "expensive" means for you:

https://www.eevblog.com/product/hvp70/

Thanks. Aha, this reconciles now. It is indeed expensive.  It does not seem like something trivial.

[james_s]

If you keep an eye on ebay you might get lucky, I have two different Tektronix HV differential probes that I don't think I paid more than $100 each for. There are also DIY projects if your needs are basic which for this application I think they are. In concept a differential probe is very simple, the devil is in the details if you want more than a few tens of MHz but something like a 20MHz differential probe is probably something you could build fairly easily.

[max.wwwang]

Due to this discussion, had some read into the earthing systems. It's good learning. But in reading this reference (Earthing System & Protection in Low Voltage Installations), I don't quite understand the difference between the TT system and the TN-C system. Since there will normally be local earthing at the installation, they look the same to me (Figure 12 and Figure 16). In Figure 16, I've added the green dash line for the local bonding between N and PE.

Another observation, the diagram for TN-S (Figure 11) in this reference is different from the diagram for the same TN-S system in this reference ( EARTHING: YOUR QUESTIONS ANSWERED ) ("TN-S", which I think is correct).

The topic of earthing/grounding is actually very complex and interesting.

[max.wwwang]

If you keep an eye on ebay you might get lucky, I have two different Tektronix HV differential probes that I don't think I paid more than $100 each for. There are also DIY projects if your needs are basic which for this application I think they are. In concept a differential probe is very simple, the devil is in the details if you want more than a few tens of MHz but something like a 20MHz differential probe is probably something you could build fairly easily.

That's good to know. Thanks. I never knew this before!

[james_s]

https://hackaday.io/project/169390-a-10x-100mhz-differential-probe

https://resources.altium.com/p/differential-oscilloscope-probe-project

[tautech]

But differential does not sound very expensive (maybe it is?), whereas according to tautech, isolated scope is difficult to make and expensive? Why is this?   

You will have to decide what "expensive" means for you:

https://www.eevblog.com/product/hvp70/

Thanks. Aha, this reconciles now. It is indeed expensive.  It does not seem like something trivial.

It is if you value your life !
We try to keep a few of the infamous DP-25's in stock and can easy whip NZ RS pricing by some margin.
http://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/17072/psn/19265

[tautech]

Due to this discussion, had some read into the earthing systems. It's good learning. But in reading this reference (Earthing System & Protection in Low Voltage Installations), I don't quite understand the difference between the TT system and the TN-C system. Since there will normally be local earthing at the installation, they look the same to me (Figure 12 and Figure 16). In Figure 16, I've added the green dash line for the local bonding between N and PE.

Another observation, the diagram for TN-S (Figure 11) in this reference is different from the diagram for the same TN-S system in this reference ( EARTHING: YOUR QUESTIONS ANSWERED ) ("TN-S", which I think is correct).

The topic of earthing/grounding is actually very complex and interesting.

TT is the NZ system where the PE/N is bonded at the pole/distro transformer with its own PE peg field and again at the customer installation, meter board and if separate the breaker board too.
Typically just one PE peg is used and both at meter and breaker boards the PE and N are tied together.
This image is a good find. 

[TimFox]

Although a real "neutral" is connected to "ground/protective earth" at some point in the building, due to substantial current flowing in the neutral (white in US) wire between your outlet and the breaker/fuse box where that connection is made, you can have a few volts of AC between the relevant contacts of your outlet when measured with a normal AC voltmeter.

Common-mode definition:  if you have any two voltages A and B, both measured with respect to the same point (for example, the PE contact in your outlet), then by elementary algebra you can define

"D" = difference = differential mode = A - B,   and
"C" = common-mode = (A+B)/2

Thereafter, you can compute A and B from the measured D and C:  A = 2C + D  and  B = 2C - D

Thanks. That's clear and helpful. But I think you mean:
A = C + D/2
B = C - D/2

Oops--I stand corrected.  I need more caffeine in my algebra.  I'll add a note to my post.

[max.wwwang]

TT is the NZ system where the PE/N is bonded at the pole/distro transformer with its own PE peg field and again at the customer installation, meter board and if separate the breaker board too.
Typically just one PE peg is used and both at meter and breaker boards the PE and N are tied together.

From this 1993 NZ code of practice (NEW ZEALAND ELECTRICAL CODE OF PRACTICE For POWER SYSTEMS EARTHING) now still available on WorkSafe website (I have not got into the detail of it yet), and a post from what appears to be a NZ hardcore expert (TT Earthing Systems - Interest by New Zealand ), it appears the system in NZ is (or was) MEN (Multiple Earthed Neutral), but was considering a transition to the TT system. From what you are saying, it seems like this transition has been done at certain point?

This image is a good find. 

Glad to hear you like it.    Just keep in mind it's not the original one, it has been adapted by a dumbass (that's me). I added the green dash line. For the original one, you will need to go to the reference above.

[IanB]

I added the green dash line.

If you add the green dash line, it's not TT anymore. Why would you do that?

[max.wwwang]

I added the green dash line.

If you add the green dash line, it's not TT anymore. Why would you do that?

Was not trying to correct an 'error', was only trying to illustrate what I was saying -- if, according to what was said by someone above that almost everywhere there is such a connection, then I couldn't see a difference between the two systems (TT, and TN-C). I've highlighted this above to avoid misleading.

By the way, do you mean in TT system, N is not tied up to PE in the household switch box?

[IanB]

By the way, do you mean in TT system, N is not tied up to PE in the household switch box?

Yes, exactly. That lack of a connection is what characterizes a TT earthing system. If you were to make such a connection it would be against code and it would no longer be a TT earthing system.

You can find a description of all the different arrangements here:

https://en.wikipedia.org/wiki/Earthing_system

[max.wwwang]

The original question (puzzle) is still not answered (explained) satisfactorily (to me at least). That is ---

[Open question 1]
Why turning the leads of the probe around will hide the signal (about 40V peak, when the load is not connected)?

Note:
1) when there is load (a dummy pure resistive load), this phenomenon does not exist, a signal can be detected both ways (though without fluctuation but rather stable +/- about 40V).
2) the circuit under test is fully isolated from the mains and mains PE.

Another question that has come out of the discussion and has also not been satisfactorily answered ---

[Open question 2 --- edit: this has been partly answered]
When an isolation transformer is needed in a repair job, which I understand is usually connected to the equipment under test, can the isolation transformer be used between mains and the scope to achieve the same effect of isolation?

[Answer]
According to the contributors to this discussion ---

This was probably once used by experienced people decades ago. But is definitely dangerous so must be avoided, particularly because of the availability nowadays of isolated scopes and differential probes which make this dangerous trick unnecessary.

The major risk is that, by doing this, when you clip the ground lead of the probe to a point in the circuit of the device under test (DUT) that has a voltage above the earth (say 110V, DC or AC), the metal case of the scope, as well as the ground lead of other probes of the same scope with become live as well (at the same potential). This is not only a threat to your own safety but also to people around you, people who o rescue you (in case the worst case eventuates), none of who would likely be aware of this hazard.

The key is that this does not require an earth fault inside the scope. This risk exists with a scope in fully working order without any fault because of the way scope (that is not an isolated type) works. This is different from using the isolation transformer with the DUT, in which case the metal case of it will not become live unless there is an earth fault inside it (that is, contact of the live wire with the metal case).

Yet another question related to earthing system is ---

[Open question 3 --- edit: this has now been answered satisfactorily]
In the TT system is there a connection at the household end between N and PE as indicated with a green dash line in the figure below?

[Answer]
A big, fat, firm NO. In TT system, there is no such direct connection in the household switch box.

Furthermore, in the five earthing systems described in the IEC 60364 standard, which are TN-C, TN-S, TN-C-S, TT, and IT, only TN-C or TN-C-S systems have a direct connection between N and PE at the consumer end (usually in the household switch box).

[james_s]

[Open question 2]
When an isolation transformer is needed in a repair job, which I understand is usually connected to the equipment under test, can the isolation transformer be used between mains and the scope to achieve the same effect of isolation?

No! Do not ever do this! People have been shocked and even killed this way. it can result in the entire scope becoming live.

[max.wwwang]

[Open question 2]
When an isolation transformer is needed in a repair job, which I understand is usually connected to the equipment under test, can the isolation transformer be used between mains and the scope to achieve the same effect of isolation?

No! Do not ever do this! People have been shocked and even killed this way. it can result in the entire scope becoming live.

Thanks, can you please elaborate, and respond to my questions (or counter argument) in reply #24 above? 

[james_s]

No, sorry, I can't give you an entire EE education, some of this you're going to have to figure out on your own.

[IanB]

Is there a connection at the household end between N and PE as indicated with a green dash line in the figure below?

There may or may not be, it depends on the local electrical code. But in most localities the protective earth and neutral conductors are indeed electrically bonded on entrance to the property. They may, or may not, also be connected to a ground rod.

[tautech]

[Open question 2]
When an isolation transformer is needed in a repair job, which I understand is usually connected to the equipment under test, can the isolation transformer be used between mains and the scope to achieve the same effect of isolation?

No! Do not ever do this! People have been shocked and even killed this way. it can result in the entire scope becoming live.

Certainly such a practice is very dangerous however with the normal isolation transformer with a pass through PE it really achieves little as each probe's reference lead is still bonded to mains earth therefore supplying the DUT via an isolation transformer makes far more sense.

Take that PE away and you have a floated scope, something that was done by experienced users decades ago however today it is universally frowned upon and I imagined outlawed in many businesses.

Today we have differential probes that remove the danger of needing to connect to an elevated reference point for not much coin at all, certainly far less that the cost of a funeral. 

[max.wwwang]

No, sorry, I can't give you an entire EE education, some of this you're going to have to figure out on your own.

Fair enough!    Will do my homework. 

[max.wwwang]

Certainly such a practice is very dangerous however with the normal isolation transformer with a pass through PE it really achieves little as each probe's reference lead is still bonded to mains earth therefore supplying the DUT via an isolation transformer makes far more sense.

Take that PE away and you have a floated scope, something that was done by experienced users decades ago however today it is universally frowned upon and I imagined outlawed in many businesses.

Today we have differential probes that remove the danger of needing to connect to an elevated reference point for not much coin at all, certainly far less that the cost of a funeral. 

I think I get all that you are saying here.

True, there is no point in isolating a scope but still tying its PE separately to the mains earth. As you said, that still puts the ground lead of the probe at the ground level which will likely short circuit whatever is above or below the ground level of DUT.

Aha! Are you saying in the good old days experienced people used to use this trick?! It sounds like this is not entirely a dumb idea then!

Don't get me wrong, I now know the existence of potential probes and isolated scopes. I am also not desperately trying to put my life in danger. I am just fiercely attacking the problem (and in the meantime grilling you guys) and trying to distil the essence out of it for the sake of knowledge and deeper understanding.

I'm not going to walk away satisfactorily with an advice "you should do this not that" but without knowing/understanding WHY.

[Psi]

Putting an isolation transformer on a scope or other test equipment is can be somewhat acceptable if you understand what side effects it creates and why it's so dangerous, and you take adequate precautions , like using test equipment with no exposed metal and only using 1 probe.


The main issue is that if you isolate the scope and then clip the scope ground lead onto 110/220 phase (or any HV source.)
Then the scope case and all metal terminals are now LIVE !!!

And of course if you put CH1 gnd clip onto a HV source then all the other scope probe ground clips are also now LIVE!!

You can see why it's so dangerous and it requires such extreme care, and why its killed many people.