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[SOLVED] possible issues with two circuits and two grounds

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d123

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Hi,

This is a question about hypothetically measuring a cable for continuity that is too long for a DMMs test leads, maybe 3m long or 1km long, whatever you prefer.

Regarding a long-distance continuity tester/function that can operate with a cabled signal over one wire but with no return cable between Tx board and Rx board (I know/think such devices don't exist), why can't they work if both circuit halves (send end and receive end) use the same voltage batteries and voltages, the same parts, and have a near-identical layout on both boards, especially the power and return paths?

Wouldn't 5V look like 5V and 0V like 0V to both boards, so the signal would arrive from circuit 1 to circuit 2 looking and feeling like ~5V to circuit 2 input pin?

Just wanted to check what the reasons for not doing this are:

two circuits two different grounds.jpg

Searched a few times for online information but nothing useful good has come up, I wanted to know the contras of such a set-up.

I know grounds need to be connected together between different boards, and have measured horribly wrong signals on and between two boards where they weren't, but I just can't see why identical, isolated circuits would not function correctly in this respect.

Any explanation affirming or refuting the reliability for this two PCBs and two grounds block diagram as in the image, much appreciated.
 

KlausST

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Hi,

as you already said: Its just hypotetical and so far away from being realistic that it makes no sense to spend much time on thinking about it. (My personal opinion)

Reviewing your informations: "identical isolated circuits", but they by far are not identical. One is ai rather low ohmic driver, where the other is a high ohmic receiver.

Or did I mis-interpret something?

Klaus
 
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wwfeldman

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electrical potential, or voltage, is measured relative to a specified reference, as you know

the 5V on the left is 5V because you measure it relative to the reference on the left
the 5V on the right is 5V because you measure it relative to the reference on the right

taking the 5V from the left and connecting it to the right is meaningless unless you also bring the reference with it

think about it in terms of current - all of the current from the left that goes to the right must go back to the left to have a closed loop
connecting the returns together through the earth "may" work, but it would be like adding a large resistor in the return:
very minimal current flows, and you drop all of the voltage across the "ground"

if it rains, the impedance of the ground changes.

presumably the cable has two wires. use one for your "signal" and the other for "return"

one way to check a long cable for conductivity is to treat it like a flashlight.
battery at one end, with one side to wire, other side to bulb and from bulb back to battery
at the far away end, connect the two wires of your cable
 
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betwixt

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They are certainly different but that doesn't really matter anyway. The critical points are:
1. the ground potential difference must not reverse the polarity at the receiver end or it will damage the IC.
2. the ground potential must be less than that needed to change the logic level perceived by the receiver end.
3. in the case of a break or short in the cable, it must cause a change in logic level at the receive end.

The last point is important because a break may leave the CMOS input floating and that could create a false report.

I think I would tackle this by sending a low frequency AC down the cable so it can be safely DC isolated at both ends. Then detect how much signal arrives at the receiver. A break or short would both drop the signal level though so as a fault indicator it would work but it may not be able to tell what caused it. Ideally a TDR would be used but it makes it far more complicated. There are several other methods with varying complexity. I assume there is some continuity between grounds even if it isn't perfect.

Brian.
 
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d123

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Hi,

Okay, thanks. I asked as it's related to a real-world situation, and this article: Continuity: So Easy to Check, Except When It's Not discusses the same issue, for example in a house with single wires crossing the house (or even all are the same colour, now I think of it) that a person needs to know which is which.

In my case, I'd have liked to turn off the AC main circuit breaker to disconnect an acquaintance's house from the AC mains and - after checking to see no voltage is present with a DMM - continuity test a couple of cables - one of which is probably buried a few cm deep under concrete and is maybe about 15m long - to see:
a) if a specific one starts and ends where I think it does, and
b) if there is a short between the two or three wires in the buried cable as someone may have split more than the outer sheath when they accidentally hit it with a chisel (no-one was hurt or anything).
I know where the phase and neutral finish, in a circuit breaker in a house, presumably the same circuit breaker that tripped when the cable was hit, but after a visual inspection I have no idea which of a few same-colored cables in a spaghetti-like junction box is the other end of that probably damaged cable. There's a fault and a light, a plug and something or other else don't work (since then I would assume).

Anyway, if it just can't work due to wfeldman's explanation, then that's that. Really, I already knew, it's just annoying me that it isn't/doesn't appear to be possible and it would be ever so useful occasionally. Especially a bidirectional one (say an h-bridge push-pull or a sinewave) that with continuity e.g. lit an LED at each end, save walking a lot when guessing which cable is which in an unpowered state or having a long ground cable/adapting COM cable to make it as long as needed.

One is a rather low ohmic driver, where the other is a high ohmic receiver.

Interesting, I hadn't actually thought about that aspect, thanks. I guess the output and input impedance could be matched (?).

- - - Updated - - -

Hi,

electrical potential, or voltage, is measured relative to a specified reference, as you know

the 5V on the left is 5V because you measure it relative to the reference on the left
the 5V on the right is 5V because you measure it relative to the reference on the right

taking the 5V from the left and connecting it to the right is meaningless unless you also bring the reference with it

Thank you very much. It's been annoying me all afternoon, I understand it but couldn't put the concept into words.

- - - Updated - - -

Hi,

They are certainly different but that doesn't really matter anyway. The critical points are:
1. the ground potential difference must not reverse the polarity at the receiver end or it will damage the IC.
2. the ground potential must be less than that needed to change the logic level perceived by the receiver end.
3. in the case of a break or short in the cable, it must cause a change in logic level at the receive end.

The last point is important because a break may leave the CMOS input floating and that could create a false report.

I think I would tackle this by sending a low frequency AC down the cable so it can be safely DC isolated at both ends.

That's great, thanks. So it could be done with care, then, super. You make some interesting points I hadn't considered, either. The (infrequent) joy of electronics: turning an idea that seems impossible into a functional reality.
 

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I asked as it's related to a real-world situation
I see the problem, that post #1 doesn't describe the "real-world situation" completely and is probably asking the wrong questions.

I presume, we are talking about DC (or low frequency AC) circuits, not electromagnetic or electrostatic problems. In this case, the two grounds are either connected, with some amount of wiring resistance or not connected. If you consider the possibility of a foreign voltage source generating a voltage difference between the grounds, it should be mentioned in the test setup. If the grounds are unconnected, the signal transmission doesn't work and you also can't measure continuity without an additional return path.

- - - Updated - - -

The "real world situation" discussed in post #5 is different and can't be modelled by the post #1 setup appropriately.

There are a few methods to identify unknown cable connections.

DC measurements need a return path as correctly mentioned, they can work if you utilize multiple wires in a bundle or cable.

AC measurements have more options, you have cable capacitance, residual (capacitively coupled) voltage measured at unconnected wires of a live cable, magnetic field of loaded wires.

The telephone guys are using signal injectors connected to a wire pair, allowing to identify the respective wires at the remote end over several cm distance.
 
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betwixt

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I know where the phase and neutral finish, in a circuit breaker in a house, presumably the same circuit breaker that tripped when the cable was hit, but after a visual inspection I have no idea which of a few same-colored cables in a spaghetti-like junction box is the other end of that probably damaged cable. There's a fault and a light, a plug and something or other else don't work
Normal UK wiring would be done with a ring mains connection, implying two live wires connected to each circuit breaker. If one socket wasn't working, it means it is either a spur from the ring or there is only one wire to the CB. That narrows things down but doesn't explain why a light isn't working as lighting circuits should be on their own wiring and CB.

Regardless, the easiest way to trace a fault like that is to apply an audio tone across the Earth and Neutral then probe at the socket with a loudspeaker. It is unlikely all three wires are open circuit so you can repeat with the tone between Earth and Live in case the fault is an open Neutral wire. I strongly recommend turning the main switch off and also turning all the individual circuit breakers off before attempting anything like this. If the circuit breakers are within hearing range, you can also do it the other way around, use a testmeter with a continuity beeper, connect it across L and N then short out each socket with only one breaker closed at a time. Listen for the one that doesn't beep. Tedious but it works.

Brian.
 

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Hi FvM,

Thanks. Okay, fair enough, I'd actually thought that the block diagram ('though simplistic) was representative of actual use so your feedback is appreciated. re no return path, I'll try to refrain from asking "...but why don't fairies and unicorns exist?" style questions again, the unavoidable need for a return path got the better of me yesterday.

Thanks for the alternate methods, I've used a cable tracer/pair finder some years ago, such a good tool...when there's a return path. It's a shame ground signals can't be transmitted wirelessly, but vice versa could work I suppose (using wire under test as ground/return and transmitting via wi-fi or similar). The cable capacitance sounds interesting. I calculated what voltage could be obtained from injecting a small current into a 1km cable to estimate cable length and saw that that approach won't work, the generated voltage would be in the uV range for even 1km.

Thanks a lot!

- - - Updated - - -

Hi Brian,

I imagine textbook/regulatory standards wiring around the world is not always what is found in the field, unfortunately. Thanks for all the methods of checking, didn't know those, they are useful. Don't worry, I'll be petitioning the nearest substation to shut down if I try to check, not just turning off the MCB, I'm a coward - we tend to live a little longer supposedly.

I'm thinking of taking the "analog" ignoramus' way out and connecting a cable (with phase and neutral, if we can get hold of one long enough) to the circuit breaker in question and wiring its other ends where the other wire dies, if someone can tell me with certainty which it is in the multi-coloured spaghetti junction box. If everything works again, then it was that cable; if it makes no difference, then it's time for an electrician.

I've learnt quite a few interesting and useful things I hadn't even thought of from the embarrassing and stupid question ("There's no such thing as a stupid question, only stupid people who ask them." - that's how the saying goes, isn't it?), so many thanks to everyone. I'll mark this as solved as you have all given me quite a lot to absorb and the question is solved.

Many thanks.
 

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