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# Electrical Wiring and Test

## 14 posts in this topic

Hi All, pls help with some questions i have. do refer to attach drawing and i repeat my questions below: 1. Any problem with connection electrically (apart from funny wiring connection)? 2. If the 24VAC (+) from transformer 1 and 24VAC (+) from transformer 2 is jumpered, will there be a problem? If so, what? Doesn’t look like there’s problem with like short circuit or signalling problem for me though 3. What would be the voltage between the 24VAC(+) of the 2 transformers? I think ~0VAC, correct me if I am wrong. 4. What would the voltage reading be if using a multimter set at VAC between 24VAC (+) and the analog output (when signal is 10VDC out). Please help out with calculation if possible. 5. What would the voltage reading be if using a multimeter set at VAC between 24VAC (+) and the analog output (when signal is 0 VDC out). Note AC and DC measured with AC setting 6. What would happen if accidental shorting of the 24VAC (+) to the analog outout signal terminal? 7. What would happen if accidental shorting of the 24VAC (N) to the analog outout signal terminal? I think the analog output terminal will blow or internal card fuse blow, correct me if I am wrong. 8. What is the voltage reading between 230VAC (L) and 24VAC (L)? Am sure it is 206VAC but my fundamentals need reassuring!! Thank you in advance. Visio-Drawing1.pdf

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Seems to me this looks an awful lot like homework. IF that is the case just say so AND tell us what you think and WHY. Even if wrong it will still be a good learning experience. Dan Bentler

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hi dan, trust me, it isn't a homework. it's something i encountered at work and need some consultation before i really go touch anything. anyway, as u can see, in my post, i hv put in some of my thoughts but some i really have no idea. measured some silly voltage previously but couldn't make sense of it. tracing the live wires were a bit of a hassle. i now hv no access to the site but its eating at me. something i jsut need to know for future reference. thanks.

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first of all the 24volt power supplies do not appear to be AC outputs they are 24VDC as it states that they are halfwave rectified. There should be no problem paralleling the power supplies All voltage meaurements should be made with reference to 0Volts or ground Switch your multimeter to a suitable DC range NOT AC you should read something like 24v x 1.41 for a halfwave rectified output Do not short analogue to power supply you will probably damage the analogue card. Why you need to know the voltage difference between the 230VAC and the 24VDC power supply output is beyond me I Hope this helps Steve

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thanks for the reply steve!! sorry, my mistake. the transformer is not halfwave rectified (forget that statement in the drawing). the controller which the analog output (both cards) resides is the one internally halfwave rectified. I would think still ok to parallel the power supply isn't it? thus my using AC setting to test the circuit. the wiring in the cabinet was not so tidy and difficult to trace the cables. i got funny readings (cannot really recall what but something like ¬50 VAC and 120VAC) when testing some points. I was suspecting my measuring between 230VAC, 24VAC or the 10VDC points. Thus my questions like expected voltage readings between: 1. 230VAC (L) and 24 VAC (L) 2. 24VAC and 10VDC (using Vac setting on multimeter) thanks again.

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Again, what is happening on the other side? You're not paying attention to your neutrals, and phase relationships matter as well. What's the difference between 230 VAC on one phase leg and another? Hint...it's not 460 VAC and it's not 0 VAC. This isn't specious...this is the kind of question you are asking. Draw out the WHOLE circuit, including neutral references. When you approach a cabinet where the wiring has been turned into a food item (spaghetti), you need to approach it with a pencil and paper and carefully reverse engineer a schematic. Simple voltage measurements can sometimes help but they are not the answer to everything. If the wiring is really bad or strung out across multiple panels, purchase a good quality, relatively high frequency circuit tracer and know how to use it. The cheap telco (<\$50) tracers work OK over short distances but not with multiconductor wire bundles or longer distances. Good grounds are essential when tracing too, not just to maintain high power quality. I also highly recommend you get a copy of the IEEE red book (IEEE 141, recommended practice for power distribution) from somewhere which is the general overview. If you can afford it, get the Emerald book (IEEE 1100, electronic circuit power), Grey book (IEEE 241, commercial power distribution), and the Green book (IEEE 142, grounding). If you have an unlimited budget, I think you can get the entire colored books series (around 12-15 documents covering all aspects of power distribution) for around \$1500 USD on CD-ROM. The cost is extremely high but over the years I've found that doing power distribution correctly makes all kinds of other maintenance problems disappear.

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Just looked at your circuit and two things jump out at me. On the bottom you show an input device?? It appears that you are intending to power this and the other device on the left off the same transformer. You are deriving your neutral reference on the 24 VAC transformer from the upstream system ground. I don't see any references to PE (potential earth) ground beyond this point so everything else must be ungrounded (or maybe, SHOULD BE!). When you are using DC voltages are signals, proper grounding is essential because the high impedance inputs are highly subject to induced signals. Ground all shields on ONE END ONLY. Preferably you have ONE common ground reference separate from your power circuits to avoid inducing anything via a ground loop....so you should strive towards referencing everything back to a single common ground reference. As drawn we have a problem with this right away on the input transformer side. I would have made this a real goal right away. Second, I'm guessing that the device shown on the bottom is a 3-wire input device?? That's what it looks like, that the input power neutral reference and signal commons are the same?? If not, then the wiring is wrong and you need commons connected to commons, and power neutral connected to power neutral. Do NOT mix them...that goes back to my first paragraph. Third, you are powering it from the transformer on the left yet supplying a neutral reference from nowhere...not good. This circuit screams ground loop. Fourth, you are assuming that the signal common and neutral are the SAME across the output device on the right. Again, not good practice. If the input device on the bottom is truly a 3-wire device, the correct way to wire it is that your power neutral and signal commons are connected together. Just treat it as a 4-wire device but the signal common and neutrals are the same. I highly doubt that it is 3-wire if it's a 0-10VDC system though because this arrangement is rather unusual. It is common to see 3-wire devices on 4-20mA loops because the inputs are low impedance and everything is DC. Since this is mixed AC and DC, I'm not used to seeing this strange wiring combination. I have no doubts that it will work in some cases, but it's not right.

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This may be a stupid question.... but why don't you just move the 24VAC power wire from transformer #1 to transformer #2 and then get rid of everything on the left? If someone ever connected one of those transformers to a different phase then they are on now you would be screwed really really bad. It's weird that they are using a 0-10 volt DC output signal tied in with a AC neutral for a common... usually stuff like that has a power input and a seperate signal common. Anyway, I would follow Paul's advice and make sure everything is grounded correctly or you're going to have all kinds of weird floating voltages and if you need any kind of precision with your 0-10 volt output you won't get it with floating voltages!

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Thanks for the information.

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The shield connections on the signal side are not shown in my drawing but they are right ie terminated on one side only to ground. Internally the card 24VAC ground and the signal commons are common (tested for continuity before) so everything on the card (commons) should be ground referenced as well right? Yes, the output component is a 3 wire device with 0-10VDC signal and 24VAC powered. My neutral is referenced to ground based on my above statement, isn't it? Correct me if I am wrong.

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What you said is what I thought should be done but not done due to some constraints.

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Perhaps instead of us trying to second guess the problem you could post some part or component numbers, so we can do some research for ourselves and supply a more definitive answer to your dilemma. Steve