How do I connect PTs and CTs for 400A 3ph service

Usually the utility does this but this time it is for information metering an we will do it. It will be a type A 2 1/2 element meter with CTs and PTs.
Seems to me I remember that the "dot" on the CT doughnuts goes toward the supply then X1 X2 to the meter current coils. I am looking for information on how to wire this stuff up. Can anyone help me out?
To send me e-mail remove the sevens from my address.
Chrisd
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Unfortunately every CT manufacture does it different. So connect everything through a shorting block. With 2 wires from each CT to the shorting block. Place the shorting block in an hot accessible area and install everything. Remember to fuse the wiring to the meter to local codes.
If you screwed up and got the wiring backwards, no harm just install the shorting screws and reverse the wiring to the meter. Your done.
If you do it the cheap way, with a common for all the CT's then you are faced with a shut down and reinstalling the wiring. The shorting block is 20-30 bucks and is a life saver. Your aware that you can not install CT's on a powered circuit with out them being shorted,,,,,,, you knew that right?
If this meter is for utility accuracy make sure that the CT's and PT's are utility grade IE less than 3-5% accuracy. 5% is the limit for utility grade out here in the west, your meter will add 1-3% as well. CT's used for tripping circuit breakers can be as high as 10-15%. You do not mention the voltage, over a 1000 volt you need a different grade of CT's and PT's. The clearances can be interesting above a 1000 volts. Most installations I have done below 1000 volts do not use PT's. Be safe and be careful.
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remove the urine to answer

on
grade
grade
Most
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|
[...]
|> Unfortunately every CT manufacture does it different. So connect | everything |> through a shorting block. With 2 wires from each CT to the shorting block. |> Place the shorting block in an hot accessible area and install everything. |> Remember to fuse the wiring to the meter to local codes. | ----------------------- | Fusing the wiring from a CT to the meter???? | If the fuse went - then you have an open secondary on the CT- not good. | Omit the fuse- safer that way. The primary circuit protection is what will | limit the CT current- safely. | Fusing the secondary of a PT is OK and normal practice..
Heh. Would probably arc inside the fuses unless the fuses are rated for many kV (doubt anyone would put that kind of money on such fuses).
Hopefully the previous poster _meant_ in "fuse the wiring to the meter to local codes" that "meter" would refer to the whole setup for metering the power, and the fuses would be the ones handling the load current. If you're paranoid about them making this mistake, ask to confirm the meaning.
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| If you do it the cheap way, with a common for all the CT's then you are | faced with a shut down and reinstalling the wiring. The shorting block is | 20-30 bucks and is a life saver. Your aware that you can not install CT's on | a powered circuit with out them being shorted,,,,,,, you knew that right?
And once you have the CTs installed and shorted, how do you go about using them? Do you now un-short them? While the power is still up?
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wrote:

If you have to ask, then you better get someone knowledgeable to do this for you. CTs produce lethal voltage across an open secondary when current flows through the primary side. The shorting bar prevents this until the meter is connected.
Ben Miller
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
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| wrote: |> And once you have the CTs installed and shorted, how do you go about using |> them? Do you now un-short them? While the power is still up? | | If you have to ask, then you better get someone knowledgeable to do this for | you. CTs produce lethal voltage across an open secondary when current flows | through the primary side. The shorting bar prevents this until the meter is | connected.
Well, that's two people that misunderstood my question. So I guess it's my fault for wording it badly. So I will try it from another angle:
Why is it safe to unshort the secondary once the meter is attached?
And: what is the voltage?
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snipped-for-privacy@ipal.net wrote:

The question you ask makes it clear you do not understand how to use CT's. That is not said as an insult, but rather to underscore what other's have told you: you need to get someone who knows CT usage to help you out. I'll try to shed some light on it - but *please* don't mess with these unless you know before hand exactly what will happen.
Think of the whole circuit, in "electronic" detail:
CT=======Meter is, electronically, the secondary of a transformer with a resistance connected across it, so it can be redrawn as CT=====R
The CT is designed to achieve a specific current (NOT a specific voltage as in a "regular" transformer) in the secondary when a specific current flows in the primary. This concept sometimes causes those learning about CT's some difficulty, but it is critical to understand it: a current will exist in the secondary with or without a load connected to the secondary.
Now, that current transformer will produce some specific current when a specific current flows in the primary. Lets use the 2.5 amps as the current produced. What value of R is required to keep the voltage safe? Assume whatever voltages you want, within reason, and work ohms law to get a feel for how the voltage changes depending on the resistance. Say, for example, the resistance is 1 ohm. The voltage therefore has to be 2.5 volts. Now assume that the resistance is 11 megohm, as it would be if you wrongly set the meter (DMM) to the voltage scale. Remember, E = IR, so E would equal 2.5 amps times 11,000,000 ohms!
If you use the meter properly, it would be set on the amps scale, where a low resistance shunt is connected in the circuit. That shunt is why it is safe to remove the shorting bar with the meter properly connected.
There is another circuit to consider:
CT====BurdenResistor====meter.
In the above case, the burden resistor keeps the voltage low, and the meter can be, and is, used on the voltage scale.
Bottom line - you never want a current transformer on an energized circuit without either a proper load or a shorting bar connected across the secondary. An open, or improperly loaded secondary circuit on a CT is a definite "NO-NO".

The output of a CT is a specific value of current for a specific input current - the secondary CURRENT is proportional to the primary CURRENT. The primary voltage is fixed, and the primary current varies. The secondary current varies when the primary current varies. The secondary voltage depends on the secondary current AND the resistance connected across that secondary.
See, that's the thing. When using CT's *you* need to select the voltage. You either make it low enough where you don't care about it at all (with a shorting bar), or you select it and design it into the system by proper selection of the burden resistor. You need to understand this and be able to do it before you can mess with the things safely.

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On Tue, 10 Feb 2004 05:38:34 GMT snipped-for-privacy@bellatlantic.net wrote:
[lots of theory omitted ... read the previous post for it]
I already understand the theory. What I was asking about are procedures. For example, how do you verify that the burden resistor is not open before you remove the shorting bar? IMHO, it's safer to de-energize the whole circuit before installing it (in new construction, that would generally be easily doable). I wouldn't want to be pulling that shorting bar when the burden load is faulty.
I've seen electrical contractors installing live wiring on 480Y/277 bus bars; very stupid if you ask me. Of course, other tenants might not have been happy to have their electric off for the 5 minutes it would have taken to tap in. OTOH, that's not an electrician I would hire (my boss's landlord hired him).
A CT will not go to infinite voltage due to factors like resistance and inductance. But it can go very high depending on what those factor happen to be in practice. I am looking at setting up a circuit to measure current on my power feed for analysis purposes. I have not decided what method I will use, yet. You might say it is safer to insert a "resistor" of 0.0001 ohms and measure the voltage drop across it. Perhaps so. And perhaps it will be better for what I intend, which is to measure the current waveform (I have not evaluated how much a CT will distort this since I have not yet looked into the construction ... the specific transformer parameters). The project is still a ways off, but I'm looking around for what I can find out in order to make that decision.
Theory learned in physics class, and practices put into place by politics and other factors are two different things. That a CT is a current driven device is physics. What particular practices are put into place around that theory are politics ... e.g. the result of multiple people contesting their ideas to see whose prevails. Hopefully the good ones do, but that doesn't always happen. Put 100 engineers in a room to solve a problem and you start with 101 different solutions. So why do all the CT ratios I see are compared to 5 instead of 1? That's not theory; it's practice (and I don't yet know why). So why do we have 110-120 volts in the US, and they have 220-240 volts in Europe? Physics won't decide that; politics does. It may have been the "politics" of a few engineers at power companies. I do know Edison's first DC system was 110 volts, so that could certainly have set a precendent (an AC system replacing it would prefer the same voltage to use the same light bulbs ... that's compatibility ... as practical as it is, it's still politics). Theory is stuff like that 0.0001 ohm shunt above will dissipate 4 watts when the current is 200 amps (politics can't change that).
My question was one of practices ... a politics category. And apparently one that some engineers want to keep others from learning about ... perhaps to protect their jobs.
I'll ask it again in a different form, focusing on specifics that were not clear before. How do you verify that it is safe to remove the shorting bar on a CT secondary when the circuit it is monitoring is energized and loaded, and you can't measure that you get the correct resistance across the burden resistor, and was not damaged into an open condition during installation, when an ohm meter test would show zero due to the shorting bar still present?
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If you are truly worried about it, you disconnect the load from the shorting bar, check it with a meter, reconnect it and remove the short!
CTs and shorting bars are used safely everyday, everywhere. A CT is NOT going to set fire to the curtains in a millisecond, it just can't deliver that much power. It can produce hazardously high voltages, but not at high currents. Take a look at the saturation curves of some CTs for a real education. In practice, you start to loosen the shorting strap, and if you see or hear arcing, you tighten it back up!
For your application you should perhaps investigate current transducers rather than CTs. Small, Hall effect devices that typically have better frequency response than CTs and eliminate this whole (non)issue of secondary voltage.
I find it kind of amazing that you are designing a product like this from scratch; there are any number of UL listed assemblies available with all kinds of network connectivity. Check out the offerings by Allen-Bradley (PowerMonitor)and Square D (PowerLogic). I would bet that either of these would do everything you need and more.
wrote:

bars;
happy
in.
him).
current
The
out
have
to
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will
that).
perhaps
bar
loaded,
burden
present?
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| If you are truly worried about it, you disconnect the load from the shorting | bar, check it with a meter, reconnect it and remove the short!
If I'm going to do that, I might as well de-energize the whole thing, and remove the short before re-energizing.
| CTs and shorting bars are used safely everyday, everywhere. A CT is NOT | going to set fire to the curtains in a millisecond, it just can't deliver | that much power. It can produce hazardously high voltages, but not at high | currents. Take a look at the saturation curves of some CTs for a real | education. In practice, you start to loosen the shorting strap, and if you | see or hear arcing, you tighten it back up!
Something that can't deliver more than 1 ma at expected current draw might be safer to handle. Fire isn't my first worry, though it is certainly a valid one.
| For your application you should perhaps investigate current transducers | rather than CTs. Small, Hall effect devices that typically have better | frequency response than CTs and eliminate this whole (non)issue of secondary | voltage.
Any particular good keywords to google for to find these?
| I find it kind of amazing that you are designing a product like this from | scratch; there are any number of UL listed assemblies available with all | kinds of network connectivity. Check out the offerings by Allen-Bradley | (PowerMonitor)and Square D (PowerLogic). I would bet that either of these | would do everything you need and more.
I've seen some around, but none have a usable output interface. The output but be optically isolated no matter what method of measurement is used, and a standard and/or documented data path and format (no proprietary software). One I did look at simply provided minute by minute watt/hour usage, which is way off from what I'm doing. Another seemed like it might have more data being handled, but required the use of their software (for Windows, which immediately rules it out), which negates the flexibility of designing my own logic.
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wrote:

secondary
www.sypris.com is a starting point for not too stringent requirements.
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| | wrote: |> |> | For your application you should perhaps investigate current transducers |> | rather than CTs. Small, Hall effect devices that typically have better |> | frequency response than CTs and eliminate this whole (non)issue of | secondary |> | voltage. |> |> Any particular good keywords to google for to find these? | | www.sypris.com is a starting point for not too stringent requirements.
It appears they only sell the raw sensors in large bulk. Maybe Digikey has them in single quantities.
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wrote: <snip>

It might be helpful if you told us what you are trying to do, not how you want to do it. So far you have not mentioned what you want to accomplish. You have posted how you plan to do it (isolated input to a computer, sampling rate, etc).
If we know what you are trying to accomplish we may know of instruments that can provide what you need without the need for all of the custom work that you are proposing.
If it has to do with harmonics monitoring and or harmonics compliance, there are many products available.
Just a thought.
Charles Perry P.E.
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wrote:
| It might be helpful if you told us what you are trying to do, not how you | want to do it. So far you have not mentioned what you want to accomplish. | You have posted how you plan to do it (isolated input to a computer, | sampling rate, etc). | | If we know what you are trying to accomplish we may know of instruments that | can provide what you need without the need for all of the custom work that | you are proposing. | | If it has to do with harmonics monitoring and or harmonics compliance, there | are many products available.
I want to get the voltage and current waveforms into a Linux based computer for the purpose of processing it in ways that I want to be able to program. The software I will write will be analyzing the data for things that I want to be able to customize without having to know them all today. At first it may be to simply measure load usage. But then I will add on other analysis, including harmonic, reactive, noise, RF, BPL, and then perform logging and remote control of devices.
My objective isn't to do a specific analysis, but rather, it is to make this data an input so I can explore what analysis and controls I can do. Once the data is in the computer in real time, then it has moved from a realm I am not an expert in (hardware) to one that I am an expert in (software).
I want to keep the hardware minimal, though I also want to protect computer hardware from added risk of any new connection to power. The computer _may_ even run entirely isolated, using a battery, or if not that certainly a UPS.
I'm not interesting in buying an instrument that does the analysis for me, nor am I interested in using someone else's software that does the analysis (although if it an open source version, I may be curious to look at it to get ideas).
Right now my first thought is to feed an analog waveform into a sound card and explore from that. Eventually I see doing multiple channels of data in digital form (still to be decided), and multiple channels of device control.
One long term objective is to manage load from selective devices based on the level of other uncontrolled loads. If other loads are low, more of the managed devices can be on at the same time. If other loads are high, fewer of the managed devices can be on at the same time. That objective would be to maintain a controlled peak load, especially when the power source is not from the power company.
But that's not the only objective; it's just an example. I won't even be able to envision all the possibilities until I get started and see how much data I really get and what I can analyze from it.
So basically, the first objective is to just get the data into the computer.
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wrote:

<snip>
Actually, quite often an in-line resistor/shunt is *not* safer. If the primary circuit is high-voltage, the leads used to sense the voltage drop across the shunt will also be at line voltage. One of the advantages of CT's is the secondary circuit is *low* voltage (<0) and is easier to run.
<snip>

True, it is 'practice', not 'theory'. The 'practice' is that most commercial metering use 5 amp current circuits. A typical kW or MW meter will be designed for 120V and 5A at the meter terminals. So most PT's are ratio of <line-voltage> : 120 and CT's are <line-current> : 5

have
to
is,
This is probably the 'most correct' answer for why we have 120/240 in US.

perhaps
Occam's razor. Don't assume nasty conspiracies when the simpler answer is you just haven't made your question clear enough for people to understand.

bar
loaded,
burden
present?
You don't. You have to make the initial installation with the system dead. As part of the initial checkout, you verify the shorting links work correctly. If you have to work on the metering, you either kill the circuits, or trust that the shorting links still work correctly. If you have any doubt about the shorting links, you don't trust them and have to reschedule the work when the system is dead.
If the shorting links are separate from the meter and normally open, you should see the meter reading drop to near zero when the links are closed. If the reading doesn't change, then obviously the link isn't shunting current away from the meter. Some meter cases have 'finger blades' that you pull to disconnect the meter and short leads at the same time. So when you pull the 'blade', the meter goes to zero, even if the shorting fingers fail. These types of cases have been around since Westinghouse, shorting-link failures are very rare in this type of unit (after all, it isn't rocket science, they designs are simple and robust).
Some CT's have a thin (mica?) member between the secondary terminals. If inadvertantly open-circuited while energized, the voltage spike will 'puncture' the insulation member and allow an arc to jump across the terminals right at the CT. Have to replace the CT to repair it, but the high voltage won't be sent through metering lead damaging other components (or people). Not *all* have this feature.
daestrom
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wrote:
| Actually, quite often an in-line resistor/shunt is *not* safer. If the | primary circuit is high-voltage, the leads used to sense the voltage drop | across the shunt will also be at line voltage. One of the advantages of | CT's is the secondary circuit is *low* voltage (<0) and is easier to run.
Which is why my plan is to build the unit to run entirely isolated, with an optical output of some kind, possibly as simple as a pulse modulated LED, or at best a fiber optic cable.
| True, it is 'practice', not 'theory'. The 'practice' is that most | commercial metering use 5 amp current circuits. A typical kW or MW meter | will be designed for 120V and 5A at the meter terminals. So most PT's are | ratio of <line-voltage> : 120 and CT's are <line-current> : 5
Which makes perfect sense now that the practice is known.
|> My question was one of practices ... a politics category. And apparently |> one that some engineers want to keep others from learning about ... | perhaps |> to protect their jobs. | | Occam's razor. Don't assume nasty conspiracies when the simpler answer is | you just haven't made your question clear enough for people to understand.
That could be. Too often people are put off be highly detailed, precisely expressed questions (they get tire of reading it after 100 words or so). So I more often try to be brief ... too brief in some situations.
| You don't. You have to make the initial installation with the system dead. | As part of the initial checkout, you verify the shorting links work | correctly. If you have to work on the metering, you either kill the | circuits, or trust that the shorting links still work correctly. If you | have any doubt about the shorting links, you don't trust them and have to | reschedule the work when the system is dead.
My trust in technology is limited when it can kill. Things break. Things come loose. Things don't have expected capacity, etc.
| If the shorting links are separate from the meter and normally open, you | should see the meter reading drop to near zero when the links are closed. | If the reading doesn't change, then obviously the link isn't shunting | current away from the meter. Some meter cases have 'finger blades' that you | pull to disconnect the meter and short leads at the same time. So when you | pull the 'blade', the meter goes to zero, even if the shorting fingers fail. | These types of cases have been around since Westinghouse, shorting-link | failures are very rare in this type of unit (after all, it isn't rocket | science, they designs are simple and robust).
Still, a failure can kill. I think we can accept that technology is not 100% perfect. Goals are in terms of MTBF and percentages, etc. Life is just once.
| Some CT's have a thin (mica?) member between the secondary terminals. If | inadvertantly open-circuited while energized, the voltage spike will | 'puncture' the insulation member and allow an arc to jump across the | terminals right at the CT. Have to replace the CT to repair it, but the | high voltage won't be sent through metering lead damaging other components | (or people). Not *all* have this feature.
Sounds like a good idea.
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snipped-for-privacy@ipal.net wrote:
<snip>

My answers below are strictly from theory. I am not qualified to discuss the politics or policies, etc. These answers are solely to tell you how it can be done in response to what you asked, and are not intended as recommendations.
In most cases, the burden IS the meter. It has a large shunt that can carry many times more current than the CT can produce. It won't burn out. However, in answer to your question about how to test: If you must, you can verify the ammeter will present a very low resistance with an ohmmeter, then connect the ammeter and remove the shorting bar to take your current measurements.
In cases where the burden is not the meter:
CT===shortingbar===burden===meter.
Those burdens don't burn out, either. They are chosen with a wattage hugely higher than they will ever dissipate. But, if you must check them:
Get two identical burdens to the one that is installed - check both with an ohmmeter. Bridge the existing burden with one of them, and remove the shorting bar. Record the meter measurement. Put the shorting bar back, and bridge a second extra burden across the circuit, open the shorting bar and take your measurement. If the second measurement is 1/2 the first, then the original burden resistor is open.
If you are talking about Harry Homeowner using his DMM to test things, that's a horse of a different color. Is that what you have in mind?
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wrote:

<snip>
bar
loaded,
burden
present?
You keep mentioning a burden resistor but most modern revenue grade meters do not have burden resistors. Most have a solid connection from pos to neg current input (one meter uses a u-bolt).
Testing depends on your physical installation. You can lift the leads to the meter at the shorting block and measure the resistance (should be very, very low). If you use a meter test switch it is possible to isolate the meter while shorting the CT at the same time and then you can measure the resistance of the meter circuit. If you use PK plugs, you can insert a plug that shorts the CTs while opening the meter and then measure the resistance.
No one de-energizes a circuit just to lift shorting bars on CTs.
Charles Perry P.E.
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wrote:

very,
plug
resistance.
I have another question related to this discussion. Well ok... I admit it, this discussion got me to double check my CT installation BEFORE I power it up:-) It appears ( at least from what I have seen in the documentation from GE) that CT transformers have an orientation to them?? So if the wire is passing through one way ( or reversed ) the phase angles of the transformer will either be correct (current lagging by 15 deg) or 180 deg out?? The documentation says that you can reverse the wiring to the terminals to correct this, but then seems to leave out the pertinent information, like which way through, for what terminal to ground. It also states that the accuracy of the metering will be effected if they are connected wrong, how much will this effect the system? And is there an easy way to test this Before I power the system up? Looks like at this point I have a 50% 50% chance of having it right but the way things have been going....... :-(
Thanks
William.....
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