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

|> I don't think it is safe to remove the shorting block with the power on |> without special safety steps. Somehow, I bet you don't do any. |>

| That statement shows your utter lack of understanding of the situation. | Which indicates that you should hire someone who does understand.

If I hire someone, it will be someone without your attitude.

But in fact I do understand the situation perfectly.

| The shorting bar can quite safely be removed with the CT in service, as long | as your wiring is correct and the meter is connected properly. You do not | seem to be qualified to assess either of these conditions.

And what makes you think that?

| Save yourself the injury and hire someone who knows what they are doing. | CTs can kill.

Your attitude shows me why I should not hire people like you.

| As for the "tude", you are the one who stated that noone understood your | question, when we all understood perfectly well.

Since you never answered it, either you didn't understand it, or you have an attitude problem by trying to answer some other question. Maybe you just mixed up my question with what the OP asked.

Of course, at this point, anything you tell me is now suspect.

You asked "I know it needs shorted. The question was whether the step of unshorting it so the meter functions can be done safely with power on. If there is a danger there that shorting mitigates, are you saying a meter will do it, too?"

I answered that.

Then you asked "Just how much voltage is coming off that thing (for some given current and CT you specify)?"

And I answered that.

Hmm, you have a reading comprehension problem.

As for the voltage produced under normal load...there is no way to say without knowing your exact circuit, and where you will measure it. If the shorting block is close to the meter and the meter is a modern microprocessor based meter and you are measuring at the shorting block, then the voltage will be less than a volt.

Not sure why I bother explaining to you since you don't really want an answer.

I pity anyone unfortunate enough to work for you.

Charles Perry P.E.

----------------------- 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..

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

------------ Yes- the shorting block is a safety feature in the situation where the meter is not connected. Removal of the block after the meter is properly connected (even under power) is not a problem.

------------

-------------- Consider a Ct rated 400/5 A. At 5A with a burden or meter of impedance 0.1 ohm, the voltage will be about 0.5V. If the burden is 0.5 ohm then at rated current the voltage will be 2.5V. It is desirable to keep the burden low. Now consider the CT open circuited. It is an unloaded step up voltage transformer in that condition. If it was perfect, and the supply was 1000V then you would have 400000/5 =80000V across the secondary as the primary circuit would see a near infinite impedance. This won't happen because the core will saturate at a relatively low voltage but there may still be an appreciable voltage on the primary. Even if it was 100V, the secondary voltage may still be well in excess of 1000V (not 8000 as the flux path will be different so less coupling will occur. Still dangerous as many widows (of those who knew better) can testify (which is why others have rightly cautioned you).

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

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?

| I've been looking through my collection of "scary aftermath" photographs for | a few shots of the destructive results of failed/improperly serviced/applied | CT installations that I thought I had saved. Unfortunately I can't locate | them. | | Perhaps if anyone has access to photographic documentation of the | destructiveness of such a failure, and could post them to | alt.binaries.schematics.electronic it could emphasize, and illustrate the | seriousness of uninformed service/experimentation involving CTs that many of | you have explained so well.

It won't reach a very wide audience there, anymore. Why not put together a web page that shows the dangers, explains the risks, explains the theory, and describes the proper practices to stay safe.

Though not exactly on topic, you might want to see this page:

On Tue, 10 Feb 2004 17:53:04 -0500 John Gilmer wrote: | | |>

|> The two CT failures I can clearly remember (and used to have pictures of) |> were a result of the secondary circuit being opened while the switchgear | was |> energized, and under load. | | | 1) What was the voltage level in the primary?

Does it really matter? If there's current being monitored, there's current on the CT secondary. I'd be more interested in the secondary voltage (not a specific number, really, but whether the secondary was open or not).

| 2) Did the insulation in the CT break down?

I would bet more than that.

It's been long enough that I can't recall, but I believe they both were 480 Y 277 v.

I would suspect that insulation breakdown was a factor, but in both cases they were mounted on a bus section that shorted to ground as a result of degradation of the CT. The damage was significant enough that it would have been difficult to quantify as to the exact sequence of events. In both situations people admitted to creating a condition where the CT secondary was left in an open circuit condition. The failure wasn't immediate.

At this point I would be interested in disassembling a CT that had failed due to an individual opening secondary, while remaining effectively whole, and examine the extent of the damage.

Has anyone done so that would be willing to share their findings?

Louis--

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.

|> Of course, at this point, anything you tell me is now suspect. | | I pity anyone unfortunate enough to work for you.

Why? Because I will require them to de-energize the circuit AND test that any CT circuits are correctly wired with correct ohmage, before re-energize?

I happen to think your method is NOT safe.

| Yes- the shorting block is a safety feature in the situation where the meter | is not connected. Removal of the block after the meter is properly connected | (even under power) is not a problem.

My position is that removing the shorting block on an energized and loaded circuit is unsafe. The reason for that position is that the shorting block prevents testing the secondary wiring for correctness.

| Consider a Ct rated 400/5 A. At 5A with a burden or meter of impedance 0.1 | ohm, the voltage will be about 0.5V. If the burden is 0.5 ohm then at rated | current the voltage will be 2.5V. It is desirable to keep the burden low. | Now consider the CT open circuited. It is an unloaded step up voltage | transformer in that condition. If it was perfect, and the supply was 1000V | then you would have 400000/5 =80000V across the secondary as the primary | circuit would see a near infinite impedance. This won't happen because the | core will saturate at a relatively low voltage but there may still be an | appreciable voltage on the primary. Even if it was 100V, the secondary | voltage may still be well in excess of 1000V (not 8000 as the flux path | will be different so less coupling will occur. Still dangerous as many | widows (of those who knew better) can testify (which is why others have | rightly cautioned you).

You're talking theory, not procedures. Tell me how you verify the circuit is correct (for example, test that the burden has not opened while the shorting block is in place).

| |> 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? | | 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.

If you can isolate the burden from the shorting block, then you risk the mistake of removing the shorting block with an incorrectly restored wiring to the meter, resulting in an open circuit.

| No one de-energizes a circuit just to lift shorting bars on CTs.

They put themselves at risk by doing that. Shorting blocks are in my opinion, a false sense of security. My general safety training taught me to employ _every_ safety measure _and_ to never depend on any one of them working correctly when injury or death is a risk. You find such safety philosophy very common in firearms safety courses, for example. I most certainly will de-energize the circuit to install or change any current transformers or associated circuitry. And any remote reading methods (what my future project will include) will be optically isolated by some length of fiber.

My next interest involves power circuit interruption upon CT failure.

|> 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.

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.....

re-energize?

Then every electric utility in the world is using an unsafe condition. Odd that the rest of the world disagrees with you.

Charles Perry P.E.

transformer

The CT documentation will tell you how that particular manufacturer marks the proper orientation of the CT. Basically if the current goes IN one terminal (or side of a window CT) it will come OUT of a particular terminal on the secondary. This will be marked with dots or dashes or H1, etc. You have to read the documentation. If you are unsure, you call the manufacturer.

The power measurement will be very inaccurate since 1/3 of your power will be measured as flowing in the opposite direction.

You can verify it once there is power by looking at the phasors.

If you pay attention to the documentation and call the manufacturer with questions you can increase your odds significantly.

Charles Perry P.E.

re-energize?

I have a question for you. What do you think is the "correct ohmage" of a CT circuit?

Charles Perry P.E.

|> |> Of course, at this point, anything you tell me is now suspect. |> | |> | I pity anyone unfortunate enough to work for you. |>

|> Why? Because I will require them to de-energize the circuit AND test that |> any CT circuits are correctly wired with correct ohmage, before | re-energize? |>

|> I happen to think your method is NOT safe. |>

| Then every electric utility in the world is using an unsafe condition. Odd | that the rest of the world disagrees with you.

Two known facts:

1. Lots of electric utility techs do get killed. Utilities frequently get cited in some way for inadequate procedures, too.

1. Utilities are frequently in situations where they must work on energized circuits. Their procedures will, and must, be different than those the rest of us can follow.

Odd that you won't explain why you think it is safe. And don't go into the theory; I already understand the theory. Explain to me why you think that there is no chance (to the extreme that you would bet your life on it) that the circuit which is currently shorted is not faulty in some way (and which cannot be adequately tested due to the short which is in place).

You're probably the kind of person who looks down the barrel of a gun to see if it might need cleaning (if you even do such a thing) when you are sure it can't be loaded, without first disassembling it.

Without particulars of the circuit, the only answer is "it depends". It will depend on things like peak load/overload RMS current and type of metering device to be used. There should also be a consideration of fault currents that could damage the CT secondary circuitry.