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

Sort of. You design the CT circuit so that it does not exceed the VA rating of the CT. This means keeping the impedence as low as possible. Difficult with old induction disk meters and relays, easy with microprocessor based meters and relays. Another reason to avoid using metering equipment that requires a burden resistor.

True

You design the secondary circuit to handle peak current. One of the tests utilities require of meters and relays in relaying CT circuits is to expose them to repeated overcurrent (worst case fault current) and ensure that the meter's (or relay's) internal current circuit does not open.

Charles Perry P.E.

|> 1. Lots of electric utility techs do get killed. Utilities frequently | get |> cited in some way for inadequate procedures, too. | | False statement. The electric utility industry has a better safety record | than almost any manufacturing industry.

Then your research is incomplete.

|> 2. 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. | | True, but no one is saying you should work on a 35kV energized circuit. It | is a CT circuit and with some basic understanding and training, meter | changeouts, meter testing, and relay testing (all of which often include | shorting and un-shorting the CT) can be done safely.

A CT can be dangerous even on a 120 volt circuit.

| I explained in another post how you can test the circuit. You ignored the | post.

The test was unsafe. I'm not reading it right now but I recall it involving additionally opening the circuit. That just makes it worse.

|> 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. | | Pretty stupid analogy. You come here and ask a question about a subject | that you know little about and then you decide that everyone who has | experience in the field is doing it incorrectly!?

You've got it set in your head that I don't know the theory, and there's probably no way to get you to realize that I actually do because you have become closed minded about it, probably based on your misreading of the original question (which was not written for people that would make such assumptions).

I do know theory. What I did not know and and what I asked about is the procedures involved. But you wanted to jump on the theory aspect and just assumed that anyone asking any question whatsoever doesn't know any theory and therefore should not be told so that they are kept in the dark. All you really wanted to do is jump on someone for not knowing theory, and perhaps just as likely, try to drum up some business for yourself (which you can be sure will never happen from me or anyone I know).

My area of interest in this (and many other areas) *IS* safety, and I have become convinced now that yours is most likely not.

Safety involves doing not just one thing to prevent an accident, but to do everything you can to prevent an accident. It does *NOT* mean that you should use your understanding of the nature of the risk to decide what is a minimal amount of protection you can get away with; only fools do that. It is to understand all the risks and modes of failure, and be sure each and every one of them is addressed simultaneously.

Maybe you should take an NRA gun safety class, and then apply the PHILOSOPHY of that kind of safety to everything you do. And that means you do every step you can to avoid, prevent, or block any risk. I've personally seen the result when 3 of 4 safety stops failed. Your approach to minimalizing safety is not what I would allow on any gun range I'm running; you'd be kicked out.

I'll stick with maximal safety in anything I do, including electrical. At least I do have the luxury of doing that (as many linemen for the power company do not) ... e.g. de-energizing and testing the circuitry (and yes, I really do test circuits even after I have opened the main breaker, before I touch them).

|> It will |> depend on things like peak load/overload RMS current | | Sort of. You design the CT circuit so that it does not exceed the VA rating | of the CT. This means keeping the impedence as low as possible. Difficult | with old induction disk meters and relays, easy with microprocessor based | meters and relays. Another reason to avoid using metering equipment that | requires a burden resistor.

Given a specific maximum current (and it seems the design maximum is 5 amps, which I think is excessive, but that is probably established from the days of mechanical rotory meters), and that VA rating (which I have not seen on any CT but presumably should be available from the manufacturer), you can figure the appropriate voltage and resistance.

For what I am considering current measurement for, I don't need anywhere near

5 amps. It will be an electronic measurement, so 1 milliamp would be plenty. While a shunt will probably be better, if I do end up using a current transformer, I'll be looking for a couple of them that can do 1 milliamp or less.

What I didn't know before this thread, and what I found out separately from this thread, is that 5 amps is the standard (and explains the ratio rating). That's not theory (theory doesn't say you have to use 5 amps); it's politics (someone decided on making 5 amps common).

|> There should also be a consideration of fault currents |> that could damage the CT secondary circuitry. | | You design the secondary circuit to handle peak current. One of the tests | utilities require of meters and relays in relaying CT circuits is to expose | them to repeated overcurrent (worst case fault current) and ensure that the | meter's (or relay's) internal current circuit does not open.

Another rating I have not seen on CTs. At least circuit breakers do have peak interruption ratings (and fear too many people never consider as an issue).

You should consider yourself lucky that your local utility does not feel the way you do or your residence and business would be out of power quite often. Relays and meters are being tested and or changed somewhere on the system every day. If the power circuits were de-energized for this you would have large amounts of the population out of power at any given time.

Your problem is that you do not understand simple electrical work practices. You claim to understand the theory. Perhaps that is your problem. Too much theory, not enough practical knowledge.

You keep throwing up gun safety. I have had gun safety training. No one considers it safe to look down the barrel of a gun. Working on energized electrical circuits is NOT the same thing. If you think it is then you have real problems.

Charles Perry P.E.

5 amps is nominal, not maximum. Most meters and relays can use 20 or 30 amps max before clipping begins. For metering you design circuits so that the normal load current is near the middle of the range. Most CTs for metering can actually be accurate at 3 or 4 times their rating (so 15 or 20 amps).

That is tiny. What is the value of the primary amps that you are measuring? If it is high (100s of amps) you will not find a CT with an output of milliamps that is accurate.

5 amps in North America, 1 amp in the EU.

You have to look at the CT documentation.

For devices going into EHV stations (765kV) we tested devices to withstand

100 amps on for 30 cycles, off, then on, then off for a predetermined number of operations related to worst case fault clearing. This is way overkill. Doesn't quite fit with the "we don't care about safety" attitude that you think we in the utility industry have.

Charles Perry P.E.

That sounds great, but there are tasks that must be done with the CT energized, like burden testing, ratio testing, etc. What Charles is trying to tell you is that there are procedures and devices that allow you to do this safely when the circuit is energized. We are talking here about meter maintenance & testing, or CT testing from the secondary side. Obviously if you are talking about installing a CT, then that is another story. That generally involves installing a short length of primary bus bar, so by all means kill the power and ground & lock out the circuit.

There are test switches in many installations that can be used to short the CT, and also allow you to connect a test instrument in series with the meter. This allows you to monitor the current or make other measurements safely. You access the circuit with a "duckbill" plug that separates a set of spring contacts in a make-before-break fashion. An additional switch closes the circuit across the contacts, for more positive connection when you are not testing. There are other shorting mechanisms in use, including interlocks that automatically close the current circuit when you pull a meter. We are talking here about commercially available products designed, sold, and widely used specifically for this purpose. These are not Radio Shack switches with a couple of alligator clip leads.

Can an accident ever happen? Yes. Is this a totally unsafe industry, not at all. I have occasion to work alongside utility electricians from time to time, verifying meter and CT accuracy. I have seen their procedures and PPE.

There is nothing wrong with asking the questions that you are asking, but you have essentially told everyone who has responded that they don't know what they are talking about. It is unfortunate that you don't want to become better educated on this subject.

Ben Miller

| You should consider yourself lucky that your local utility does not feel the | way you do or your residence and business would be out of power quite often. | Relays and meters are being tested and or changed somewhere on the system | every day. If the power circuits were de-energized for this you would have | large amounts of the population out of power at any given time.

Utilities work under different requirements. Their safety procedures have to be different than what I work under.

| Your problem is that you do not understand simple electrical work practices.

I don't need to understand how it is done by a utility because I do not have the requirements of a utility.

| You claim to understand the theory. Perhaps that is your problem. Too much | theory, not enough practical knowledge.

So why do you think I was asking a question about practical knowledge? Duh! Did your light just come on?

| You keep throwing up gun safety. I have had gun safety training. No one | considers it safe to look down the barrel of a gun. Working on energized | electrical circuits is NOT the same thing. If you think it is then you have | real problems.

I have looked down the barrel of a gun. The gun was disassembled at the time.

I have worked on circuits up to 200 amps, 240 volts. They were de-energized at the time, and further, I tested to make sure they were de-energized for certain. I have seen electricians work on circuits up to 1000 amps, 480 volts while they were live.

I will always de-energize a circuit to work on a current transformer connected to it, whenever that situation happens. I don't work for a utility, so I am not going to be doing the cases where this isn't an option.

You just don't grok the concept of employing every safety method available.

| There is nothing wrong with asking the questions that you are asking, but | you have essentially told everyone who has responded that they don't know | what they are talking about. It is unfortunate that you don't want to become | better educated on this subject.

The first responses were not attempting to provide any information. Instead, they were strictly saying that I should hire someone else. It sounded to me like someone trying to guard secret knowledge and drum up business. Further, I was accused of not knowing theory, when in fact there was no basis in the question to come to that conclusion. In effect, the first replies I got were saying that it was wrong to ask the questions I was asking.

|> For what I am considering current measurement for, I don't need anywhere | near |> 5 amps. It will be an electronic measurement, so 1 milliamp would be | plenty. |> While a shunt will probably be better, if I do end up using a current | transformer, |> I'll be looking for a couple of them that can do 1 milliamp or less. | | That is tiny. What is the value of the primary amps that you are measuring? | If it is high (100s of amps) you will not find a CT with an output of | milliamps that is accurate.

The primary amps will be something around 200 to 300 rated, with typical loads of around 10 to 80. Early tests will involve much smaller amps, like on the order of 1 or less.

|> What I didn't know before this thread, and what I found out separately | from |> this thread, is that 5 amps is the standard (and explains the ratio | rating). |> That's not theory (theory doesn't say you have to use 5 amps); it's | politics |> (someone decided on making 5 amps common). |>

| 5 amps in North America, 1 amp in the EU.

I hadn't needed to know that, but thanks for the info. I don't need to know their voltage is 400Y/230 either, but it's fun to know that. But maybe a CT made for the EU market might be more readily adapted.

I actually won't need the accuracy; I'll be able to calibrate my reading based on other factors, such as changes in load. The project involves measuring the current waveform, not metering usage. A CT may introduce unwanted inductance (I haven't done the calculations since I don't know specifics of CTs I may be able to use) at the harmonics I want to detect.

|> Another rating I have not seen on CTs. At least circuit breakers do have |> peak interruption ratings (and fear too many people never consider as an |> issue). | | You have to look at the CT documentation. | | For devices going into EHV stations (765kV) we tested devices to withstand | 100 amps on for 30 cycles, off, then on, then off for a predetermined number | of operations related to worst case fault clearing. This is way overkill. | Doesn't quite fit with the "we don't care about safety" attitude that you | think we in the utility industry have.

Is that 100 amps on the CT secondary?

I'm not saying the electric utility industry does not care for safety. What I am saying is that it is an unsafe industry where every precaution that can be taken, must be taken. There are certainly many cases were work must be done on energized lines. Where that is NOT the case, de-energizing the lines should be done for no other reason than that extra level of safety. Then, even with the lines de-energized, short them to increase your chance of being alive after an accidental re-energizing, which is something that can happen, and has happened to someone I know (he lived because he was not actually in contact with the line, but was royally pissed that it happened). I didn't ask him what the line voltage was, but he described it as "then suddenly the line started buzzing" and from that I'll let you guess the voltage.

At those current levels (assuming 600V or less) you would be better off buying a power quality instrument and using a clamp-on CT directly on the line. You mention harmonics. You need to be aware that some CTs have very poor accuracy at frequencies other than the fundamental. It sounds like your are going through a lot of trouble to measure something that can be done quite easily with the proper equipment.

Try

(if you need three phases at the same time) or (if measuring one phase at a time is ok).

Charles Perry P.E.

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?

| At those current levels (assuming 600V or less) you would be better off | buying a power quality instrument and using a clamp-on CT directly on the | line. You mention harmonics. You need to be aware that some CTs have very | poor accuracy at frequencies other than the fundamental. It sounds like | your are going through a lot of trouble to measure something that can be | done quite easily with the proper equipment. | | Try

(if you need three phases at the same time) or | (if measuring one phase at a time is ok).

I need sensing that will feed all four wires to a computer which will be doing real time analysis. A sample rate of 48000 Hz will be fine. I'll be sensing voltage, too.

At that sampling rate, you won't be looking at anything over 24 khz. The better clamp-on probes that are made for scopes or power quality work have bandwidths of 100 kHz or more. These should work fine for what you are doing, and they are easy to install since you don't need to disconnect any wiring. Fluke, AEMC, and others all have them.

Of course, in the interest of safety, be sure that whatever you use has a CAT III minimum safety rating at or above the line voltage involved, assuming that you connect it indoors after the main breaker (recommended). If you connect it ahead of the main breaker, then it needs to be CAT IV. This applies both to the probes and the instrument that you connect them to. This will be a problem for a PC. You will need a signal conditioner that is designed for this to act as a buffer.

Ben Miller

How do these things feed to a computer at that higher rate? Ethernet?

| Of course, in the interest of safety, be sure that whatever you use has a | CAT III minimum safety rating at or above the line voltage involved, | assuming that you connect it indoors after the main breaker (recommended). | If you connect it ahead of the main breaker, then it needs to be CAT IV. | This applies both to the probes and the instrument that you connect them to. | This will be a problem for a PC. You will need a signal conditioner that is | designed for this to act as a buffer.

I plan to connect optically. I haven't decided the best way, yet. I do want things digitized at the sensing point, and be optical from there (because I want optical, but keeping analog linearity over optical is not easy to do).

Has a series of data logging products that are of reasonable quality, and very affordable. You may find sensors, and/or a system that will fit your needs.

Louis--

********************************************* Remove the two fish in address to respond

|

Has a series of data logging products that are | of reasonable quality, and very affordable. You may find sensors, and/or a | system that will fit your needs.

Thanks for the lead. Maybe some stuff they have might be useful for some things. But for the power measurement I'll be doing I need at least 16 bit resolution, 48000 samples per second, optical isolation rated into some kV, and computer input documented for my own programs to access (i.e. not using their software at all).

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 (.... So why do all the CT ratios I see

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 : 120 and CT's are : 5

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

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.

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

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.

The secondary current, for any particular burden, is very effectively limited by the saturation of the CT core. In fact, in some uses in motor starters, etc, the saturation is a design factor in tailoring the time delay of the overcurrent protection. If you run a CT near saturation you can produce a situation where 3X primary current does not produce 3X secondary current. Good trick for achieving NEMA class 30 motor overload protection using standard class 20 relays.