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

[Charles Perry]

On Tue, 10 Feb 2004 05:38:34 GMT [email protected] wrote:

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.

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

Charles Perry P.E.

 

[William]

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.

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.....

 

[Charles Perry]

|> 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.

Charles Perry P.E.

 

[Charles Perry]

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.

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.

It also states that the
accuracy of the metering will be effected if they are connected wrong, how
much will this effect the system?

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

And is there an easy way to test this
Before I power the system up?

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

Looks like at this point I have a 50% 50%
chance of having it right but the way things have been going....... :-(

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

Thanks

William.....

Charles Perry P.E.

 

[Charles Perry]

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

Charles Perry P.E.

 

[Charles Perry]

|
| |> On Tue, 10 Feb 2004 22:43:28 -0500 Charles Perry
|>
|> |> 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.

False statement. The electric utility industry has a better safety record
than almost any manufacturing industry.

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.

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).

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

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!?

 

[Charles Perry]

|
| | <snip>
|> 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 have a question for you. What do you think is the "correct ohmage" of a
| CT circuit?

Without particulars of the circuit, the only answer is "it depends". True

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.

and type of metering
device to be used.
True

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.

Charles Perry P.E.

 

[Charles Perry]

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).

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.

 

[Charles Perry]

|> 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.

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).

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.

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.

|> 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 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.

 

[Ben Miller]

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).

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

 

[Charles Perry]

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.

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 www.dranetz-bmi.com (if you need three phases at the same time) or
www.fluke.com (if measuring one phase at a time is ok).

Charles Perry P.E.

 

[[email protected]]

[email protected] wrote:

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?

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?

 

[Ben Miller]

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

 

[Louis Bybee]

| 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 www.dranetz-bmi.com (if you need three phases at the same time) or
| www.fluke.com (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.

http://www.onsetcomputer.com 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

 

[daestrom]

On Tue, 10 Feb 2004 05:38:34 GMT [email protected] wrote:

... You might say it is safer to insert a "resistor" of 0.0001
ohms and measure the voltage drop across it. Perhaps so.

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 (<=120) and is easier to run.

.... 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).

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

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).

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

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.

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 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

 

[Charles Perry]

wrote:

It appears they only sell the raw sensors in large bulk. Maybe Digikey
has them in single quantities.

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.