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

 

[SQLit]

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

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.

 

[Charles Perry]

| 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?

Are you seriously asking these questions? If so, never go near a CT
circuit.

You never leave the secondary of a CT open when the primary is energized.
You use a shorting block to short the secondary when you need to remove a
meter (for example). Once the meter is installed, you simply remove the
shorting bar from the block. This can be done safely with the circuit
energized.

Charles Perry P.E.

 

[Ben Miller]

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.

Ben Miller

 

[[email protected]]

Are you seriously asking these questions? If so, never go near a CT
circuit.

You never leave the secondary of a CT open when the primary is energized.
You use a shorting block to short the secondary when you need to remove a
meter (for example). Once the meter is installed, you simply remove the
shorting bar from the block. This can be done safely with the circuit
energized.

Charles Perry P.E.

Charles, You can help me out on this:

CT=============================Burden=meter

That is the way it is installed now - about
100 feet between the CT's (one on each phase,
30 KW 208 rooftop mounted heater) and the
burden resistors. My knee-jerk reaction is
that that is wrong, and it should be:

CT=Burden=============================meter

I don't remember the values, but the burdens were
sized right to keep the maximum V low and were
oversized for dissipation - 50 watts when the max
was something like 5 watts - as I recall. The
question is about the placement. Is it legitimate
to have that distance? If a burden opens ...
well, I don't want to think about what could happen,
I'd rather limit the "physical exposure" to the
much shorter path. Am I all wet?

Thanks!

 

[Charles Perry]

| Are you seriously asking these questions? If so, never go near a CT
| circuit.
|
| You never leave the secondary of a CT open when the primary is energized.
| You use a shorting block to short the secondary when you need to remove a
| meter (for example). Once the meter is installed, you simply remove the
| shorting bar from the block. This can be done safely with the circuit
| energized.

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?

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

Hire someone who knows what they are doing!!!!!!!!!!!!!!!!!

The meter acts as the"short" when it is in the circuit, but you should hire
someone who knows what they are doing.

The voltage across an open CT secondary can be thousands of volts, but you
should hire someone who knows what they are doing.

Charles Perry P.E.

 

[Charles Perry]

Charles, You can help me out on this:

CT=============================Burden=meter

That is the way it is installed now - about
100 feet between the CT's (one on each phase,
30 KW 208 rooftop mounted heater) and the
burden resistors. My knee-jerk reaction is
that that is wrong, and it should be:

CT=Burden=============================meter

I don't remember the values, but the burdens were
sized right to keep the maximum V low and were
oversized for dissipation - 50 watts when the max
was something like 5 watts - as I recall. The
question is about the placement. Is it legitimate
to have that distance? If a burden opens ...
well, I don't want to think about what could happen,
I'd rather limit the "physical exposure" to the
much shorter path. Am I all wet?

Thanks!

I think the burden resistor is best where it is presently located. You will
have much less interference than if you had the resistor further away.
There would be a slight increase in safety by burdening close to the CT but
your readings will likely be more accurate with the burden where it is.

Personally, I don't use many meters that require a burden to measure
current. Most of what I use are meters that take 5A input current.

Charles Perry P.E.

 

[Ben Miller]

Why is it safe to unshort the secondary once the meter is attached?

Because the meter provides the low resistance path for the CT secondary
current.

And: what is the voltage?

Normally? A few volts at most. For example, if the meter burden is 0.5 Ohm,
then the voltage at full load (5A secondary current) will be 2.5 volts.

Open? The CT at full load is trying to produce a 5A secondary current. With
an infinite resistance, the voltage will theoretically go to infinity trying
to push the current through the circuit. The actual voltage is limited by
the magnetic characteristics of the CT core, but it could be from one to
several thousand volts.

I don't think any of us misunderstood the question. We are just sensitive to
the hazard that exists for an untrained individual, and are showing concern
for your safety. People are electrocuted by CTs.

Ben Miller

 

[[email protected]]

I think the burden resistor is best where it is presently located. You will
have much less interference than if you had the resistor further away.
There would be a slight increase in safety by burdening close to the CT but
your readings will likely be more accurate with the burden where it is.

Personally, I don't use many meters that require a burden to measure
current. Most of what I use are meters that take 5A input current.

Charles Perry P.E.

Thanks!

 

[[email protected]]

| |> 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?

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".

And: what is the voltage?

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.

 

[Louis Bybee]

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.

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.

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

 

[.]

Ok while we are on the subject heres my next question and thanks to
everybody who answered my last one:

If I had a 600/347Y 400A service and
Lets say you have CTs at 400:5 and PTs at 360:120
connected like the picture on the left in this link.
http://www.geindustrial.com/products/reference/BBpg34,35.pdf

The above values of transformers is what I have been made aware is
commonly used for this type of service.

The CT ratio is 80:1 and the PT ratio is 3:1
This would mean that the CTs would deliver 0-5A to the meter current
coils and the PTs would supply 115.6V to the voltage coils of the
meter. I'm guessing the meter would be specifically calibrated to use
115.6V instead of 120V such as you would expect on a 480V service.
Let me know if I am out to lunch on that one.

Next, since the CT secondaries are Y connected, and so are the PT
secondaries, the meter multiplier would be the product of the two
ratios or in this case 80x3=240. So everything on the meter's register
would have to be multiplied by 240 right?




To send me e-mail remove the sevens
from my address.

Chrisd

 

[John Gilmer]

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.

OK. And what was the mechanism of the failure in your experience.

EMWTK

Unfortunately I can't locate
them.

To bad. Although one blown up panel looks about the same as another blown
up panel.

 

[Louis Bybee]

OK. And what was the mechanism of the failure in your experience.

EMWTK

To bad. Although one blown up panel looks about the same as another blown
up panel.

Not necessarily! Some are a lot more dramatic than others, and some are
purely caused by "pilot" error and not equipment failure. You know, the
fingers/tools where they don't belong, or haven't the expertise necessary to
complete the task without a catastrophic result syndrome!

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. One was due to poorly secured termination
connections verified by multiple remaining loose connections that weren't
destroyed, and deemed to have never been tightened properly. The other was
caused by an individual that removed a secondary connection lead without
proper preparation, and training. The end result in both cases was
switchgear that required considerable man hours to repair before being
returned to service. Amazingly there wasn't any personal injury in either
case (with the possible exception of reputations).

A PT failure (the assumption due to lack of material left to evaluate) I
used to have pictures of was assumed to have been caused by a technician
attempting to change the PT while the switchgear was energized. The
resulting damage was exacerbated by improper over current protection. The
switchgear in this case had to be replaced. It was deemed costlier to repair
than replace. The amazing thing was that the technician was burned, but
wasn't killed! Unfortunately he had little recollection of the events just
prior to the fireworks so little was gleaned other than proper safety
procedure wasn't followed. The summation by the investigation team was that
most likely the service wasn't properly deenergized, and bonded to ground
before work was commenced. It never was clear why the technician was
attempting to replace the PT. If someone knew they weren't talking.

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

 

[John Gilmer]

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?

2) Did the insulation in the CT break down?

 

[Charles Perry]

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.

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.

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

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

Charles Perry P.E.

 

[Charles Perry]

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.

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.

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

I pity anyone unfortunate enough to work for you.


Charles Perry P.E.

 

[Don Kelly]

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..

 

[Don Kelly]

| Are you seriously asking these questions? If so, never go near a CT
| circuit.
|
| You never leave the secondary of a CT open when the primary is energized.
| You use a shorting block to short the secondary when you need to remove a
| meter (for example). Once the meter is installed, you simply remove the
| shorting bar from the block. This can be done safely with the circuit
| energized.

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?

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

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

--------------
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
[email protected]
remove the urine to answerhttp://ka9wgn.ham.org/ |

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

---

 

[Louis Bybee]

1) What was the voltage level in the primary?

2) Did the insulation in the CT break down?

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