ECN Forum

Current Transformer Safety!

Posted By: Trumpy

Current Transformer Safety! - 05/21/04 09:54 AM

How many of you guys, know how to work safely in and around Current Transformers?.
What are the procedures for working in area's where these are in a panel?.
What are the procedures for installing them safely, where cannot turn the supply off?. [Linked Image]
Posted By: aland

Re: Current Transformer Safety! - 06/03/04 09:40 PM

Have not had a great deal of experience with C.T's other than for monitoring P.F equipment Do remember one thing never work on them with an open circuit, the terminals tothe CT should be short circuited. Dependant on the CT ratio and the magnitude of the current flowing the open circuit voltage can be very very high.
Posted By: Trumpy

Re: Current Transformer Safety! - 06/04/04 09:25 AM

Darn good point Aland.
Current Transformers entail a great part of my day-to-day work, with High Current loads, spread over 400V phases.
I have had to cut live Busbars with an Insulated Hacksaw, certified to VDE standards,to a voltage of 1200V.
Keep the hacksaw moving, or it will weld!
Measuring currents on 11kV and 22kV Bushing type CT's give me the willies!.
CT's need thier own sort of Safety treatment!. [Linked Image]
Posted By: aland

Re: Current Transformer Safety! - 06/04/04 05:31 PM

Trumpy,You have my greatest respect, I dont mind live working but I dont fancy cutting Live bar section with a hacksaw insulated or not.
Posted By: Bjarney

Re: Current Transformer Safety! - 06/06/04 05:41 PM

CTs may be considered as opposites to voltage or potential transformers in some electrical characteristics. Where loads or “burdens” for potential transformers are almost always placed in parallel [shunted] with PT secondaries—loads or “burdens” for CTs are almost always connected in series [looped.]

Just as it’s problematic to short out an energized PT secondary, forcing damaging overcurrent through its windings, one should not open-circuit an energized current-transformer secondary—for this effectively forces ‘excess voltage’ on CT windings. Also, though not an immediate hazard, it is poor practice to leave in-place CTs with open-circuited secondaries, if they later can be energized in this condition.

Another thing to avoid is fuses in CT circuits, for that may easily create an undesirable open-circuit condition in the CT loop. It is good practice to fuse PT circuits on their primaries and in a number of cases, on secondary sides. {added paragraph}

In general, one consideration is that as the mass of copper and steel in a CT increase, the hazardous open-circuit potential typically increases. For CTs and PTs, increased size and electrical ratings usually dictate increased hazard an likely decreased tolerance to forgetful practices.

Besides the practice of not open-circuiting CT secondaries with any primary current present, it’s wise to also not leave grounding jumpers off of CT secondaries. [Note that the ground-jumpering need holds true for PTs, too.] Floating secondaries can develop high electrostatically induced voltages to ground, presenting hazards to personnel and equipment.

Noise to watch out for working around CTs is “singing” or a higher-pitched squeal, that can be caused by open-circuited CT cores operating in saturation. Also, a sputtering or crackling noise can originate from intermittent CT-secondary connections. Because winding voltages are typically very low in normally operating CT loops, voltage measurements on secondary wiring will not be effective for determining if primary circuits are safely deenergized. If a screw on a terminal block for secondary wiring is loosened, and any sputtering or crackling sound is heard, suspect current flow and the hazards that may grow if ‘loosening’ is further attempted.

Open-circuit conditions in CT-secondary wiring can be limited through the use of shorting-style terminal blocks. Typical components are the “SC”-series blocks at www.marathonsp.com/PDF%20Files/HDTB%20-%201500%20Series.pdf
[Linked Image]

A stodgy old {but detailed and free!} publication on instrument-transformer characteristics is www.geindustrial.com/products/applications/GET-97D.pdf [~6MB]




[This message has been edited by Bjarney (edited 06-07-2004).]
Posted By: Trumpy

Re: Current Transformer Safety! - 06/07/04 03:37 PM

Bjarney,
Great post mate!.
The dangers of working with PT's and CT's was bought home to me one day a few years back.
Working in a rather warm switchroom in the middle of summer.
I thought I would roll my forearm sleeves up to get rid of the sweat.
I was wearing ASTM Class 2 Gloves and Outers.
One hit on a live CT terminal between the top of my gloves and the end of my overalls.
230V through the arm!.
aland,
I only work in situations where I know I will be safe, anything else, they can all just jump in the lake!. [Linked Image]
Posted By: Trumpy

Re: Current Transformer Safety! - 06/18/04 10:15 AM

Bjarney,
The Current Transformer is a very common thing here in Commercial and Industrial installations.
A 3 phase meter has a 50A per phase load current rating, above that a CT arrangement is used to step the current down to a level that the meters can use.
Ratio's of 100:5, 200:5, 300:5, 500:10, 700:10, 1000:20 are common here at 400V
11kV bushing type CT's are used for Merz-Price Over-Current Protection, these things really give me the willies!. [Linked Image]
Posted By: Bjarney

Re: Current Transformer Safety! - 06/19/04 04:08 PM

The 700:10 and 1000:20 ratings are interesting. In North America the highest rated primaries are still referred to as having “5 ampere,” secondaries, but with “rating factors” of 2.0 or 4.0 which is multiplied by the 5-ampere rating, so essentially operating at 10 or 20 amperes. Practically, they would be similar.
Posted By: Trumpy

Re: Current Transformer Safety! - 07/10/04 10:35 AM

Bjarney,
Do these CT's have a stated "Burden Rating"?.
This has to be taken into account too. [Linked Image]
Posted By: Bjarney

Re: Current Transformer Safety! - 07/11/04 03:34 AM

Trumpy — Yes indeed, ANSI current transformers have burden ratings, but are rated in ohms, where I understand IEC uses voltamperes. With Ohm's law, though, they work out to be very similar.

An ANSI meter-class CT rating might be 0.3B0.5—that works out to a maximum primary-to-secondary ratio error of 0.3% for a ½-ohm burden or secondary “loop”.
Posted By: RODALCO

Re: Current Transformer Safety! - 01/07/06 11:06 AM

Quote
Ratio's of 100:5, 200:5, 300:5, 500:10, 700:10, 1000:20 are common here at 400V


Trumpy:
Very interesting are the 500:10,700:10 and 1000:20 values. Are these metering CT's or protection CT's? [Linked Image]

I have done a lot of CT metering work in the North Island and the secondaries were always referred to as 5 Amps for 400 Volts metering. The Watthourmeters are very accurate between 0.5 and 6.5 Amps loading. With current in the 10 to 20 Amps range the current coils in the 5 Amp meter would be saturated and big under readings recorded on the watthourmeter.
Most CT meters class 1 and 2 were rated at 5 Amps ( Sangamo, Ferranti, English Electric, Landis&Gyr ).
Some Email meters had nameplate ratings of 2.5 / 10 Amps. ( the only ones I'm aware off ) which were designed to take 10 Amps from the CT's.
CT accuracies were CM, BM or class 1 and common maximum burdens were 5VA or 10VA for CT's.
A Trivector kWh / kVArh meter would have a burden of about 4 VA.
Auckland Central used thermal MDI meters in series with a standard kWh meter which required 10 or 15 VA rated CT's
Ratio's used were 100,150,200,250,300,400,500,600,800,1000,1200,1600,2000,3000 / 5 Amps.
In installations over 1600 Amps often summation CT's were used because double feeders were provided for in large pumpstations or shopping malls.
The multitap CT's were most accurate on the highest tapping while being tested.( these CT's were e.g. 150/250/400/5A ).

From 2000 onwards through the MARIA standards a lot of multitap CT's are being replaced to single tap class 0.5 or 0.2 CT's for the T.O.U. metering.
Posted By: RODALCO

Re: Current Transformer Safety! - 01/07/06 11:25 AM

A thing I forget to mention is the testblock.

The CT secondaries were earthed at the common point at the testblock. Also here were the shorting links provided to short out the CT's and the PT fuses to isolate the voltage to the meter if a meter change was required for a different tariff.

The non standardising of CT wiring in different networks was a bit of a worry sometimes. Waitemata Area had CT's earthed, and Auckland not, Hamilton had different colour CT wires while Thames Valley had the testblocks the other way around.
Through the new MARIA standards at least earthing of CT secondaries is now mandatory which will improve metermen safety and on new installations the wire colours are standardised too.
Current S1-S2 (M-L) Blue-Yellow(White) and Potential Red. The 3 core usually TPS sleeved with the appropiate phase colour.
Neutral Black and Earth Green / Yellow.

So a typical CT installation had 11 wires coming from the CT chamber.
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/07/06 09:18 PM

And which direction does an AC current flow in, exactly?

The polarity markings on a transformer are to determine proper phasing. On a schematic diagram, this is usually indicated by a dot at one end of each winding. When the marked end of the proimary goes positive, so do the marked ends of all the secondaries...

[This message has been edited by NJwirenut (edited 01-07-2006).]
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/08/06 01:25 AM

Quote
The dot represents the line side, and the current must pass through in that direction.


In an AC circuit, the current flows in BOTH directions, reversing at 2X the line frequency.

The markings are there to make sure that the METER connected to the CT secondaries turns in the proper direction. It makes no difference at all to the CT which side of the donut is line or load. If you were to flip the CT connections (primary AND secondary) the system would operate just fine.

In a simple system using a CT to monitor a single conductor with an ammeter across the CT secondary, these "polarity" markings become completely meaningless. The "polarity" (or "phasing") of the connections only becomes important when multiple legs are being metered by a single device. Then you need to maintain the proper phase relationships between the current and voltage signals from all 3 phases in order for the meter to register correctly.
Posted By: RODALCO

Re: Current Transformer Safety! - 01/08/06 03:28 AM

In New Zealand the newer CT's (1990's onwards) are marked:

Primary: P1 lineside - P2 loadside.

Secondary: S1 polarity to meter - S2 return from current coil from meter. (S3 - S4 may be there on multitap CT's).

Older CT's were marked as:

Primary: P1 lineside - P2 loadside.
or manufactures CT label on primary side.
Sometimes with a dot.

Secondairy: com. 150 / 250 / 400 on a multitap CT.
also markings as M - L were used, M to meter, L from meter. ( these were single tap CT's ).

It is very important that the correct phasing relationship exists between the voltage circuit and current circuit.

e.g. Red potential, Red current S1 - S2 and so on.
With Yellow potential for example and Red current S1 - S2 the meter will go backwards.
Red current and S2 - S1 meter will go backwards too. + various other combinations possible.

On CT checks with testblock links open and meter connected and 3 phase load a meter potential was livened one at the time and the meter has to make a forward movement of the disc or pulsing LED. Then one fuse was inserted, second fuse, third fuse and an increase of meter has to be noticed. If not there may be wrong wiring.

Also the primary current / secondary current has to be measured with accurate clamp meters and the ratio confirmed.

Say for 250 / 5 CT tap.
Primary current 200 Amps / secondary 4 Amps.
usually there are some minor reading errors in the clamp meter but close enough to see if the CT is at the correct tap. ( 201A / 3.92A ). multiplier x 50.

For TOU metering a half hour load check was done and primary readings compared with readings obtained from standard.
Posted By: RODALCO

Re: Current Transformer Safety! - 01/08/06 06:51 AM

It's important to do a job properly !!
CT's in the correct way.

I case of multitap CT's it is very important that P1 - P2 sides S1 - S2 sides are connected up correctly if high accuracy is required.
On the lower tappings there is a compensating wire in parallel with the main CT current wire. This one is of smaller diameter. It's there to allowe for compensation to have good acuracy.
Some classes are stated as followes
500/750/1000/5 Amps class 1.0/1.0/0.5
The compensation is for the 500 and 750 tap.
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/08/06 03:40 PM

XtheEdgeX:

Your original quote:

Quote
And don't forget that ct's are directional. There is a polarity mark on them, to distinguish the direction of the flow of current.


CTs are NOT "directional". Current flows equally well through them on either half of the sine wave cycle. If you want to see a "directional" device, how about a diode? The polarity markings on a CT have nothing to do the "direction of the flow of current". The current flows in BOTH DIRECTIONS in a transformer, because it is an AC device.

The markings are there to establish the PHASE RELATIONSHIP between the primary and secondary sides.

No, I haven't wired any substations, but I have installed a lot of CTs on control systems, motor drives, energy management systems, and the like. And I always follow the polarity markings when doing so. if nothing else, it makes for easier troubleshooting down the line. Nobody is arguing the need for following industry standard practices, only your overly simplified explanation of the reasons BEHIND these practices.

Knowing the WHYS as well as the HOWS behind the work we do is what separates the electrician from a monkey pulling wire through a pipe.



[This message has been edited by NJwirenut (edited 01-08-2006).]
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/08/06 09:42 PM

Quote
The ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced.
The ratio of the current transformer can be modified by altering the number of secondary turns by forward or backwinding the secondary lead through the window of the current transformer. By adding secondary turns, the same primary current will result in a decrease in secondary output. By subtracting turns, the same primary current will result in greater secondary output.


So exactly what there makes a CT "directional"? It behaves like ANY OTHER TRANSFORMER.

Adding secondary turns to a normal transformer will increase secondary VOLTAGE, while dropping secondary CURRENT. Adding primary turns will DROP secondary voltage, while increasing current (subject to limitations of wire size and core saturation).

The fact that added windings (such as those through a CT window) all need to be wound in the same direction is nothing specific to a CT, either. Again, it has to do with the relative phase relationships between the windings. The same way that the LV windings on a buck/boost transformer can be connected to add or subtract voltage, by connecting them series-aiding or series-opposing.

Not an issue of "direction of current flow", but one of relative phase relationship.
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/08/06 10:27 PM

Quote
If you wind the conductor through the window one direction, you get a result, if you wind the same conductor through the window in the opposite direction, you get a different result, making a CT directional to the flow of the current.


Take a 100:5 CT. Place a 5A ammeter across the secondary. Run 2 loops of wire through the window, and pass a 50 A current through the wire. What does the ammeter read? 5A, of course.

Now reverse the direction of the 2 loop winding. What does the ammeter read now? Still 5A? Hmmm...

The CT itself isn't "directional". It is just that the majority of applications involve more than one CT, and the MEASURING EQUIPMENT connected to the CT (and PT) secondaries depends on a precise phase relationship in order to read correctly.
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/08/06 11:33 PM

Quote
You are wrong in that assumption.


WHAT assumption?

Quote


Hmmm. That page shows the EXACT example I gave, of a 100:5 CT converted to 50:5 (or 25:5) with additional primary turns. I don't remember saying ANYTHING about added secondary turns, but the Simpson page gives a siimilarly good, clear explanation of those installations.

[quote]And what about the bar type CT's here. You better mount them with polarity in mind. ]http://www.kilowattclassroom.com/Archive/CurrentTransf.pdf[/quote]

Yeah, kinda hard to put an extra turn through one of those, ain't it? But again, when used with a single ammeter as an indicator, it really doesn't make a bit of difference which way the thing is oriented. The polarity markings are only important when the downstream equipment cares about the phase relationship between multiple phases, or between volatge and current signals from the same phase. This would include MOST CT applications, as they are USUALLY used in sets of 3, to feed a polyphase metering setup.

I don't think we disagree AT ALL about the need for observing proper polarity in a CT metering installation. But where I did disagree is in calling a CT a "directional" device. It works just the same no matter what "direction" the current is flowing, same as any other transformer. But the equipment DOWNSTREAM of the CT usually cares very much about such issues.

[This message has been edited by NJwirenut (edited 01-08-2006).]
Posted By: kdal

Re: Current Transformer Safety! - 01/11/06 01:13 AM

XtheEdgeX
I think where this misunderstanding started is when you stated, "And don't forget that ct's are directional. There is a polarity mark on them, to distinguish the direction of the flow of current."
The "direction of the flow of current" part is misleading. When you say "polarity" and the "direction of flow" it sounds as if you are talking DC.
I have to agree with NJwirenut.
There is no direction of flow with ac. current. This is more of an orientation issue as in line and load.
Do you not agree that Ac current has no direction of flow ? If you say that it does have a direction of flow could you please tell me which way it flows.
Posted By: NJwirenut

Re: Current Transformer Safety! - 01/18/06 06:01 PM

Gotta love people who delete all their posts when they are questioned about one of their statements...

Kind of destroys any future value in the thread for someone looking through the archives. [Linked Image]
Posted By: RODALCO

Re: Current Transformer Safety! - 01/18/06 06:32 PM

Quote
Gotta love people who delete all their posts when they are questioned about one of their statements...


It's a pity that all the statements re the CT issue from XtheEdgeX have been edited out by himself.

Editing is for spelling mistakes!!

Ithought that only a moderator could do that and so it should be. It destroys the discussion going on at that time.
Posted By: Trumpy

Re: Current Transformer Safety! - 01/18/06 06:53 PM

Ray,
Quote
I thought that only a moderator could do that and so it should be. It destroys the discussion going on at that time.

Members can edit their own posts, for things like spelling errors and the like.
However, removing the whole thing, as Mr X has chosen to do is a bit on the nose.
However, only a moderator can remove the posts entirely. [Linked Image]
Posted By: XtheEdgeX

Re: Current Transformer Safety! - 01/19/06 01:31 AM

I removed my posts in an attempt to clean up the mess I started, but yet again, I aggravated some. I obviously got off on the wrong foot here. All I was trying to say, was that a properly installed CT has the polarity mark facing the source, in order to have the secondary outputs to the meter be in the proper orientation, for the meter to read correctly. I don't think I deserved the third degree because my first post was too simplified. But, from now on, if I post, I will be sure to choose my words more carefully, as to not be misleading. Sorry for getting on everyone's nerves.
Posted By: Trumpy

Re: Current Transformer Safety! - 01/19/06 01:56 AM

XtheEdgeX,
I accept your apology and sure we have all said things on this BB that we may regret, but to go through and delete your entire postings from a large thread like this has left the whole thing rather dis-jointed.
{end of subject, lets just keep this thread on topic, everyone}. [Linked Image]
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