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#149894 - 05/21/04 02:54 AM Current Transformer Safety!
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
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?.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#149895 - 06/03/04 02:40 PM Re: Current Transformer Safety!
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
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.

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#149896 - 06/04/04 02:25 AM Re: Current Transformer Safety!
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
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!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#149897 - 06/04/04 10:31 AM Re: Current Transformer Safety!
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
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.

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#149898 - 06/06/04 10:41 AM Re: Current Transformer Safety!
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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


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).]

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#149899 - 06/07/04 08:37 AM Re: Current Transformer Safety!
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
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!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#149900 - 06/18/04 03:15 AM Re: Current Transformer Safety!
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
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!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#149901 - 06/19/04 09:08 AM Re: Current Transformer Safety!
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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.

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#149902 - 07/10/04 03:35 AM Re: Current Transformer Safety!
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Bjarney,
Do these CT's have a stated "Burden Rating"?.
This has to be taken into account too.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#149903 - 07/10/04 08:34 PM Re: Current Transformer Safety!
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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”.

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