CT Shorting Block connection

[andy32821]

Hi,

I need to install a ct shorting block on an existing GE EPM 9900 energy meter. Most of the commercially available shorting blocks direct me to tie together and ground the ct negative leads. The GE manual recommends shorting blocks as an option, but the installation block diagram of the energy meter show 3 completely independent ct's. (3 phase 4 wire wye system.)

Is there any advantage to shorting the negative leads? Any way this could affect the meter?
I'm installing the shorting block on the next shutdown, I don't want to cause an additional shutdown to correct the wiring. The goal is to be able to replace the meter in the future without a power outage.

Thanks,
Andy

 

[LarryFine]

Shorting blocks prevent individual CT's from becoming PT's when the secondary circuit is opened, which is a result of removing the meter.

3ph CT secondary circuits normally have the "negatives" tied together and grounded through the supply neutral, like any 3ph wye circuit.

One has nothing to do with the other.

 

[jim dungar]

It typically does not really depend on the number of CTs or system configuration, but rather on how your instruments are wired.

Some people prefer to make the ground/common connection as close to the CT as possible, thus all shorting bars just need to be 4-pole style.
For other CT installations, especially where multiple instruments will be connected in series, it is often better to use 6-pole shorting blocks up to the final device.

 

[infinity]

We use these at the CT end, all other connections are near the metering equipment.

 

[RumRunner]

Hi,

I need to install a ct shorting block on an existing GE EPM 9900 energy meter. Most of the commercially available shorting blocks direct me to tie together and ground the ct negative leads. The GE manual recommends shorting blocks as an option, but the installation block diagram of the energy meter show 3 completely independent ct's. (3 phase 4 wire wye system.)

Is there any advantage to shorting the negative leads? Any way this could affect the meter?
I'm installing the shorting block on the next shutdown, I don't want to cause an additional shutdown to correct the wiring. The goal is to be able to replace the meter in the future without a power outage.

Thanks,
Andy

Shorting the “negative” together in a multiple array of CT's is a recognized practice. This is the traditional modality in terms of simplicity and even labor-saving that gives an overall economic advantage. ..a darling of contractors.


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Anything to save money.


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I am not keen in calling one leg of the CT terminal “negative” because it gives the impression of a DC source.. . .but since you initiated the post… I will use it for the sake of decorum.


​

Generally, when you have a current transformer you should maintain a closed circuit. It could be either a shorting block or the meter must be always connected.​

Leaving the CT secondary leads open--could pose a hazard.. . . a shocking hazard. It acts like a power transformer.
In a situation where there is a need to replace the meter, it is important to short the two terminals to avoid this shocking hazard.


​

The GE recommendation of providing shorting block is a good option. You did not mention the “tying together of negative terminals” in the GE manual.
For the GE engineers, having three independent energy meters provide flexibility in the field. It insures that each set of leads to the meters can be handled independently without having the other meters accidentally disturbed or disconnected during maintenance.


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Shorting the negative leads or even the two leads should not affect the meters.

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You don't have to shut the power off to replace the meter/s as long as the CT leads are properly shorted before disconnecting the meters.


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All of the above of course is contingent upon the proficiency of the electricians working on the system. . . and your knowledge of the system.
​

 

[tkb]

We use these at the CT end, all other connections are near the metering equipment.

View attachment 22096

What is the purpose of the shorting block at the CT’s?
I have never seen it before, only at the meter.

 

[infinity]

What is the purpose of the shorting block at the CT’s?
I have never seen it before, only at the meter.

In the case of the ones in the photo they serve two purposes, one to provide a splice point for the CT conductors (which were too short to reach the metering equipment) and secondly to provide points within the switchboard section where the CT's could be shorted so that they may be worked on in the future.

 

[tkb]

In the case of the ones in the photo they serve two purposes, one to provide a splice point for the CT conductors (which were too short to reach the metering equipment) and secondly to provide points within the switchboard section where the CT's could be shorted so that they may be worked on in the future.

What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

It seems like an extra unnecessary expense when butt splices would do.
Out of all of the CTs that I have installed over the years, we never had to go back to do any work on them, just the meters.

 

[kingpb]

STATES test switches work quite well, also.

 

[jim dungar]

What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

There have been several project I have had to connect relaying and power quality metering where I was not involved in the installation and the CTs were installed backwards to what I needed. Having access to all 6 CT leads, at a shorting block allowed us to effectively change the CT polarity as needed.

 

[RumRunner]

What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

It seems like an extra unnecessary expense when butt splices would do.
Out of all of the CTs that I have installed over the years, we never had to go back to do any work on them, just the meters.

A lot of CT manufacturers don't follow marking conventions when it comes to designating the “polarity” of the secondary.

Sometimes, the schematic symbols differ from the actual marking on the CT itself.
In the schematic. . . one end of the coil has a square dot. . . on others, it has a solid round dot.
This designates where the “positive” lead goes together with the source lead.

This is where the face of the CT is marked “source”.

The secondary wires (the ones connected to the meter) are color-coded. It is usually black and white. White being “negative”.



There is a way to check the polarity in case all these markings are missing. . . by using a 9 volt battery.

But you have to know the procedure on how to do this. . .don't just connect your battery to a 480 volt AC.

 

[LarryFine]

Just to throw this in, every CT shorting device I've seen installs directly onto the terminals on the body of the CT itself.

 

[jim dungar]

Just to throw this in, every CT shorting device I've seen installs directly onto the terminals on the body of the CT itself.

And yet probably none of the window style 600V CTs I have ever specified has had integral shorting devices. Which is good because most of the CT placements have not been easily accessible, especially in energized gear.

 

[LarryFine]

It's been years, but I remember a bent metal bar that probably came with the CT, and was removed after connecting meter wiring.

 

[ATSman]

In line shorting devices ant CT theory

In line shorting devices ant CT theory

In response to the OP I just want to add that there was a case we ran into where the shutdown of load would take
a pile of paperwork in order to change the power meter where no shorting block existed. We talked the customer into installing a shorting block to facilitate future repairs.
We used an in-line shorting device that used a needle and shorting wires that would pierce the wire when clamped on it to short out the 3 signal wires to neutral coming from the CTs. It really worked slick and allowed us to install a shorting block and new meter without a power shutdown. The devices were then removed without any damage to the control wires. I forget the name of the device but you can google it.

The other subject has to do with the open circuit voltage at the CT secondary terminals. I recall Westinghouse stating that depending on the load current thru the CT, the terminals can cause the CT to act as a PT (as others have mentioned) and the voltage can rise to infinity. I can attest to this when I was thrown against a chain link fence of a MV substation when I was installing a test plug on a 50/51 protective relay. The Port Engineer who I was working with replied " What's the matter Tony, did you trip?" :rant:
He had no Idea what shocked me!

 

[infinity]

What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

It seems like an extra unnecessary expense when butt splices would do.
Out of all of the CTs that I have installed over the years, we never had to go back to do any work on them, just the meters.

There have been several project I have had to connect relaying and power quality metering where I was not involved in the installation and the CTs were installed backwards to what I needed. Having access to all 6 CT leads, at a shorting block allowed us to effectively change the CT polarity as needed.

Exactly this, we had several service switchboards where the CT's were wired backwards and needed to be fixed while the service was energized. Simply inserting the shunts back into the shorting blocks made this a simple fix.

Regarding butt splices, our metering vendor would never accept them in lieu if shorting blocks for this reason.

 

[meternerd]

Well...since I'm a retired 40+ year utility meter/relay tech, I have a couple of comments from a utility perspective. In my world, CT's are used for metering and also for input to protective devices such as over current and differential relays. I won't get into medium or high voltage CT's, though the same principles apply. As mentioned several times, open circuits of energized CT's can be damaging or deadly. Even when not connected in a circuit, they could potentially become a voltage source if left open circuited around strong magnetic fields. Most come from the factory shorted with jumpers or shorting type terminals that have the terminals shorted. Shorting jumpers are only removed when the CT circuit is complete. But there are numerous reasons for needing to work on energized CT circuits. Series type meter test setups, relay operation diagnostics, etc. All require energized access to the CT secondary circuits. Metering and relay CT's are usually connected to test switches but likely connect to shorting blocks as well. CT's used for relaying are not normally accessible when in service. The secondary leads run to a shorting terminal block somewhere in the circuit. As mentioned, 4 and 6 pole blocks are common. 12 pole blocks are also used. Most metering rated CT's come with a shorting bar installed that can be closed with a sliding jumper to allow for work on wiring while the CT's remain energized. Using a 6 pole shorting bar as an example, three terminals are wired to CT polarity terminals and three are wired to CT non-polarity terminals which are usually grounded either through a separate ground wire or the connection of the wires to a ground at the CT's that keeps a ground on the shorting bar. Either way, the only safe way to work on a CT was with the terminals in the circuit and grounded or shorted and grounded.

Of course there are exceptions, but that's the world of us utility meter/relay geeks. If this is too basic and therefore too boring, forget what I said. But trust me, open circuiting a high ratio CT from a 230KV substation transformer gives "terrifying" a new meaning. Don't ask how I know.

 

[meternerd]

Hi,

I need to install a ct shorting block on an existing GE EPM 9900 energy meter. Most of the commercially available shorting blocks direct me to tie together and ground the ct negative leads. The GE manual recommends shorting blocks as an option, but the installation block diagram of the energy meter show 3 completely independent ct's. (3 phase 4 wire wye system.)

Is there any advantage to shorting the negative leads? Any way this could affect the meter?
I'm installing the shorting block on the next shutdown, I don't want to cause an additional shutdown to correct the wiring. The goal is to be able to replace the meter in the future without a power outage.

Thanks,
Andy

In answer to your question, and disregarding all of the useless info I posted regarding utility work..........Yes, a shorting block makes life easier. Jumper all three of the CT non-polarity (your negative) leads together at the CT then ground them there or run one wire to the shorting block or run them separately to the shorting block and jumper and ground them there. Six wires from the shorting block to the meter. Just make sure you wire it to keep the shorting strip grounded so that when you install the shorting screws, they short AND ground the CT's on the polarity side as well. You didn't ask about polarity, so I assume you know the marking conventions.

 

[meternerd]

Sorry.....

Sorry.....

I don't know how to delete posts, so please disregard both of my former posts........I re-read your question and I wrote a bunch of useless information. I found the wiring diagram of your 9900 meter and I'm not clear what you have. You mentioned three separate CT's. Is that not what you have? If you do, then wire it just like the diagram shows with the shorting block between the CT's and the meter. Connect the CT polarity to one side of the terminals on the shorting block. Connect the other side to Ia, Ib and Ic on the meter. If you use a 4 terminal block, In will connect to the three "negative" non-polarity wires and ground. If you use a six terminal block, just jumper the three terminals together with a ground then to In. Polarity matters according to the diagram. Sorry for all the techno-baloney!

 

[ike5547]

Hi.

I'm being asked to install a cheapo electronic kWh meter in a separate room remote from the the CT locations (about 20 ft. away). The voltage is 120/208 3-phase. The CT's are 200:5.

Pretty much all of the discussions regarding shorting blocks and grounding of the CT's I've read so far have involved higher voltages. In the past I've installed these types of meters (E-MON D-MON) adjacent to the primary supply (low voltage) and in sight of the main circuit breaker(s). I wasn't too concerned.

My question is: How relevant would grounding the secondary of the CT's and shorting block be in my case? All conductors will be in steel conduit point a to z.

Thnx