Disconnecting current transformer leads from a meter.

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JdoubleU

Senior Member
I have a power quality meter that needs to be sent back to the factor for calibration. What is the best way to disconnect the CT leads on the meter without having to shut down the building.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
That aside I would say the only safe OSHA compliant way is to shut the building down. (And if you install the shorting blocks you would only have to do this once.)
How does the shorting block really change anything as far as shutting the power down and the electrical safe work practices?
 

kwired

Electron manager
Location
NE Nebraska
Can you get split core type CT's?

As mentioned you may have to shut down this time but can maybe install different equipment so that if there is a next time you wont have to.

I guess reinstalling your calibrated equipment will be the next time.
 

zog

Senior Member
Location
Charlotte, NC
I have a power quality meter that needs to be sent back to the factor for calibration. What is the best way to disconnect the CT leads on the meter without having to shut down the building.

Do not open circuit the seconday of a CT with load on the system, you need to short the seconday of the CT, there should be shorting blocks to do this and if not you will need to shut down the system.
 

JdoubleU

Senior Member
Do not open circuit the seconday of a CT with load on the system, you need to short the seconday of the CT, there should be shorting blocks to do this and if not you will need to shut down the system.

How do the shorting blocks work? I have never used them.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
How do the shorting blocks work? I have never used them.
They're used like a make-before-break switch. The shorting wires/jumpers/bars bridge the CT's metering terminals before(!) disconnecting any metering wires.

Don't worry, there shouldn't be any spark, and the metering circuit's neutral should be bonded, but treat the terminals as hot anyway.
 

JdoubleU

Senior Member
So the shorting Block just completes the circuit. Will there me something I have to switch so that they are shorted out. I am not there so I am just asking questions blindly.
 

stevebea

Senior Member
Location
Southeastern PA
So the shorting Block just completes the circuit. Will there me something I have to switch so that they are shorted out. I am not there so I am just asking questions blindly.

The shorting block should have threaded shorting screws to short the secondary of the CT. You can see in the pic screws stored in the block to be used for shorting.
 

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JdoubleU

Senior Member
disregard my last message. These meters do have shorting blocks. So do I just just shut power off the meter, Install the screws, and then at that point I can disconnect the leads that go to the meter.
 

GMc

Senior Member
disregard my last message. These meters do have shorting blocks. So do I just just shut power off the meter, Install the screws, and then at that point I can disconnect the leads that go to the meter.

You don't have to shut the power off to the meter first, in fact you can verify the ct was shorted if you leave the meter powered and watch the current drop to 0.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The issue of killing the power and the required PPE will be based on the location and voltage of the CT circuit and its shorting terminal block. Many are located in parts of the equipment where you have an arc flash exposure and putting the screws in the shorting block is still working on energized equipment, if the voltage exceeds 50 volts, and it is a rare case where you are permitted to do that.
 

stevebea

Senior Member
Location
Southeastern PA
disregard my last message. These meters do have shorting blocks. So do I just just shut power off the meter, Install the screws, and then at that point I can disconnect the leads that go to the meter.

Is the secondary (shorted) side of CT jumpered to ground? Is there a disconnect ahead of the Power Quality Meter? After all the above is done, only then is it safe to disconnect the instrument (meter) side of the shorting block.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
........And hopefully nobody threw away the screws after they removed them! They should be in the holders on the side of the block if yours is like what is pictured.
 

G._S._Ohm

Senior Member
Location
DC area
"A current transformer should never be open-circuited while main current is passing through the primary winding.

If the load is removed from the secondary winding while the main circuit current is flowing, most of the primary winding current becomes magnetizing current, but the vector angles change in such a way as to keep the total current in the primary the same as before. Because the main circuit is now mostly magnetizing current, the flux in the core shoots up to a high level and a very high voltage appears across the secondary.

Due to the high turn ratio usually found in these transformers, the voltage in this condition can reach a dangerously high level, which can break down the insulation.

It also becomes a hazard to personnel.

The high flux can saturate the core and result in strong residual magnetism left in the core, thereby increasing magnetization current and introducing error in the transformation ratio.

We strongly recommend that one put a short on the secondary winding before removing the secondary load while the main current is flowing through the primary winding."
 

SG-1

Senior Member
You don't have to shut the power off to the meter first, in fact you can verify the ct was shorted if you leave the meter powered and watch the current drop to 0.

If the current drops to zero the current path is probably open, not shorted. The shorting screws will cause the current read by the meter to drop because there is now a parallel path. I would expect that if the meter is functioning it will still read some current. A clamp-on type amprobe can be used to verify each phase reading has been reduced after the shorting screws are in place, if the meter is dark.

Get a current reading before & after installing the shorting screws. The measurement will not change on the wires between the shorting block and the CTs. It will be lower on the wires between the shorting block & the meter.
 

GMc

Senior Member
If the current drops to zero the current path is probably open, not shorted. The shorting screws will cause the current read by the meter to drop because there is now a parallel path. I would expect that if the meter is functioning it will still read some current. A clamp-on type amprobe can be used to verify each phase reading has been reduced after the shorting screws are in place, if the meter is dark.

Get a current reading before & after installing the shorting screws. The measurement will not change on the wires between the shorting block and the CTs. It will be lower on the wires between the shorting block & the meter.

Have you ever worked with shorting blocks? The blocks are used to Short the 2 terminals of a particular CT. When the 2 terminals are shorted, example CT-1 for Phase 1, the current read on the meter for phase 1 will read 0
 
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