Current Transformer Secondary - Lock Out Tag Out Mechanism

[Protection Engineer]

Hello,
I am wondering if anyone has come across a method to "Lock Out" a Current Transformer secondary circuit? I spoke with a colleague who had attended a conference many years ago. He mentioned seeing a locking mechanism that could be placed over the shorting bar in a breaker cabinet or shorting blocks in the control/relay house. Maybe a clamshell type shape. Has anyone come across this? I tried Googling around for an answer, but could not find one. Our company is improving our CT safety procedures. We are currently implementing a Tag Out approach, but would be interested in a Lock Out mechanism as well.

Thanks for your time.

Justin

 

[JG2828]

ABB has an option to do it at the FT1 test switch. I have never seen an option to do it at the terminal block.

https://library.e.abb.com/public/237bcda3f2b7eb14c1257b130056ca70/FT1 DB41-077April2010_2.pdf

 

[davidbeach]

Remember though, that opening the blades of a current position on an FT block Does NOT break the electrical connection between the CT circuit and the device connected to the top of the FT block.

 

[scottf]

davidbeach beat me to it....

Shorting a CT's secondary is not isolating the circuit. It does keep the circuit from having high voltages present due to open-circuit conditions. However, in theory, if the secondary circuit doesn't have ground reference, the entire circuit can float up in voltage.

With that said, it's hard for me to imagine a method to lock-out a CT secondary circuit. You would almost not want to based on the dangers of accidentally opening the CT secondary circuit.

 

[waross]

I always took "Lock-out, Tag-out" to mean securing the circuit in a safe condition.
For a CT that would be grounding and shorting, with some type of locking mechanism to prevent tampering.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Protection Engineer]

@JG2828 - thanks for the response; unfortunately, we use FT-1 switches at the relay racks only, which is typically the end of the CT secondary run. Ideally, this lock out mechanism would be applied at the PCB cabinet, or closest to the source as to leave the rest of the circuit available for work.

@davidbeach - agreed, while the current is shunted away from the end device, the current "stab-in" harps are still providing an electrical connection beyond the FT-1 switch.

@scottf - agreed, the circuit is not isolated, but "made safe" for work downstream of the short with proper precautions. Our standard is to ground the CT secondary at a single point where it initially terminates in the relay/control house. This lock-out method would just be a physical barrier blocking an unknowing worker from removing shorting screws. I think it's merely a clamshell that can wrap around terminal blocks and have a lock placed; not a truly isolated circuit. The lock-out method I described would still require a shorted secondary, so an open CT event should not be any more likely.

I think the question we've been throwing around is, we utilize Clearances and LOTO for energy source controls, why not apply that to CT's? Of course, a CT is a different beast, but from a worker's safety perspective, perhaps the same procedures should be followed? (or as close to it as we can get)

 

[thermionic1]

One of my colleagues made something very similar a few years ago with a 3D printer. Now there appears to be a company making in bulk. We use these to LOTO individual FT poles open, or shorted. You may be able to contact the company and see if they can make something for you. These work in open and closed position.

BTW, I'm sure it's obvious, but one way to LOTO the secondary is to LOTO the Primary. Nowadays, 95% of the time for an upgrade, modification or replacement is handled that way in a utility environment. In MV/LV that's racking out the breaker. In HV it's breaker open, disconnects open and locked / tagged.

FT LOTO

 

[stevenal]

Put your lock on the breaker cabinet door.

 

[JG2828]

Remember though, that opening the blades of a current position on an FT block Does NOT break the electrical connection between the CT circuit and the device connected to the top of the FT block.

Yes, very good point. But then again, using a shorting terminal block does the same thing, unless you remove the cables from the secondary side of the terminal block also.