Current Transformer connectors
- Thread starter james
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bphgravity
Senior Member
- Location
- Florida
Re: Current Transformer connectors
Not per the NEC. Follow all manufacturer's requirements and instructions.
Not per the NEC. Follow all manufacturer's requirements and instructions.
wanderer20001us
Senior Member
Re: Current Transformer connectors
If you've ever seen the damage an open-circuit CT can produce, you will use ring terminals on CT secondary circuit connections.
If you've ever seen the damage an open-circuit CT can produce, you will use ring terminals on CT secondary circuit connections.
bphgravity
Senior Member
- Location
- Florida
Re: Current Transformer connectors
An open grounded neutral connection on a multiwire circuit/service can be just as destructive, yet I don't see any special precautions being made for that possibility?
(other than 300.13(B)
)
[ October 18, 2005, 11:18 AM: Message edited by: bphgravity ]
An open grounded neutral connection on a multiwire circuit/service can be just as destructive, yet I don't see any special precautions being made for that possibility?
(other than 300.13(B)
)[ October 18, 2005, 11:18 AM: Message edited by: bphgravity ]
Re: Current Transformer connectors
The difference of an open neutral of a 120/240v suppy and an open conductor on a CT circuit is that as open neutral will be 240v max but the voltage induced across an open circuited CT skyrockets. Think of it this way, a 100:5 CT with a 480v conductor passing through the center essentially ends up to be a 20:1 ratio, the 1 being the 480v conductor, which would mean that the voltage across an open CT may reach as high as 480x20 as I understand it. Dangerous.
The difference of an open neutral of a 120/240v suppy and an open conductor on a CT circuit is that as open neutral will be 240v max but the voltage induced across an open circuited CT skyrockets. Think of it this way, a 100:5 CT with a 480v conductor passing through the center essentially ends up to be a 20:1 ratio, the 1 being the 480v conductor, which would mean that the voltage across an open CT may reach as high as 480x20 as I understand it. Dangerous.
rcwilson
Senior Member
- Location
- Redmond, WA
Re: Current Transformer connectors
Many of us specify ring tongue terminals on CT circuits to prevent a burn down if the wire comes loose. If a Voltage Transformer or normal power or control circuit wire gets loose, a fuse or CB will usually open to limit the damage. But an open CT circuit can blow up the CT and lead to switchgear failure. The wires are more secure with a ring terminal. They can't slip out or get pulled out accidentally as easily.
If an electrician starts loosening the screw on a ring tongue CT termination, the ring holds the terminal on the screw and he might see the little arcs that start jumping across the threads. This might alert him to tighten it back up and recheck what he is doing.
Many of us specify ring tongue terminals on CT circuits to prevent a burn down if the wire comes loose. If a Voltage Transformer or normal power or control circuit wire gets loose, a fuse or CB will usually open to limit the damage. But an open CT circuit can blow up the CT and lead to switchgear failure. The wires are more secure with a ring terminal. They can't slip out or get pulled out accidentally as easily.
If an electrician starts loosening the screw on a ring tongue CT termination, the ring holds the terminal on the screw and he might see the little arcs that start jumping across the threads. This might alert him to tighten it back up and recheck what he is doing.
Re: Current Transformer connectors
I posted this before (maybe on ECN). This is a picture of a lose CT lead. We had an entire hospital shut down changing over the service to this new switch gear. We had the MRI building back up first and as we were working I kept hearing arcing and the smell of burnt electrical was in the air. When I opened this cabinet I saw the sparking and smoke coming from this terminal block. What had happened was the lose wire you see in the pic was stuffed behind the block in contact with the metal mounting plate. (I took the picture after pulling it out.) Of course I had to land it energized. Luckily there was no damage to the CT.
I posted this before (maybe on ECN). This is a picture of a lose CT lead. We had an entire hospital shut down changing over the service to this new switch gear. We had the MRI building back up first and as we were working I kept hearing arcing and the smell of burnt electrical was in the air. When I opened this cabinet I saw the sparking and smoke coming from this terminal block. What had happened was the lose wire you see in the pic was stuffed behind the block in contact with the metal mounting plate. (I took the picture after pulling it out.) Of course I had to land it energized. Luckily there was no damage to the CT.
- Location
- Illinois
- Occupation
- retired electrician
Re: Current Transformer connectors
For those who haven't seen a CT terminal block before. If you look at the picture in Nick's post you will see a metal strip running the length of the terminal block. The strip has holes in it that line up with tapped holes in the terminal below. There will be screws provided that you install in these holes to short out the CT so that you can work on the load side of the CT circuit without open circuiting the CT.
Don
For those who haven't seen a CT terminal block before. If you look at the picture in Nick's post you will see a metal strip running the length of the terminal block. The strip has holes in it that line up with tapped holes in the terminal below. There will be screws provided that you install in these holes to short out the CT so that you can work on the load side of the CT circuit without open circuiting the CT.
Don
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