Grounding X2 on a control transformer

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More questions for grounding X2

More questions for grounding X2

250.20 appears to say I have to ground the control transformer if its a 480 to 120volt. If it's 240 to 120 thats less than 150 volts to ground. Such as a dual voltage transformer. So do I still have to ground it? If I do ground the secondary side why can't I ground X1?
Is it because it is an industry standard? Because it doesn't say X2 in the code book.

250.21 (A) (3) seems to imply that I don't have to ground the control transformer if it meets a,b,&c. If I don't ground the secondary side of the control transformer how will my control fuse blow if I touch the X1 to ground?
 
You may bond either the X1 or the X2 terminal it is your design choice, neither the NEC nor UL508A address this issue.
 
Bowhunter said:
250.20 appears to say I have to ground the control transformer if its a 480 to 120volt. If it's 240 to 120 thats less than 150 volts to ground. Such as a dual voltage transformer. So do I still have to ground it? If I do ground the secondary side why can't I ground X1?
Is it because it is an industry standard? Because it doesn't say X2 in the code book.

250.21 (A) (3) seems to imply that I don't have to ground the control transformer if it meets a,b,&c. If I don't ground the secondary side of the control transformer how will my control fuse blow if I touch the X1 to ground?
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Don't have a code book in front of me, but I can't imagine someone not wanting to ground X2, other wise you will get a 60v neutral. Now this is old code memory but I believe ever time you establish a secondary with a neutral, it is defined as a "seperately derived system" and shall be grounded..Otherwise the circuit has no reference to ground.....correct me if Im wrong

It is industry standard that X1 is hot and X2 is neutral on 120v secondaries, or at least I have never seen it any other way. In the event of a ground fault in the secondary wiring, without a grounded X2 it would not blow the secondary fuses and perhaps be a shocking hazzard to a inosent bi-stander...
 
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jim dungar said:
You may bond either the X1 or the X2 terminal it is your design choice, neither the NEC nor UL508A address this issue.
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Not questioning your statement in any way, but just a question,have you ever seen a hot X2? In every control cabinet, MCC, and dwg I've every seen...X1 is hot, X2 is bonded neutral
 
Look at it this way if you dont ground the control xformer of say a conveyor belt in a factory.
And lets say a forklift bumps the conduit in the middle of the conveyor and shorts out the emergency stop button wires at the far end of the conveyor.
Now if it were grounded it would blow the control fuse and stop the conveyor.
However if the emergency stop was bridged AND ungrounded as someones arm was being ripped off by the conveyor.
If someone tried pushing the emergency stop button it would not work and when the accident were investigated and they found the control xformer not to be grounded you would be sued for sure if you did the install.
 
I'm not sure that you can comply with 250.21(3) a thru d. (you must comply with all), so I think your answer is 250.20(B)1. Where the system can be grounded....etc. JMSO
 
Mule said:
Not questioning your statement in any way, but just a question,have you ever seen a hot X2? In every control cabinet, MCC, and dwg I've every seen...X1 is hot, X2 is bonded neutral
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Yes, I have.
The last time was just a few months ago. It was in a 5kV synchronous motor starter built in the late early 80's.

Some motor control center manufacturers ground their X1 and fuse the X2. This arrangement allows the wiring in their starters to be routed easier.

This is definitely not the current norm, and I would not normally do it, but there is no code or standard preventing it.
 
Mule said:
Don't have a code book in front of me, but I can't imagine someone not wanting to ground X2, other wise you will get a 60v neutral. Now this is old code memory but I believe ever time you establish a secondary with a neutral, it is defined as a "seperately derived system" and shall be grounded..Otherwise the circuit has no reference to ground.....correct me if Im wrong

It is industry standard that X1 is hot and X2 is neutral on 120v secondaries, or at least I have never seen it any other way. In the event of a ground fault in the secondary wiring, without a grounded X2 it would not blow the secondary fuses and perhaps be a shocking hazzard to a inosent bi-stander...
Click to expand...

You will not have a neutral, you will have 120 v L-L whether one end of the winding is grounded or not. Unless the winding is center tapped there would not be 60 v measured anywhere.

Roger
 
roger said:
You will not have a neutral, you will have 120 v L-L whether one end of the winding is grounded or not. Unless the winding is center tapped there would not be 60 v measured anywhere.

Roger
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I agree and if you do ground one winding everything that is grounded on the machine could act as a current carrying conductor.
 
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