Bonding of control transfromer

[reyamkram]

When did bonding the control transformer rule, start, or what cycle of the NEC did the bonding of X2 on the control transformer start??
because I have some machines where the control transformers are not bonded,

Thank you, for any and all information.

 

[ptonsparky]

It is not always required.

 

[tom baker]

I dont know the answer. But if X2 is not bonded, its harder to troubleshoot, as the 120 doesn't have a reference. The SBJ is what allows a line to case fault to clear.

 

[jim dungar]

When did bonding the control transformer rule, start, or what cycle of the NEC did the bonding of X2 on the control transformer start??

I wouldn't be surprised to find it's been only for the past 30 years or so.
It was probably pushed heavily by the control and machine panel standards like UL508A, JIC (in the old days), and NFPA 79.

I know one major manufacturer that routinely grounded the X1 side of their transformers.

 

[tom baker]

It is not always required.

I had to look for this, 250.21 A 3 gives an exception for control circuits, with qualified persons servicing the installation.
I have been doing motor control work since 1980 and have installed SBJ on X2 per NEC.
I looked in my oldest code book, 1956 and didn't see it or actually any language on transformers
Maybe JREF (mod) will know

 

[tom baker]

When did bonding the control transformer rule, start, or what cycle of the NEC did the bonding of X2 on the control transformer start??
because I have some machines where the control transformers are not bonded,

Thank you, for any and all information.

Under the 2020, and likely earlier, you would need to meet the requirements in 250.21 A 3.
Do you know when control panels were made? Or drawings?