View Single Post
Old December 6th, 2002, 04:48 PM   #5
Eric Nelson
Lifetime Supporting Member + Moderator
United States

Eric Nelson is offline
Eric Nelson's Avatar
Join Date: Apr 2002
Location: Randolph, NJ
Posts: 4,346
120vAC Control Circuit Transformer...

I always ground one side of a control transformer (X2 out of habit). I look at it as a new power source, so by grounding X2, you create a "neutral" conductor. Although, there's no requirement that it MUST be grounded...

From NFPA 79...

9 Control circuits

9.3 Grounding of control circuits

Grounded or ungrounded control circuits shall be permitted
as provided in 19.7. Ground faults on any control circuit shall
not cause unintentional starting or dangerous movements, or
prevent stopping of the machine.

19 Grounded circuits and equipment grounding

19.7 Control circuits

Control circuits shall be permitted to be grounded or un-
grounded. Where grounding is provided, that side of the
circuit common to the coils shall be grounded at the control
transformer if alternating current or at the power suppy termi-
nal if direct current.

Exception No. 1: Exposed control circuits as permitted by 9.2.1, Ex-
ception No. 2, shall be grounded.

Exception No. 2: Overload relay contacts shall be permitted to be
connected between the coil and the grounded conductor where the con-
ductors between such contacts and coils of magnetic devices do not ex-
tend beyond the control enclosure.

9.2 Control circuit voltages

9.2.1 Alternating-current (ac) control voltage shall be 120 volts
or less, single phase. Where the supply voltage is greater than
120 volts, the control voltage shall be provided from a trans-
former with an isolated secondary winding.

Exception No. 1: Other voltages shall be provided, where necessary,
for the operation of electronic, precision, static, or similar devices used
in the control circuit.

Exception No. 2: Exposed, grounded control circuits shall be permit-
ted where supplied by a transformer having a primary rating of not
more than 120 volts, a secondary rating of not more than 25 volts,
and a capacity of not more than 50 volt-amperes

Exception No. 3: Any electromechanical magnetic device having an
inrush current exceeding 20 amperes at 120 volts shall be permitted to
be energized at line voltage through contactor or relay contacts. The
contactor or relay contacts shall break both sides of the line voltage cir-
cuit to the magnatic device. The relay coil shall be connected to the con-
trol circuit.

I guess that answers that...


  Reply With Quote