UL 508A requirements for an ungrounded 24 VDC circuit

[larsahl]

Hi,

What are the UL 508A requirements for an ungrounded 24 VDC circuit that does not contain any field wiring? vs. circuit that does contain field wiring?

There is a bit of confusion with the additional requirements in 66.3.2.
Shall there be seen as any type of circuit with and without field wiring?

66.3.2 The secondary winding of a control transformer or the secondary of a power supply is not required
to be grounded, as specified in 16.1, when the secondary supplies only devices included as part of the
controlled machine, not other machines or circuits, and is provided with a monitoring device that:

a) Provides an audible or visual indication when a ground fault occurs in any ungrounded
conductor, such as a panel mounted indicator light or display, or one that interrupts the circuit in the
event of a ground fault, such as a ground fault protective device; and

b) Is arranged with the control circuit so that the machine cannot restart with a detected ground
fault.

 

[tortuga]

Hi,

What are the UL 508A requirements for an ungrounded 24 VDC circuit that does not contain any field wiring? vs. circuit that does contain field wiring?

You quoted the section for a circuit that does not contain any field wiring.
66.3.3 covers a circuit for field wiring:
" A transformer that supplies circuits that are not part of the controlled machinery shall comply with
Section 16, Transformer Secondary Grounding."
Then 16.1(a) covers
"When the secondary voltage is less than 50 volts;
and
1) The supply to the primary is over 150 volts to ground;
or
2) The supply to the primary at any voltage is ungrounded;"

Hope that helps.

 

[petersonra]

I would caution you that sections 65, 66, and 67 or UL508a only apply to industrial control panels for industrial machinery. If it is not an industrial machine per the definition of such, these sections just do not apply. A lot of people seem to miss this.

Personally, I just ground all DC power supplies.