Grounding of 24 VDC Power Supplies

[tom baker]

I don't have a current copy of UL 508. Is there a requirement for the DC power supply common in UL 508 to grounded?
The NEC in Art 250 does not require grounding of DC power supplies under 50 volts.

 

[bthielen]

Re: Grounding of 24 VDC Power Supplies

Quickly breezing through the book I found this. Hope it is what you are looking for.


UL508A, September 1, 2005

16.1 A secondary circuit that contains field wiring terminals and is supplied from a power transformer, control transformer, or power supply shall have the secondary grounded under any of the following conditions:

a) 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.


Bob

 

[tom baker]

Re: Grounding of 24 VDC Power Supplies

Yes that helps.
I'll have to get a current copy of UL508A

 

[frenchelectrican]

Re: Grounding of 24 VDC Power Supplies

i hate to butt in here but one question i want to clearify it well ,,

UL508A, September 1, 2005

16.1 A secondary circuit that contains field wiring terminals and is supplied from a power transformer, control transformer, or power supply shall have the secondary grounded under any of the following conditions:

a) 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.

Do this cover both AC and DC system i am try to check out this question to make sure we are on right track here

Merci , Marc

 

[eric stromberg]

Re: Grounding of 24 VDC Power Supplies

I was a licensed Fire alarm superindent for the state of Texas for 7 years. In that time, i installed hundreds of fire alarm panels. All these panels had 24 Volt power supplies (fed from 120 VAC). The outputs of the power supplies were not grounded, per se. This is because the fire alarm codes require that ground faults be detectable on either side of the power supply. The power used for the fire alarm circuits was rectified but not filtered. The + side of the power supply was connected through a diode to a filter capacitor and then to chassis ground. This resulted in a + voltage that measured 16 volts to chassis and a - voltage that measured 40 volts to chassis. None of this would have been possible had either side of the power supply been required to be grounded.