UL 508A - Control Panel Grounding Point for AC and DC ci

[kyle_panel]

i am asking for electrical expert to help me here. pls see below

give specification
120/1/60 incoming power input, then being step down by a control power supply(approx 50watt ) to 24VDC to feed PLC. This control panel is mixed with 120VAC and 24VDC control voltage components (AC-contactor, overload, relay, DC-PLC)

should i ground both AC ( from G terminal of power supply) and -V ( from -24VDC terminal) to the baseplate of the control panel??? Both circuits contains field wiring terminals.

I believe it some cases, it just need to ground the 120V AC circuit but not the 24VDC, right?

note: control power supply contain terminals of L,N,G,-V,+V

thanks

 

[petersonra]

Re: UL 508A - Control Panel Grounding Point for AC and DC ci

I think you will find that for 24V control circuits you are not REQUIRED to ground them. In fact 120V control voltage is often ungrounded in industrial control panels.

That does not mean there are not good reasons to do so.

[ April 12, 2005, 06:54 PM: Message edited by: petersonra ]

 

[megloff11x]

Re: UL 508A - Control Panel Grounding Point for AC and DC ci

If you have other DC power supplies with other DC voltages, it may be prudent to ground DC(-) on all such supplies to avoid a floating voltage issue between the various DC levels. You do risk ground noise but it usually isn't an issue. An added adavantage of grounding DC minus is when you want to measure DC Volts, just put the black lead on any exposed chassis metal and the red lead on what you want to measure. No hunting for a DC ground terminal, which may have you playing "twister" to reach it.

Definitely ground the AC side "gnd-in" terminal to the power supply. If you get power noise, you may just have a bad power supply. I went through several brands once on a control computer. I had a situation where it would run fine plugged in at one part of the plant and wouldn't boot up in another.

These AC-to-DC supplies are designed to be as cheap as possible and it shows. The rule of thumb is get one rated for twice as many Watts as you need for a switching supply. I'm awaiting fan #5 on my 18-month old PC's supply here. I've had the best luck with Power-One brand, especially when we had to do CE mandated 2000V zap and brownout tests.

Another trick if the supply will do it, power it phase-phase instead of phase-neutral. The switcher supplies are usually 115/230 and running at 208-240, they will withstand a brownout better than at 115.

A good DC power supply costs more, but what is your troubleshooting hourly rate?

 

[tom baker]

Re: UL 508A - Control Panel Grounding Point for AC and DC ci

The DC circuit common must be grounded, per the NEC and UL 508. In the NEC see Art 480. If you don't ground the common then you have to fuse both DC+ and common.