SMPC vs. Control Transformer

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Johnshealey

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I'm designing a control panel with 208V 3? mains. The control voltage is 24VDC. The control transformer would only supply 120V to the 24VDC SMPS. Can I eliminate the control transformer and simply power the SMPS directly from the 208V mains? Of course it will work fine, but I'm concerned about satisfying NFPA 79 2012 Article 9.1.1.1.
 
Johnshealey said:
... Can I eliminate the control transformer and simply power the SMPS directly from the 208V mains? Of course it will work fine, but I'm concerned about satisfying NFPA 79 2012 Article 9.1.1.1.
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9.1.1.1 Where control circuits are supplied from an ac source, control transformers shall be used for supplying the control circuits. Control circuits shall not be derived from autotransformers. Control circuits supplied from windings of multiwinding power transformers shall be permitted if the output voltage of the winding supplying the control circuit does not exceed 120 volts ac and the available short-circuit current does not exceed 1000 amperes rms.

9.1.1.2 Where dc control circuits are connected to the equipment grounding (protective bonding) circuit, they shall be supplied from a separate winding of the ac control circuit transformer or by another control circuit transformer or a listed dc power supply.

9.1.1.3 Transformers shall not be required if the supply voltage does not exceed 120 volts ac and the available short-circuit current does not exceed 1000 amperes rms.

........
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Disclaimer: I am not a designer of Industrial Machinery

9.1.1 seems pretty clear. You can decide if the 120V, 1000A SCC conditions are met. That is a pretty wimpy supply. It would have to be a small xfm and some distance to get that low for the SCC. If you don't have access to software of the ETAP ilk, you could use the Bussmann point to point.

Are there other issues not yet mentioned. For example, does the switch mode power supply provide the isolation called for in 9.1.1.2? No use answering - just a design question for you.

ice
 
NFPA 79 2012 Article 9.1.1.1.

NFPA 79 2012 Article 9.1.1.1.

IceWorm has posted the article for me. I questioned this for the following reasons:
1. I've seen this practice done many times (i.e. no transformer).
2. I wondered if 9.1.1.2 was implying a DC power supply instead of a transformer.
3. I've seen other threads on this site where connecting the SMPS directly was suggested.
 
Johnshealey said:
IceWorm has posted the article for me. I questioned this for the following reasons:
1. I've seen this practice done many times (i.e. no transformer).
2. I wondered if 9.1.1.2 was implying a DC power supply instead of a transformer.
3. I've seen other threads on this site where connecting the SMPS directly was suggested.
Click to expand...

The biggest problem I see is the 1000A SCC limit. I just finished doing an Available SCC calc for four industrial control panels (NEC 409), each powered from a 120V, 15A circuit. The sources were either 208/120 or 120/240. All but one were over 1000A. All were under 5000A. Xfms were 112.5kva (largest) to 15kva (smallest). I would be surprised if the available SCC from a xfm supplying 208/120 to your NFPA 79 Industrial Machine was less than 1000A

1. Okay - so?
2. Don't know. See disclaimer. However, it says either (paraphase) 'separate winding or xfm, or listed power supply'. What do you have?
3. Okay - so were the ps listed?

ice
 
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Listed Powwer Supply

Listed Powwer Supply

The power supply in qustion is a Sola HD (Emerson) model SDN 5-24-100C and is cULus listed. I'm not convinced that this means that it is an OK substitution for an isolation transformer. I think I would need to be certain of the SMPC's circuitry. It might just be simpler to find room for a transformer; thjat way I can't go wrong.
 
Johnshealey said:
The power supply in qustion is a Sola HD (Emerson) model SDN 5-24-100C and is cULus listed.

I'm not convinced that this means that it is an OK substitution for an isolation transformer.
I agree

I think I would need to be certain of the SMPC's circuitry.
yes

It might just be simpler to find room for a transformer; thjat way I can't go wrong.
yes
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However, I'm still wondering how you know the supply has Available SCC < 1000A?

ice
 
SCC < 1000A?

SCC < 1000A?

I don't. So that's another reason not to do it. There are plenty of 480V to 24VDC and other such mains-to-DC-control power supplies out there. Is every application of them against the rules?
 
Transformer Added

Transformer Added

I've aded a transformer to this panel. I usually do large main enclosures and use a 5-10kVA transformer outside the enclosure. This is a very small panel with one motor starter and two heater contactors. One of my coworkers reads article 9.1.1.1.as applying to AC control circuits and 9.1.1.2 as applying to DC control circuits. We formerly had 120VAC control circuits for nearly everything, but have recently moved to 24VDC.

Thank you for your interest and comments. I'll continue to explore this question, but will move on with this current project.
 
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