UL508A and 24VDC Panels

[mspuddy@electro-matic.com]

We are starting to do more and more 24VDC powered panels. We are requested to mark these UL508A (we are a UL508A panel shop) my question is marking, there is no SCCR for these 24VDC panels, correct ? What do I put on my nameplate for SCCR ?

 

[Jraef]

Technically, you don't need to show SCCR on a strictly 24VDC panel at all.

SB4.2.1 All power circuit components, including disconnect switches, branch circuit protective devices,
branch circuit fuseholders, load controllers, motor overload relays, terminal blocks, and bus bars, shall
have a short circuit current rating expressed in amperes or kiloamperes and voltage.

But assuming your labels are already pre-made and formatted, for the requirements that have to show;
"Short circuit current: ___kA rms symmetrical, ___V maximum"

I would put "N/A" or "No Power Components" (if it would fit) for the SCC, and "24" in the V maximum part.

 

[winnie]

Given the available fault current from a large battery, I am surprised that there _isn't_ a SCCR requirement for 24V systems.

Obviously it only takes a tiny bit of impedance to drop the fault current at 24V, and 24V probably won't sustain an arc, but a large 24V fault can still cause lots of magnetic stress.

-Jon

 

[Adamjamma]

Imagine how many farmers now shaking their heads.. 24v won’t sustain an arc? You have never used ac switches on 12 or 24vdc then... or stick welded a gate using two batteries..l just clamp the stick 8n the negative and clamp positive to the gate..l pull down a shield... it works... used to be pretty as well...

 

[GoldDigger]

Given the available fault current from a large battery, I am surprised that there _isn't_ a SCCR requirement for 24V systems.

Obviously it only takes a tiny bit of impedance to drop the fault current at 24V, and 24V probably won't sustain an arc, but a large 24V fault can still cause lots of magnetic stress.

-Jon

24VAC will not sustain an arc because the arc can quench during the current zero moments and will not restart when the voltage returns to maximum without deliberate motion of what is serving as electrodes to restrike the arc. A DC arc does not have to survive zero current intervals and so can sustain (and weld) quite well.

 

[Adamjamma]

But he was talking about 24 Vdc panels

 

[Jraef]

He said this was for 24VDC Controls, not power from a large battery or anything like that. 24VDC Control Power will usually come from Class 2 power supplies, limited to 100W of total power per circuit. Nothing in there is going to be spitting out molten copper and blast pressure or provide 4 cm² of heat energy. Come on guys...

 

[GeorgeB]

He said this was for 24VDC Controls, not power from a large battery or anything like that. 24VDC Control Power will usually come from Class 2 power supplies, limited to 100W of total power per circuit. Nothing in there is going to be spitting out molten copper and blast pressure or provide 4 cm² of heat energy. Come on guys...

Machine panels for plastic molding machines are commonly 24DC, but with the average hydraulic valve load of about 25 watts, and there being often a dozen or more simultaneously energized, that 100W won't go far. There will usually be 3 to 8 motor starters as well, plus the array of pneumatic valves.

A 3-phase powered supply of 20 to 40 amps at 24DC is not uncommon.

The question is where do they put the supply ... and how is it protected.

 

[winnie]

He said this was for 24VDC Conttols, not power from a large battery or anything like that. 24VDC Control Power will usually come from Class 2 power supplies, limited to 100W of total power per circuit. Nothing in there is going to be spitting out molten copper and blast pressure or provide 4 cm² of heat energy. Come on guys...

Just goes to show how our assumptions can confuse our response.

The original post doesn't say anything about controls. It just says 24V panels. I made the assumption that these were 24V power distribution systems, eg. for solar powered equipment or for boats or the like.

But I agree with Jraef, this is much more likely some sort of power limited control system.

Only the OP really can say

-Jon

 

[lightmedic]

Provided you are on Class 2 or limited power AND that voltage is originating elsewhere you should not need anything. If you are generating the Class 2 or limited power within the enclosure from an AC source, you should plate that SCCR. If you are feeling lazy and there are no contractual requirements, there is the courtesy 5kV, but that may cause issues for the client.
The best advice is to call your inspector on the labeling and beg for the SCCR book on how to get that number higher by design.

 

[petersonra]

UL508a suggests, but does not require control circuits not exceed 15A. It would not need to be a class 2 circuit to be a control circuit. It can be a class 1 circuit as well.

Class 1 Power-Limited Circuits. These circuits shall be
supplied from a source that has a rated output of not more
than 30 volts and 1000 volt-amperes.

(B) Class 1 Remote-Control and Signaling Circuits. These
circuits shall not exceed 600 volts. The power output of the
source shall not be required to be limited.

As you can see, for a 24VDC class 1 circuit, you could have a fair amount of current.