SCCR existing equipment no fusing

I have an existing piece of equipment which I need to calculate the SCCR for.
The kink here is a 24VDC power supply (PULS ML100.200) tied directly to the incoming 480. There is no fusing or circuit breaker between power input side of supply and the disconnect. The power supply is UL 508 rated.

Welcome to the site.
Over here that is illegal. I suggest the same would be the case in the US.
you may have to isolate the machine and fix it.
normally even in a large control enclosure there are some form of circuit protection somewhere.

clarify

There are other devices drives which are protected. The drives I can deal with the SCCR calc. But a device without protection, do I just give the 5kA of a unrated transformer. Illegal? I know it is BAD design, but as I said this is an existing piece of equipment.

msaeger,

welcome to the forum

what do you mean by sccr?

NFPA 70 - NEC and expecially NFPA 79 - Electrical Standard for Industrial Machinery points out that it must be protected.

it doesn't matter if the power supply is UL508A rated.

from the main disconnect, you use fusing to protect the wiring downstream and its devices.

Not saying this will happen, but you have the possibility for these devices to short out and power everything with 480 volts.
things may need to be just right for it to happen, but you should take actions to fix this problem in my opinion.

NFPA 79 states that new control panels shall not have control power over 120 volts.

regards,
james

Exactly my problem, while the drives in the panel have circuit breakers separating them from the disconect, the 24Vdc power supply , is supplied directly from the disconnect. First what would the SCCR, rating be for this
portion of the circuit? Can the SCCR be calculated? and how many code violations does this represent?

thanks in advance.

how does this sound for a solution?

I think I found the answer: NEC 70, 725.127

Conductors and equipment on the supply side of the power
source shall be installed in accordance with the appropriate
requirements of Chapters 1 through 4. Transformers or other
devices supplied from electric light or power circuits shall be
protected by an overcurrent device rated not over 20 amperes.

Bottom line there must be a protective device in front of power supply.

Thanks for all the responses.

google NFPA79 and you will find an old pdf of the manual.

SCCR - short circuit conductor rating?

without any protection - its wide open.
it will carry current until it melts or is no longer conducting the neutral or ground. then it starts all over again when maintenance gets into the panel trying to figure out what happened. ONLY, this time, someone becomes the path to ground.

regards,
james

James Mcquade said:

...what do you mean by sccr?...

...SCCR - short circuit conductor rating?...

...without any protection - its wide open.
it will carry current until it melts or is no longer conducting the neutral or ground. then it starts all over again when maintenance gets into the panel trying to figure out what happened. ONLY, this time, someone becomes the path to ground.

Click to expand...

James, you're close on the SCCR part, but while explaining about having no fuse you have also outlined the effects of a SCCR being under-rated.

Short-Circuit Current Rating (SCCR), or Withstand Level, is the maximum level of short-circuit current (kA) that a component or entire assembly can withstand.
The SCCR must comply with NEC 110.10.

Not to be confused with...(not saying you are James)

The Interrupting Rating (IR) is the maximum current, at rated voltage, that an Over Current Protective Device (OPCD) can safely interrupt. The IR rating of the OCPD should be no less than the highest potential current at its line terminals.
The IR must comply with NEC 110.9.

To meet SCCR requirements, both a suitable IR and SCCR needs to be considered for protective devices.

msaeger said:

...The power supply is UL 508 rated...

Click to expand...

James Mcquade said:

...it doesn't matter if the power supply is UL508A rated...

Click to expand...

UL508 pertains to the industrial safety standards for individual components. (Circuit Breakers, disconnects, switch fuses, etc.)

UL508A pertains to the industrial safety standards for assemblies of components. (Industrial Control Panels, Machines, Systems, etc.)

James Mcquade said:

NFPA 79 states that new control panels shall not have control power over 120 volts.

Click to expand...

James, the power supply is fed 2 phase 480V, the control circuit is 24VDC here, so the 120V rule isn't being breached in this case.


The PULS ML100.200 power supply is fused internally.

General Description said:

The ML100.200 is a simple and cost effective approach to convert the AC voltage of a typical three phase system into a regulated DC voltage. It only requires two phases and thereby saves terminal space, terminal cost, wires, fuses and installation time.

The ML100.200 is very compact, high efficient and easy to use. The input is internally protected, which makes external fuses unnecessary in many cases.

Internal input fuse T3A15.

Click to expand...

That keeps NFPA\NEC happy as there is a suitable OCPD in front of the PULS power supply.

But what about the SCCR?

SCCR only applies to Power Circuit Components up to 600V. NEC 110.10 requires the SCCR to be adequate enough to protect the circuit equipment from sustained short-circuit currents, which may pose a potential electrical shock hazard or fire risk.

NEC 110.10 also requires the SCCR to prevent extensive damage to the equipment within acceptable limits or criteria. i.e. once the fault damage cannot pose the above risks outside of the panel enclosure. The acceptible limits can get tricky when the situation you mentioned occurs James, opening panels to find out where the smell is coming from?

A specific fuse or circuit breaker needs to be used to provide the necessary SCCR to withstand the maximum potential short-circuit current.

Fuses tend to be higher rated, usually 200kA, but it depends on the fuse type and potential fault current. If the fuse in the PULS power supply is a class CC, J, T, R or L UL248 branch circuit type fuse, then you can give it a 200kA SCCR. If not, I would rate it similar to a UL 489 Circuit Breaker at 5kA to be safe.

Is your disconnect fusible or non-fusible?
If fused with any of the above fuse types, it can give you up to 200kA SCCR.
If un-fused, then 10kA SCCR, but only if you use above fuse types on the branch circuit after the disconnect. Otherwise 5kA SCCR.

If you were to put current limiting fuses on the incoming feed, this would then reduce the potential short-circuit current. Reducing this would instantly increase the SCCR for the whole assembly i.e. the panel. This is beneficial in many ways, as SCCR is based on the weakest link principle. If the lowest rated component had an SCCR of 5kA, this determines the overall assembly SCCR. By decreasing the potential short-circuit current at the feeder, this could potentially bring the assembly SCCR up to 100kA.

Note: Control Circuit Components, typical <15Amps, do not need a Short-Circuit Current Rating.

G.

my concern is with the power supply being fed directly from the disconnect switch. internally fused is good but the cabling would be a fairly small gauge - this means is may carry the full short circuit current to the power supply.
it is not uncommon for water or RAT Pss to cause that type of short.
the internal fusing is usually a glass style fuse soldered to a PCB

Personally, because the power supply has a 2 Phase supply requirement.
I would have a double pole CB feeding the power supply. maybe sized to protect the cable gauge rather than the current of the power supply.
Use a second CB for the DC side.

I agree Ian, always use a double pole circuit breaker or double fuse on a 2 phase supply. I'm just pointing out it has some protection, as it was thought there was none.

From a SCCR point of view, the typical internal fuse you've described would have a very low rating, so a suitable protective device should be installed between the disconnect and the power supply. The assembly SCCR is only being calculated at the moment, so this may happen at a later date.

G.

we had a 2000 amp aluminium bus bar melt
the 2000 amp Air CB failed to trip
the 3500 amp main switch failed to trip
the fault was transferred up the line to the 22kv CFS
this blew a 100 amp fuse
so that was 100 x 22000 or 2.2Mva fault

Geospark,

thanks for the post and clearing that up.
i wasn't 100% sure.

regards,
james

Geospark said:

Fuses tend to be higher rated, usually 200kA, but it depends on the fuse type and potential fault current. If the fuse in the PULS power supply is a class CC, J, T, R or L UL248 branch circuit type fuse, then you can give it a 200kA SCCR. If not, I would rate it similar to a UL 489 Circuit Breaker at 5kA to be safe.

Click to expand...

I believe the Allen-Bradley 1489-A series breakers are 14kA according to the literature I've seen, unless I'm misinterpreting it. The arc flash rules are so full of legalese it's hard to decipher what they actually mean. I spent half a day once trying to figure out if a 5x20 fuse counted as branch circuit protection or supplementary protection. After emailing a UL508A trainer he surprisingly told me it counts as branch protection and we could rate it at the manufacturer's stated 10kA.

Ok with all the information presented, I decided to be safe and go with a 5kA rating. With regards to concerns about the wire it would meet the requirements of NFPA 79, 7.2.8(1), and similiar for 60204-1.

Thanks for all the help.

Matt