SCCR Rating - fault current 1p or 3p?
SCCR Rating - fault current 1p or 3p?
(OP)
Hello,
Another question on SCCR ratings for devices. I am hopeful that this one gets answered. When a device is rated to withstand 14kA fault current, is that a single line fault or 3 phase fault.
thanks
EE
Another question on SCCR ratings for devices. I am hopeful that this one gets answered. When a device is rated to withstand 14kA fault current, is that a single line fault or 3 phase fault.
thanks
EE





RE: SCCR Rating - fault current 1p or 3p?
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RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
A single-phase fault beyond the rating will destroy one contact, which kills the breaker just as well as destroying all three.
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
10kA single-phase-to-ground
10kA phase-to-phase
10ka three-phase
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Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
The contact rating is either 10kA on one phase (which means it's not rated for more than a single phase fault), or it is 10kA on all three phases (which means it can handle 10kA on a single phase and it can handle line to line faults and it can handle 3ph faults). This is what I assume, but I'd like to know for sure.
RE: SCCR Rating - fault current 1p or 3p?
It's the design of the contacts and arc-clearing apparatus that limits how much fault current it can interrupt. It doesn't matter whether the current is passing through one, two, or all three of the contacts. Each individual contact can only interrupt the rated current.
In sum, a breaker with a simple 10kA rating is rated to interrupt:
10kA flowing through one contact (single phase)
10kA flowing through two contacts (phase to phase, or both to ground)
10kA flowing through three contacts (three phase, whether to ground or not).
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
The answer is the same. It can take 10kA through any one of its terminals, or through some of them, or through all of them.
It can't take more than 10kA through any single terminal.
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Actually it is tested to withstand the fault current from a transformer with the listed available fault current, at a specified power factor and a specified X:R ratio.
The SCCR rating and the transformer available short circuit rating give a simple and usually reliable method to match transformers, service entrance devices and components. The actual withstand current of a device is higher than the rating to compensate for the fact that the actual instantaneous short circuit current that a transformer will deliver is quite a bit higher than the available short circuit current as calculated from the transformer impedance. Google asymmetric currents and DC offset.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
It's clear that this documentation is not easy to come by, or someone would have acknowledged it by now.
Thanks for your replies.
RE: SCCR Rating - fault current 1p or 3p?
Do your fault current analysis, determine what the highest available fault current will be (note: single-line-to-ground is often the highest if the system is solidly grounded, depending on the ratio of positive sequence impedance zero sequence impedance). Choose your equipment with an SCCR that is above the highest available fault current.
One more try at an example:
A device has an SCCR of 10kA. The (grounded Y) system fault current calculations indicate that a bolted three-phase fault will produce 9.8kA, and a single-line to ground fault will produce 10.5kA. The device is not suitable, because a single-line to ground fault will exceed its SCCR rating.
Another device has an SCCR of 10kA. The (ungrounded delta) system fault current calculations indicate that a bolted three-phase fault will produce 9.8kA. In this case, the single-line-to-ground fault is zero, and the two-phase fault is much less than the three-phase value. So 9.8kA is the maximum available, and the device is suitable.
The point is that both of those devices have three phases connected to them. The SCCR of the devices must be higher than the highest amount of current that will pass through any of the three connections.
In sum, the SCCR is not based on a three-phase fault or a single-phase fault. It is to be applied to the highest available current that can flow through any one terminal. A current exceeding the SCCR through one terminal of a three-phase device will probably destroy it. A current exceeding the SCCR through two or three of its terminals will also probably destroy it.
"Highest Available Fault Current" not three-phase, not single-phase. Whichever is higher.
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Specifically, if I have a control panel with no external loads (motors and such), but I do have a panel heater and a panel light, how do those devices get rated? Are these power components, control components, or neither?
I'd appreciate a reference to documentation if any of you can find it. Thanks.
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Not all panels have main power feeds, nor do they have control transformers. Consider a panel with one PLC, a light, and a heater. There is no external loads to the panel, however the heater and the light are not control devices. I could easily argue that the panel heater is a load.
Where do these devices fit into the SCCR rating?
RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Look at the glossary at the front of UL 508A, plus diagrams 6.1, 6.2, and 6.3.
The diagrams are especially helpful.
The standard is also broken up into chapters, two of which are power circuits and control circuits. Basically, if a device's specs are in the power circuits section, treat them as power circuit devices.
Good on ya,
Goober Dave
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RE: SCCR Rating - fault current 1p or 3p?
RE: SCCR Rating - fault current 1p or 3p?
Table SA1.1 spells much of it out too.
A PLC would likely have a CCN of NRAQ from under section 45 - control switching devices.
The Pilot light likely has a CCN of NKCR from section 46 - control circuit loads.
The heater likely has a CCN of NITW2 from section 26 - Enclosure Enviromental Controls. This isn't under the control or power sections so you have to decide.
Have you considered taking a UL course on the subject?
RE: SCCR Rating - fault current 1p or 3p?
Thank you very much. I would love to take a course on this. Maybe then I could write up a proper procedure for following this requirement. It's just disappointing that UL could devise such a requirement that nobody seems to fully understand.
RE: SCCR Rating - fault current 1p or 3p?
Coincidently, I got an e-mail from UL Academy just now, regarding 508A (link below and discount code). I've taken this course along with our whole panel shop, it's good. Plus, you get to know the instructors -- they're good folks to ask for opinions when strange applications come up.
UL Academy 508A
Good on ya,
Goober Dave
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