SCCR Rating - fault current 1p or 3p? 4

[eeprom]

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

 

[jraef]

Any fault. So that would mean a ground fault on one pole of a 3 pole device (the only way to have a "single line fault").

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[eeprom]

I don't understand how the rating could mean any fault. A 3 phase fault of 50kA per phase will do more damage to a breaker than will a single phase fault of 50kA.

 

[burnt2x]

I don't understand how the rating could mean any fault. A 3 phase fault of 50kA per phase will do more damage to a breaker than will a single phase fault of 50kA.

The keyword is "damage". How badly damaged, doesn't matter anymore. You'll still be needing replacements either way, you know?

 

[eeprom]

Well, the point is to size things so that they are not damaged. So my question is...If I have a device with a short circuit rating of 10kA, does this value refer to a single phase fault, line to line fault, 2 phase to ground, three phase to ground, or all?

 

[DRWeig]

All faults.

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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[eeprom]

So, the rating is for a three phase fault?

 

[DRWeig]

All faults.

Good on ya,

Goober Dave

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[DRWeig]

In other words, 10kA flowing through any or all contacts is the limit.

10kA single-phase-to-ground
10kA phase-to-phase
10ka three-phase

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Goober Dave

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[eeprom]

I'm not sure that this makes sense to me. Despite the currents being the same magnitude, the difference in energy absorbed during a 3 phase fault is about 3 times as much as a single phase fault.

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.

 

[DRWeig]

Think about the breaker on a per-contact basis, not as a unit. Imagine it were three single-phase breakers.

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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[eeprom]

SCCR is not about interrupting current, it's about withstand current. This rating applies to breakers, but it also applies to every power component in the feeder and all branch circuits. What is the interrupting current of a motor contactor, or a power distribution block?

 

[DRWeig]

Yes -- it means how much current it can survive and still be put back into service, whether it's a breaker, a contactor, or a terminal block. In the case of a contactor or breaker, the interrupting capacity is integral to this.

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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[DRWeig]

Perhaps it would help us to explain if you would post the problem you are trying to solve?

Good on ya,

Goober Dave

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[eeprom]

When a device is rated to withstand 14kA fault current, is that a single line fault or 3 phase fault? How are these devices rated?

 

[eeprom]

Maybe this would be clearer. If I had a device rated at 14kA short circuit rating (not interrupt rating), and that device was subject to a 3 phase fault of 14kA per phase, would that device survive (according to the vendor rating)? I think it would, but I can't find any information on it.

 

[waross]

Multiply the answers for 10 kA by 1.4
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

 

[burnt2x]

What can we do? It's either we don't understand your question or you failed to follow the logic behind our replies here! If the withstand rating is 14kA, then 14kA it is; never to be be exceeded in any kind of fault! Crystal!

 

[eeprom]

Obviously the question has gone off course, which happens when I don't clearly state my question. I am looking for documentation that states that the sccr rating of a device is for a symmetric 3 phase fault, RMS. So that when I calculate the available short circuit rating at a given control panel, then I will use the worst case fault current, which would usually be a 3 ph fault.

It's clear that this documentation is not easy to come by, or someone would have acknowledged it by now.

Thanks for your replies.

 

[DRWeig]

eeprom, there is no documentation that the SCCR is based on a three-phase fault or a single-phase fault. It does not matter, it is a single number.

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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