Arc resistant switchgear - fault duration 1s?

[JuanBC]

Hello guys, I would like your opinion regarding the following issue:

Since the short-circuit level in an installation I am designing is about 60kA, I am asking for IEC LV switchgears rated 65kA/1s and also being arc resistant (65kA/1s arc duration)

One of the bidders is offering 65kA/1s switchgear tested for 0.3s arc duration because he says there is not 65kA/1s arc resistant Swgs in the market tested for 1s but I do not find it acceptable because the installation design (Cascade) will require longer coordination times (about 500-600ms)

Do you think 65kA/1s arc duration is really that strange?

I don't know why would anyone buy a switchgear rated 65kA/1s but arc tested for 0.3s....

Any insight would be really appreciated

Thanks,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[jraef]

IIRC, the IEEE C37.20.7 spec for Arc Resistant Switchgear calls for 500ms maximum arc duration ratings. I don't know if IEC requirements are different. Don't commingle short circuit withstand ratings with arc flash duration ratings, they are different events.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dpc]

Arc-rated LV switchgear is rare in the US and Canada. Remember that the 65 kA rating applies to bolted fault external to the switchgear. It has nothing to do with arcing fault resistance. It should not take much research to determine if other manufacturers can meet your requirement. just another observation - the arcing current will be considerably less than the bolted fault current maximum of 60 kA.

 

[JuanBC]

Hi jraef, thanks for posting. So based on IEEE point of view, I shouldn't set the incomer overcurrent protection to more than 500ms, right? That seems quite low if you need it to coordinate with 3 or more downstream protective devices

I know short circuit withstand ratings and arc flash duration ratings are different things but both are related to the incomer overcurrent setting

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[JuanBC]

dpc, thanks.

I like your comment on the following:

the arcing current will be considerably less than the bolted fault current maximum of 60 kA

The problem is how to calculate arc current before buying que equipment and performing protective devices coordination study

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[dpc]

If you know the bolted fault current and the type of equipment, the arcing fault current can be calculated.

 

[dpc]

Jeff,

Curious if the scope of C37.20.7 extends down to low voltage equipment?

 

[stevenal]

DPC,

Yes.

JaunBC,

For what it's worth, I spec out arc resistant gear for the added safety, but haven't used the 0.5s duration in coordination. The 0.5s (and likely more) provides some added time for a worker (already clad in FR) to move away, and the gear is going to need repair or replacement anyway. Do others here include it in coordination?

 

[FPelec]

IEC standard 61641 states that the arc duration should, in General, not exceed 0.5 s.
0.3 s is also a typical duration when the switchgear is fed by a transformer.
ABB SACE however once produced some power centres tested for a 1 s arc.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[dpc]

If you are assuming the main breaker in the switchgear will clear the fault, then 0.5 sec is reasonable. If you don't have a main breaker or are working on the main breaker and require primary fuses to clear the fault, then 0.5 seconds is optimistic. Perhaps achievable with expulsion fuses, but more difficult with current-limiting fuses. The ANSI standard for short circuit withstand test in the LV Switchgear and LV power circuit breakers is 0.5 seconds IIRC.

 

[ElecEE8]

Hi All,

About IEC what is @FPelec posted is correct.
I want to add following;
1- No any relation between arc duration and short circuit withstanding time of the switchgear.
2- Any fault shall be cleared within max. 200-250msec. this is main principle of protection relaying.
3- I do not see any relation btw arc duration and relay coordination or grading time requirement.
4- 0.3 sec arc duration is typical and technically acceptable.

Please correct me if I am wrong.

Thanks

 

[ElecEE8]

Additional info;
IEC/TR61641 criteria 1 to 7 which is switchgear and personnel protection.
Duration of the test:
The arcing time is set for 300ms. If during the test the arc extinguishes within 150ms, then the test is repeated using the same point of initiation. A further repetition is not required. For conditional short-circuit current under arcing conditions, the recovery voltage is maintained for at least 10 cycles.

Base on my experience, Most of Switchgear manufacturer are complied to only 0.3 sec by referred to above specified testing duration and methodology while all client spec. still required 0.5 sec.

 

[JuanBC]

Hello everyone,

Thanks for your comments

I reviewed many vendors available in the market and, as you indicated, 300ms in the standard value

I used to think that more than 500ms was required to coordinate the incomer with the outgoing circuit-breakers but I think that is only required in MV level

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[stevenal]

Please correct me if I am wrong.

Okay. Consider the 1st protective device on the load side of a transformer is an arc resistant breaker. The next protective device upstream is on the transformer primary. The primary device is sized to accommodate normal transformer loading as well as short term overloading and sits well below the transformer damage curve. 3s in the example I just looked up in our system. The alternative is to use a non-arc resistant breaker. I believe the arc resistant choice provides additional safety even though it might be exposed to a 3s arc.

 

[krisys]

To re-iterate what DPC and others said, the arc current is considerably lower. By the very characteristics of arc it will not sustain for a prolonged duration like 1000 ms. It extinguishes within few cycles.

To illustrate the above;
If the rated short circuit current is say, 65 kA, the arcing current could be say, 35 kA. Then the duration of arc at 35 kA will be 0.5 s * (65/35)[sup]2[/sup] = 1.72 s

The figures are indicative, to illustrate the concept.

So the offered rating by the vendor appears to be more than adequate.

 

[stevenal]

It extinguishes within few cycles.

Self extinguishing arcs? Perhaps at very low voltages.

If the rated short circuit current is say, 65 kA, the arcing current could be say, 35 kA. Then the duration of arc at 35 kA will be 0.5 s * (65/35)2 = 1.72 s

I don't think you mean the actual arc duration, but the permitted duration. Nothing in the IEEE standard speaks of adjusting the 0.5 s figure by (I*2)t.

 

[dpc]

Any fault shall be cleared within max. 200-250msec. this is main principle of protection relaying.

Not familiar with any US standard that requires this and I don't see how it is even possible to stipulate such a requirement. ANY fault would encompass a wide range of fault currents. If you are talking about clearing a fault current at the maximum rating of the breaker or equipment, this might make some sense.