Cable damage in short circuit

[manuel bonilla]

Is cable insulation damage acceptable after a short circuit has occurred. In some cases you can see cable damage curve intersect TCC breaker cable at the highest available short circuit, and this seems to suggest that at very high short circuits the cable insulation may be damaged because the breaker is actually capable of opening the short. I do not know if this damage is an acceptable result of the short, pero code

 

[manuel bonilla]

Is cable insulation damage acceptable after a short circuit has occurred. In some cases you can see cable damage curve intersect TCC breaker cable at the highest available short circuit, and this seems to suggest that at very high short circuits the cable insulation may be damaged because the breaker is actually capable of opening the short. I do not know if this damage is an acceptable result of the short, pero code

Just to correct a couple of errors on my post

Is cable insulation damage acceptable after a short circuit has occurred. In some cases you can see cable damage curve intersect TCC breaker cable at the highest available short circuit, and this seems to suggest that at very high short circuits the cable insulation may be damaged BEFORE the breaker is actually capable of opening the short. I do not know if this damage is an acceptable result of the short, PER code

 

[manuel bonilla]

Wrong forum?

Wrong forum?

I can see many reads to my post but no answer.
Could someone please provide feedback on whether I am on the wrong forum or my question is unclear? I do not post frequently and my native language is not English (is my post understandable?)... so I am unsure of the reason for which no answers or comments have been posted.

 

[Barbqranch]

Your question is clear, but I, and I expect many others, just don't have any knowledge on this topic. For low power circuits (15 to 50 amps) in the home, nobody seems to have a problem with the wire when there is a short circuit.

 

[manuel bonilla]

Cable damage in short circuit

Is cable insulation damage acceptable after a short circuit has occurred??.

In some cases you can see cable damage curve intersect TCC breaker curve at the highest available short circuit, and this seems to suggest that at very high short circuits the cable insulation may be damaged because the breaker is actually incapable of opening the short quickly enough. I do not know if this damage is an acceptable result of the short, per code.

 

[Ingenieur]

No

the protection should be designed to protect the wiring, conductor and insulation
the damage threshold is very high

At a minimum it should be megger'ed if you think it may be damaged

http://www.okonite.com/engineering/short-circuit-currents.html
a 10 awg can take 2 ka for 8 cycles
a 30 a cb will trip in 2-4 hz at that level
if the Z even allows that much i

 

[jim dungar]

Part of the reason that UL tests protective devices with 4 ft of conductors attached, is to prove that the conductors are not damaged. So if you are dealing with breakers listed to UL489, you should not be concerned with properly sized conductors.

 

[templdl]

There are 2 things that can damage wire insulation which are over current because of bering overloaded or a voltage as above the wires insulation rating in my opinion other than being physically damaged. A short circuit in and of by itself will exceed the wire's current rating but if the OCPD's instantasneous trip responds correctly the momentary overload would not damage the insulation. In industrial/commercial instances the larger wire sizes which a commonly used often respond mechanically to a short circuit and wip. So being secured is an issue where this is not something that is problem in residential settings.

 

[steve66]

Are you talking about a new installation, where a short would theoretically damage the wire, or are you talking about an existing installation where a short circuit has occurred?

 

[manuel bonilla]

Are you talking about a new installation, where a short would theoretically damage the wire, or are you talking about an existing installation where a short circuit has occurred?

I am talking about a new installation. For example, at 20kA available short circuit, a 1x20A, 22kAIC feeding #12AWG cable, the breaker TCC curve seems to show that it is unable to open in less than 1 cycle. But in under 1 cycle #12 cable would be damaged at those levels of short. You would see this where you have this 1x20A breaker physically close to 1000kVA main transformer, for example.

 

[manuel bonilla]

No

the protection should be designed to protect the wiring, conductor and insulation
the damage threshold is very high

At a minimum it should be megger'ed if you think it may be damaged

http://www.okonite.com/engineering/short-circuit-currents.html
a 10 awg can take 2 ka for 8 cycles
a 30 a cb will trip in 2-4 hz at that level
if the Z even allows that much i

For example, at 20kA available short circuit, a 1x20A, 22kAIC feeding #12AWG cable, the breaker TCC curve seems to show that it is unable to open in less than 1 cycle. But in under 1 cycle #12 cable would be damaged at those levels of short. You would see this where you have this 1x20A breaker physically close to 1000kVA main transformer, for example.

 

[Dennis Alwon]

I am talking about a new installation. For example, at 20kA available short circuit, a 1x20A, 22kAIC feeding #12AWG cable, the breaker TCC curve seems to show that it is unable to open in less than 1 cycle. But in under 1 cycle #12 cable would be damaged at those levels of short. You would see this where you have this 1x20A breaker physically close to 1000kVA main transformer, for example.

I don't think the wire would be damaged for that short cycle

 

[Ingenieur]

For example, at 20kA available short circuit, a 1x20A, 22kAIC feeding #12AWG cable, the breaker TCC curve seems to show that it is unable to open in less than 1 cycle. But in under 1 cycle #12 cable would be damaged at those levels of short. You would see this where you have this 1x20A breaker physically close to 1000kVA main transformer, for example.

Dist of feeder from 1 mva xfmr to pnl with the 20/1 in it?
length of 12 awg?

A failure scenario can always be contrived
but a design is for real world worse case

all this assumes infinite bus to boot
no primary side losses

 

[manuel bonilla]

Dist of feeder from 1 mva xfmr to pnl with the 20/1 in it?
length of 12 awg?

A failure scenario can always be contrived
but a design is for real world worse case

all this assumes infinite bus to boot
no primary side losses

I understand. I am talking 20kA on secondary, so no infinite bus.

But from what you are saying, I understand that available short circuit should be taken at the end of the #12 (...or lets say at first pullbox). At that point cable Z will limit SC substantially.

So, just to confirm, would you say that the correct practice is establishing available short circuit at first pull box? ...or at branch end (load)?

Thank you for your help.

 

[don_resqcapt19]

The one cycle short circuit withstand for #12 without damage to the insulation is 2700 amps...it drops to 1900 amps for two cycles.

 

[gray.one]

I was taught that, in low voltage systems, cable damage curves should not be plotted on the TCC. If you follow the NEC cable damage due to a short will not occur. The graphs are misleading.

On the other hand, Medium voltage cables must be plotted and protected based on the curve.


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[Tony S]

Deleted

 

[Sahib]

For example, at 20kA available short circuit, a 1x20A, 22kAIC feeding #12AWG cable, the breaker TCC curve seems to show that it is unable to open in less than 1 cycle. But in under 1 cycle #12 cable would be damaged at those levels of short. You would see this where you have this 1x20A breaker physically close to 1000kVA main transformer, for example.

Let us see what the size of cable is required for 20 kA current flowing for a duration of 1/60 second. The formula is A=[I/k]*SQRT[T] where I is current in A, k constant= 143 and T in second. The area in sqmm to be converted to cmil.

 

[manuel bonilla]

I was taught that, in low voltage systems, cable damage curves should not be plotted on the TCC. If you follow the NEC cable damage due to a short will not occur. The graphs are misleading.

On the other hand, Medium voltage cables must be plotted and protected based on the curve.


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I have 20 years working under the same assumption. But the info on cable damage vs trip time at high SC does not seem to support that. That is why I posted.

After reading all posts I think this is a very unusual situation. And that is why it is not addressed on our regular day to day knowledge as EE.

It requires a combination of large Tx, and very short small branch circuit.

 

[Ingenieur]

10 awg will fuse in 2 cycles at ~9 ka
insulation may be the least concern lol

http://www.litz-wire.com/New PDFs/F...ure_Copper_Aluminum_Magnet_Wire_R2.011609.pdf