JFletcher
Senior Member
- Location
- Williamsburg, VA
Will the wire itself withstand that, or would it explode? I've no idea the currents involved in bolted faults, or their cycle time, only that I've no desire to see another one up close and personal.
Cable damage in short circuit
- Thread starter manuel bonilla
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Will the wire itself withstand that, or would it explode? I've no idea the currents involved in bolted faults, or their cycle time, only that I've no desire to see another one up close and personal.
In my previous post 20 a link is provided for common methods of calculating the fusing point of conductors
eg 12 will fuse open in 2 cycles at~ 6 ka
JFletcher
Senior Member
- Location
- Williamsburg, VA
I think I see the math.. so, using your example, #12 would fuse open in half a cycle @ ~24ka?
- Location
- Illinois
- Occupation
- retired electrician
The temperature of the conductor will not exceed 150°C. That is the temperature most conductor insulation can withstand for a very short time without damage to insulation.
- Location
- Illinois
- Occupation
- retired electrician
This Busssmann Document shows the 1/2 cycle fusing current for #12 to be about 9800 amps. Table 4 in the document.
that seems to be insulation damage not fusing of the conductor?
Table 2 shows the maximum short-circuit current rating of the conductor based upon the opening time of the device. This table is based upon the ICEA Insulation damage level to raise the conductor from 75 degrees C to 150 degrees C.
ok
misread
table 4 not page 4
that is lower than the equations calculated value?
I see why
the table uses an ambient of 75 C
the calc uses 25
I'll do the calc at 75
Last edited:
- Location
- Illinois
- Occupation
- retired electrician
All of the tables in that document use a 75°C conductor starting temperature.
Bugman1400
Senior Member
- Location
- Charlotte, NC
Assume the worst and test the cable. Its simple and quick and will tell you for sure.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
Here’s the result of a heavy fault. Sorry it’s a marketing film but it shows the magnetic repulsion between conductors under heavy fault conditions.
https://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s
I’m preparing for a 2MVA switchboard modification at the moment. The busbars are the easy bit, keeping them where they should be under fault conditions is another matter.
https://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s
I’m preparing for a 2MVA switchboard modification at the moment. The busbars are the easy bit, keeping them where they should be under fault conditions is another matter.
To the point that I made in my post #8. There seems to be much attention being made toward to insulation being damaged thermally and mechanical damage ignored. Is it being assumed that a short circuit is long enough in duration such as more than a few cycles is asufficient enough time to damage the insulation thermally? Interesting.
But in a residence I doubt if mechanical damage to the insulation nor terminations is of a concern where the large commercial/industrial applications involving much larger conductors and currents is a major concern where ecuyring and supporting cable cable is important.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
Occasionally on UK electricians sites we get “the prospective fault current is too high, what do I do?” Unfortunately many of our younger electricians don’t appreciate the problem.
Get a town centre house adjacent to a 1500KVA substation and they’re going to be way above the breaking capacity of a standard MCB. OK, you can get higher rated MCB’s but if they’re not recognised then dangerous fault currents are possible.
Wouldn't it be then anp unacceptable instalation to begin with? Would not the OCPDs kaic be rated at no less than the available fault current of the transformer? I would like to assume that there are qualified people who are doing the installations, people that understand the relationship between a transformer's tasted available fault current and an OCPD's kaic rating. But then again, I could be wrong.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
It is something that is often overlooked maybe even ignored to keep the installers costs down. To issue the installation test certificate the MPFC has to be measured and recorded so either of the reasons for overlooking it could be valid but not condoned.
Domestic installs aren’t my forte which is why I posted the magnetic repulsion link. The bigger it gets, the worse it gets
hmy:
Domestic installs aren’t my forte which is why I posted the magnetic repulsion link. The bigger it gets, the worse it gets
hmy:manuel bonilla
Member
- Location
- San Jose, Costa Rica
The breaker should have the correct KAIC rating and this will guarantee it will open the fault. My original question is if it will open the fault before the cable insulation is damaged or is this insulation damage and acceptable consequence of the fault.
This is certainly not an issue in residential because system Z will limit the available fault. But I am thinking it may be an issue in industrial, in particular cases where your #12 circuit is just too close to the 1000kVA transformer and circuit length is short. Thats what I have come across and what the SKM seemed to show.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
If it is a breaker listed to UL489, it will protect the insulation and conductor, they are a tested combination.
As soon as the breaker contacts begin to open, the fault current begins to fall which means the conductor never sees the full available fault current.
Should you look at a breaker trip curve. It will illustrate at which instance a breaker opens. When it does the clearing time of a correctly applied and rated breaker is a.matter of the time out takes for a breaker to initiate a trip by the mag element hitting the trip mech which initiates the opening of the contacts. Once the contacts start to open the arc should be cleared in (1) cycle or less. It is to be emphasized that the mag trip element is magnetic with NO TIME DELAY. It is NOT an inverse time current sensitive element.
You appearently seem to be hung up on the possibility of thecable being damaged as a result of a short circuit/fault current. Please state at which time a thermal magnetic molded case breaker would not protect the wire other than the possibility of physically being damaged when large ampacty conductors in industrial/commécial installations are not properly installed to reduce the possibily of cable wip.
manuel bonilla
Member
- Location
- San Jose, Costa Rica
I was not aware of how UL489 was done. I think this would be the explanation. You had said this earlier but I had not understood the fact that current begins to fall as contacts begin to open.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Breakers, and fusible switches, are tested with 4 ft of conductors attached to them. Effectively, to pass the SC test the conductors must not be damaged and they cannot have pulled out of the terminations.
Yes, 4' of rated cable is 8ncluded in the UL489 testing requirements and is also included in NEMA AB4 which includes the field testing of breakers. But don't hand your hat on that it affects the instantaneous test as it more directed towards the thermal as breakers rely on the heat sink effect of that cable taking heat away from the breaker.
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