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Low Voltage Cables Sized Based on Short Circuits

07/12/2012 4:49 AM

Hi All!!

I'm an entry level oil & Gas professional.

Why LV cable sizing should not be based on short circuit withstand. Can anybody point me to any direct reference regarding this?

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#1

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 5:03 AM

It is: the cable needs to survive the short circuit event until the moment the installed circuit protective device disconnects it from the supply. British Standard 7671 and codes applicable to other nations are based upon this principle.

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#6
In reply to #1

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 6:53 AM

Can you mention the section number of BS 7671 in which I can find it. It will be a great help !!

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#7
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Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 7:10 AM

All of it. Buy a copy.

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#2

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 5:05 AM

"Why LV cable sizing should not be based on short circuit withstand " - because overcurrent protection devices are (or should be) fitted.

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#3

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 5:28 AM

All cables during fabrication are subject to short circuit testing´s... but that´s just there. I could point you a very good manual for it, but it is written in Portuguese.

Do you know that language?

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#5
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Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 6:47 AM

I don't know Portuguese but you can mail me the document. I can translate it on google translator.

Thanks !!

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Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 7:50 AM

It is highly inadvisable to post e-mail addresses, as well as contrary to CR4 rules. E-mail addresses are easily harvested by spambots, which then bombard the address with scam bait for pharmaceuticals, risqué materials and schemes for getting money out of Nigeria, thereby rendering the address practically useless.

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#12
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Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 4:32 AM

so mate, you know the drill! any way it is a big book and it would take me a lot of time to digitalise it... do your homework please.

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#4

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 6:35 AM

What are you trying to accomplish by sizing bus bars to SC rating? Protecting bus bars, equipment and load devices will only be accomplished via protective devices (switchgears). Assuming we size bus bars to carry the Isc, that way bus bar can handle Isc and transmit it to your load or equipment, aside from the invalid costly practice of doing so, your equipment are still rated with respect to your systems power parameters, therefore, you'll be jeopardizing a big part of your system. On the other hand, if you have protective gear in place to avoid such a disaster, then all you have accomplished is raise the cost of your system since your switchgear will only allow the running load current to run through your circuit.

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#9

Re: Low Voltage Cables Sized Based on Short Circuits

07/12/2012 10:41 PM

avnishnarula86,

In the USA, the National Electrical Code, in Article 310, notes that for derating purposes (related to environmental temperature and number of conductors in a conduit) the allowable ampacity of AWG-14 wire with THHN insulation is 20 amps and for AWG-12 wire (the next larger size), it is 30 amps. However, for each of these, the maximum allowed overcurrent protective devices are 15 amps and 20 amps respectively. Notes in the code do not explain this, however commentary in related books notes that the reason for the 15 or 20 amp limits is because the wires could overheat to the point of melting "fusing" if protected by a larger rated device when a short circuit occurs.

I believe this is an example of the limits you are asking about. The absence of these limits on the larger size wires (cables) suggest to me that as the conductor's cross-sectional area increases, there is a greater ability for it to absorb the thermal energy of a short circuit until the overcurrent protective device operates.

The problem emerges again with cables run in parallel. The normal current is split equally (if designed and installed correctly) between the separate conductors. Short circuits in the equipment downstream are not a problem, as noted above, because the minimum size cable for which parallel operation is OK is much larger than those noted above. However, if the fault is within one of the paralleled cables, then the short circuit current will be much larger than that cable can handle, and melting or even vaporization is possible, with resultant damage to surrounding areas. You can buy "cable limiter" fuses that are designed for installation directly on both ends of the paralleled cables. Their ratings and design will make such a catastrophic fault virtually impossible.

--JMM

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#10

Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 12:36 AM

You DO NOT allow extended short in any circumstance. It stresses and break the generator. And no cable is capable to absorb the energy dumped into it in an extended short.

I saw a 100 -150meter long 450V(?) three phase underground cable hit by a lightning. Many isulation punchthru due excessive voltages. But, many insulations melting due high currents.

No, sizing for shorts is a bad idea to start with.

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#11

Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 3:20 AM

On an HV circuit generally volt drop is not an issue. In the event you do get a fault its takes a short period of time to isolate the circuit. You need to ensure the cable can withstand the mechanical / heating forces. I've always used the graphs and charts at the front of the BICC cables book but that book is near 25 years old.

LV cables you have more issues with volt drop and generally by the time you have sized them they are well able to withstand the fault current.

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#13

Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 8:40 AM

Simply because LV cable /conductors are sized based on how safely it can carry a certain amount of current at certain temperatures only. LV cables are not designed nor intended to be used as a protective device in electrical circuits.

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#14
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Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 9:14 AM

You don't use either an LV or HV cable as a protective device!

The idea is the fuse or breaker protects the circuit.

It might be that the cable has to withstand thousands of amps in a fault state during the time it takes the breaker to clear the fault. The cable has to be of sufficient magnitude to be able to withstand these stresses. On an HV cable like this it is sized so that it can be re-used safely - 120mm² is not unsual on an 11kV cable although it may only be carrying a few tens of amps.

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#15
In reply to #14

Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 10:45 AM

"LV cables are not designed nor intended to be used as a protective device in electrical circuits." Which part of the above comment or statement did you not understand???

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#16

Re: Low Voltage Cables Sized Based on Short Circuits

07/13/2012 11:27 PM

Who said that "the LV Cable sizing should not be based on S/C withstand?" I am surprised. Aren't all cables-whether HV or LV-sized for S/C withstand also?

The formula for S/C sizing of cables is:

S = I Root 't' / K

Pl. refer to IS 732 or the latest National Electric Code of India 2011. The constant values are given in that.

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#17
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Re: Low Voltage Cables Sized Based on Short Circuits

07/14/2012 8:55 PM

The formula is as meaningless as it can get. For a starter, is I the operational current of , say 150Amps, or the short circuit's, say 15000Amps. For how long? Over what cable length? What resistance?

Example: 45kV 150A normal. 1 Ohm cable, 10meters long. 1 Ohm generator.

Normal condition: 45kVx150A= 6,750 MW transmitted

1 Ohmx1502A= 22,5 kW loss in the generator, and in the cable too. The generator may be comfortable with it, the cable may run mighty hot.

Let's short the cable: 45kV into 1Ohm+1Ohm=22,5kA

1+1Ohmx 225002A = 506 MW 250MW burned up in the generator, 250MW burning up the cable in a spectacular fireworks is a fraction of a second.

Now, a primitive scenario will not happen, because the generator's magnetics will saturate much earlier. But, the generator coils get damaged, the switch gear and cable burned beyond repair in seconds.

This is, why there are power network specialists to size every component iust right. This is, why I do not fool around with forces I do not fully understand. This is, why a haphazard quote from a rule book leaves me worse, than unimpressed.

.Other than that, do as you please.

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