Thermal stress verification of cable

[zxfabb]

Per IEC 60364-4-43, the thermal stability of cable shall be verified by the following method:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
434.5.2 For cables and insulated conductors, all current caused by a short-circuit occurring at any point of the circuit shall be interrupted in a time not exceeding that which brings the insulation of the conductors to the permitted limit temperature.
For operating times of protective devices <0,1 s where asymmetry of the current is of importance and for current-limiting devices k²S² shall be greater than the value of the let-through energy (I²t) quoted by the manufacturer of the protective device.

For short-circuits of duration up to 5 s, the time t, in which a given short-circuit current will raise the insulation of the conductors from the highest permissible temperature in normal duty to the limit temperature can, as an approximation, be calculated from the formula:

t=(k*S/I)²


where
t is the duration, in second;
S is the cross-sectional area, in mm²;
I is the effective short-circuit current, in A, expressed as an r.m.s. value;
k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. For common conductor insulation, the values of k for line conductors are shown in Table 43A.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My question is: is there a counterpart regulation in NEC? Thanks.

 

[topgone]

Per IEC 60364-4-43, the thermal stability of cable shall be verified by the following method:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
434.5.2 For cables and insulated conductors, all current caused by a short-circuit occurring at any point of the circuit shall be interrupted in a time not exceeding that which brings the insulation of the conductors to the permitted limit temperature.
For operating times of protective devices <0,1 s where asymmetry of the current is of importance and for current-limiting devices k²S² shall be greater than the value of the let-through energy (I²t) quoted by the manufacturer of the protective device.

For short-circuits of duration up to 5 s, the time t, in which a given short-circuit current will raise the insulation of the conductors from the highest permissible temperature in normal duty to the limit temperature can, as an approximation, be calculated from the formula:

t=(k*S/I)²


where
t is the duration, in second;
S is the cross-sectional area, in mm²;
I is the effective short-circuit current, in A, expressed as an r.m.s. value;
k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. For common conductor insulation, the values of k for line conductors are shown in Table 43A.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My question is: is there a counterpart regulation in NEC? Thanks.

Try 310.15(A)(3)

 

[zxfabb]

If I understand correctly, article 310.15 deals with the ampacity of conductor in normal working condition.

Do you need to verify the performance of conductor under fault conditions like short-circuit?

 

[GoldDigger]

If I understand correctly, article 310.15 deals with the ampacity of conductor in normal working condition.

Do you need to verify the performance of conductor under fault conditions like short-circuit?

Instead of describing the performance (results) standards which must be met, along with either the testing or the computation to demonstrate compliance, the NEC is based on allowed and required practices (methods) to be followed.
As such, it assumes that if the circuit protection meets one of the UL-described performance characteristics and the wire meets one of the UL-described temperature and ampacity categories, then as long as the tables of ampacity versus conductor size and type and the rules regarding the maximum rated current of the OCPD and the current interrupting capability of the OCPD are followed, the results will be satisfactory.
The performance standards for the materials are certified by UL or other independent testing labs instead of being certified by the manufacturer alone, and do not need to be independently verified by the user of those materials.
If the NEC sizing rules are followed, it will insure both normal current and fault current safety.

 

[Sahib]

Do you need to verify the performance of conductor under fault conditions like short-circuit?

May be yes. Check Art.240.4

 

[petersonra]

May be yes. Check Art.240.4

what does that have to do with the question at hand?

there just is no requirement that a user or installer verify performance levels at all in the NEC.

 

[Julius Right]

First at all, IMO, IEC 60364 ?Electrical installations of buildings? it is not a compulsory code ?as NEC- but it is a ?recommendation? and it is close to IEEE Std 241 Recommended Practice for Electric Power Systems in Commercial Buildings
more or less.
I think NEC provides the following connected to short-circuit protection:
VII. Service Equipment ? Overcurrent Protection
230.95 Ground-Fault Protection of Equipment (A) Setting
Current maximum setting 1200 A time delay 1 sec if Isc=>3 kA.
VIII. Services Exceeding 600 Volts, Nominal. 230.205 Disconnecting Means.
230.208 Protection Requirements.The same as art. 230.95
I don?t think UL is providing anything about the short-circuit cable withstanding [at least nor UL83,44 neither UL1072 ?may be other].The cable short-circuit issues is treated by IEEE.

 

[Sahib]

what does that have to do with the question at hand?

there just is no requirement that a user or installer verify performance levels at all in the NEC.

I do not think that the OP wants to conduct a short circuit test to verify the suitability of cables to be used in a site. His intention may be to check the cable for short circuit capacity just as he may like to verify it for voltage drop. The code has mentioned in the informational note of Art 240.4 the method of short circuit calculations.

Now the question is whether it is required. For that I refer you to an interesting discussion at

http://forums.mikeholt.com/showthread.php?t=145291&page=2

(Start with T.M.Haja Sahib post#13)

The missing link in last post there is here.

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Conductor_Protection_Wire_Cable_Protection_2.pdf

 

[don_resqcapt19]

The Cooper/Bussmann link in the previous post does show the maximum withstand current for conductors, however, as Jim said in the thread that was linked to in the previous post, the breaker testing standard requires that the conductor be undamaged when the breaker interrupts the maximum fault current that it is rated for.

 

[Sahib]

the breaker testing standard requires that the conductor be undamaged when the breaker interrupts the maximum fault current that it is rated for.

The code also requires it.
So the inspector may check it (by calculation at site).

 

[don_resqcapt19]

The code also requires it.
So the inspector may check it (by calculation at site).

The point is that when you are using a listed OCPD, there is no need to do any field calculations for the conductors on the load side of the OCPD.

 

[Sahib]

The point is that when you are using a listed OCPD, there is no need to do any field calculations for the conductors on the load side of the OCPD.

Fuses are listed and circuit breakers are listed. But if circuit breakers are substituted for fuses in the field, the safety of the cables they protect may be compromised and so it requires checking per paper in post#8.

 

[don_resqcapt19]

Fuses are listed and circuit breakers are listed. But if circuit breakers are substituted for fuses in the field, the safety of the cables they protect may be compromised and so it requires checking per paper in post#8.

Not true. They are both tested the same way. If the conductor is sized per the NEC and connected to the load side of a listed OCPD there is no need to do any calculations.

You have to be careful using the information in paper from post 8, as with any manufacturer supplied information. Such information is often used as tools to push users to the use of specific types of equipment. That paper, while having a lot of very good technical information, is part of the ongoing fuse v breaker war.

 

[jim dungar]

If the conductor is sized per the NEC and connected to the load side of a listed OCPD there is no need to do any calculations.

Absolutely true for <600V applications.
The NEC says that inspectors are allowed to use standards such as UL489 (for molded case breakers) and UL98 (for switches) rather than performing field calculations.

 

[Sahib]

Absolutely true for <600V applications.
The NEC says that inspectors are allowed to use standards such as UL489 (for molded case breakers) and UL98 (for switches) rather than performing field calculations.

Very nice that along with size of the OCPD, minimum size of the cable to be used with it is also stated by the standards for <600V applications. But is it necessary to do the calculations for >600 V applications? Thanks.

 

[jim dungar]

But is it necessary to do the calculations for >600 V applications? Thanks.

Yes, conductor protection should be part of applying MV relays. I would not expect an inspector to do this in the field. It is usually performed by qualified individuals with the supporting Time Current Curves supplied for reference.