In NEC land, there is no specific requirement for checking SC thermal damage limits, AFAIK. If other aspects of NEC are followed, such as sizing of feeders and overcurrent protection, thermal damage of cables during faults is generally not a problem. Our software plots the cable damage curve automatically. In decades of doing coordination studies, it's never been a factor.
I believe IEC does have some requirements, but not really sure.
If you do mean one has to check whether the prospective short-circuit current will not damage the
low voltage cable insulation in the clearing time of the protection, he has to do it.
NEC-art.240 and IEC 60364-4-43 clause 434 required this-I think.
Per NEC, it is required for transformer secondary conductors less than 1000V that are over 100 feet long without an OCPD on the secondary of the transformer and that do not have differential protection. It is also required for conductors over 1000V.
As dpc says it's easiest if you're using software that shows this automatically. If not, you might want to check the withstand capability in situations where:
-The available short circuit current is high,
-The trip time is long (e.g. no instantaneous trip),
-The amount of conductor cross section is low, which may be caused by:
--Taking advantage of higher insulation temperature ratings,
--Taking advantage of a low ambient temperature and applying a derating factor, and
--Using an installation method that allows higher amapacities (e.g. free air ratings)
You may also want to check your grounded conductor in very large installations with parallel conductors where you're at the bottom of Table 250.102(C)(1). You may also want to check in cases where you're using relays with LVPCBs instead of integral trip units.
I have experienced problems while coordinating the LV breakers connected in series like;
Main LV Switchboard - Sub Main Switchboard - Main Distribution Board - Sub distribution Board - 100A Feeder for welding sockets Now the client is asking for the grading margin between all the breakers in series. It becomes a challenge.
Another typical example is when you need to supply a small power DB directly from a Main Switch Board with available short circuit level of say, 72 kA. The cable between the Main Switch Board and the small power DB may be say 4 core x 16mm2
You are right wroggent. However, what you said eventually one has to calculate in order to check it.
In IEC world there are not all this practical permissions of NEC.
First of all even it will be an instantaneous trip provided the cable could not withstand the short-circuit current if the distance to transformer terminals will be less than 50-100 ft.
Usually the circuit breaker short-circuit clearing time is approx. 0.1 sec.
The short-circuit impedance of a transformer 2500 kVA [15/0.4 kV] could be 6% [See:
IEC 600076-5 Table 1 – Recognized minimum values of short-circuit impedance for transformers with two separate windings.
and a cable of 3*6 sqr.mm copper presents 0.0012 ohm/ft. impedance.
For 0.1 sec clearing time 6 mm^2 copper at 2.4 kA will reach 250 oC [xlpe insulation].
Transformer impedance 0.4^2/2.5*6%=0.00384 ohm
A minimum 77 ft. it is not protected by usual circuit breaker.
From 150 sqr.mm copper and up, XLPE [or EPR] insulated, this circuit breaker will be ok.
I checked what will happened if the transformer is 500 kVA and 4% short-circuit impedance:
then the minimum distance will be only 70 ft.
Incidentally, I have recommended the minim length of the cable to be installed in such cases. People were surprised what the extra length of cable is used when the downstream distribution board is in the same room. I found it difficult to convince the people.
I have suggested that time to keep the extra length of cable coiled in the cable cellar.
Molded case breakers in series generally won't coordinate with each other due to the instantaneous trip. They can be coordinated in certain circumstances, but not in general. If selectivity is mandatory, fuses will work better.
This has always perplexed me a bit. Regardless of IEC vs NEC, the same physical laws must apply. Are the conductor sizes determined by the continuous load significantly smaller when sized by IEC rules compared with NEC. No question that the NEC load calculations are very conservative, resulting in conductors much large than necessary in most cases.
At a second glance on NEC art.240.92 it seems to me the protective rules are the same,
eventually, as per IEC [except the transfer of protection to the primary side of a transformer].
However, what it will be the last test it is Table 240.92(B):
“(3) The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and ground-fault conditions” And that is the IEC philosophy too.
