How to use CL CB's let-through curves

[veritas]

I have a question regarding let-trhough curves of fuses and circuit breakers. I have researched this forum but not found any thread that exactly addresses the question I have. Consider the attached pdf.

The top let-through curve is that of a fuse. We are dealing with a 80A fuse and the prospective fault current is 30kA. According to these curves, the fuse limits the rms fault current to around 6kA which is 8.4kA peak. Thus the current limiting (CL) action of the fuse allows the downstream network to be designed to 6kA and not 30kA.

The bottom curves are those for Schneider NSX range of CL breakers. Schneider, like most LV breaker manufacturers, provide cascading curves where the CL action and let-through energy allows for lesser rated cb’s downstream than the prospective fault level at the downstream location.

So let’s say I have a cascading combination in place, breaker A upstream and lower rated breaker B downstream. What should the fault rating of the cable between them be? Let’s say I have a NSX630A breaker upstream, Iprospective = 100kA. If the peak let-through is 46kA does that mean the rms let-through is 46/1.414 = 32.5kA? So my cable only need be rated for up to 32.5kA and not 100kA? Is this correct?
I am aware that there are let-through energy curves for CL cb’s as well but I have no idea if they’re any use when it comes to determing the reduced symmetrical fault current due to the action of the CL breaker.
Am keen to hear some thoughts regarding how to use these cb let-through curves to determine the reduced symmetrical fault current, if indeed this can be done!

 

[waross]

A comment on extra cable length to reduce fault levels.
Extended cable length is a tried and true method for reducing the downstream fault levels.
For instance; Additional cable length of a feeder may be used to limit the fault current at a sub panel and allow lower rated components to be safely used.
However, extending the length of a 15m cable will lower the maximum fault current at the load but does nothing to limit the current or the cable damage in the event of a fault in the first 15m of the cable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[veritas]

waross - agreed, however in this application we are looking already congested cable trays and also the issue of further derating cables due to added length of the given cable. As the installation is above ground in conduit or in cable trays a fault cable will just be replaced. Thus the location of the fault along the cable length does not really matter then in that regard. So, extending the length of the cable is really about lowering the throughfault current.

wroggent - I normally consider not only the trip time of the 1st upstream device but also backup time and/or breaker fail where applicable. With this particular application upstream breaker should clear the fault in inst time or around 30 - 50ms. Backup is the incomer ACB set to 200ms. I allowed 50ms margin and thus the 250ms. I guess there's quite a bit of subjectivity here are to what should the withstand time be.

Also, I do not consider the effects of the CL breaker initially, I determine the I2t based on prospective current and trip time as discussed above. This shows that the cable is in trouble. Then the CL breaker is brought into the picture and saves the day.

Another point I'd like to mention - when using the let-through energy curves of the breaker or fuse, the horizontal has the prospective fault current. With an IEC0909 analysis does one use the Ik" (initial symmetrical) or Ik steady state symmetrical once the AC decays has died off?

The application I had, had numerous DOL motors connected to the MCC which resulted in Ik" being > Ik by around 9.5kA. So which one to use for the prospective fault current?

I used Ik" as this is the actual initial symmetrical current that flows, albeit for only a few cycles and then dies down to Ik. But as far as the breaker is concerned it is the symmetrical current during the 1st half cycle that counts and so Ik" gets my vote - and that's why DOL contributions cannot be ignored.

This leads to the next question - what about the peak due to the DC offset? In the NEMA world could you use the initial asymmetrical current? (thus is the rms equivalent of the initial symmetrical with DC offset combined). My initial thoughts are no. With prospective is meant symmetrical.

Another point of interest - a CL breaker does not interrupt the fault current within the 1st half cycle. It actually interrupts the fault current at the 1st zero crossing which is 10ms for 50Hz and 8.3ms for 60Hz. The breaker introduces an arc into the fault current path which dramatically reduces the fault current and at the next zero crossing the current is interrupted. This is only applicable for very high fault currents. As the fault current decreases the CL action diminishes and trip times are longer and as per the TCC for the device.