TERASAKI LV MCCB low Icw rating 3

[veritas]

The XH630NE Terasaki mccb has an Icu value = 65kA (rated ultimate short-circuit current) and Ics = 50kA (rated service short-circuit current). However, the rated short-circuit withstand current (Icw) is only 10kA for 0.3s. This tells me it is only good for a threephase throughfault of 10kA for 0.3s. Why so low? Normally the given Ics and calculated Ith values are within 15% of each other.

I have an application where IEC 60909 fault study results are Ik" = 30.3kA and Ith = 32.5kA. From a short-circuit interrupting capability no problem but the withstand current rating needs to be > 32.5kA.

Does the cb low Icw rating have something to do with it being current limiting?

Have tried my local Terasaki contact but he was just as puzzled as I was.

Thanks in advance.

 

[LiteYear]

Just a WAG, could it be because for all currents at or above 10kA, the MCCB is designed to clear well within 300ms? In other words, it'll clear a 50kA short circuit current, but if the clearance is defeated somehow it will only last 300ms at 10kA. I think the NE series have a current limiting feature, so they're probably designed to absorb a bit of the fault current energy, hence the relatively low Icw.

 

[ppedUK]

Depends if you're looking at a category A or B MCCB (IEC World). Cat A are not designed to have delayed STPU (S) settings, just long time (L) + instantaneous (I) high set trips and these MCCBs generally have low Icw ratings. Cat B MCCBs can be fitted with protection modules having selectable time delayed STPU (S) settings in addition to L + I trips and have slightly higher Icw ratings, but still nowhere near the Icw ratings of an equivalent size open construction ACB.

Struggling to think of an instance where your through-fault current would be higher than I"k.

 

[jraef]

Maybe it's a semantics difference in the terms "interrupt" and "withstand". Interrupt means it will stop the flow of fault current at that level, the be able to be reset and used again. Withstand I believe has no provision for either interrupting nor re-using, just not becoming shrapnel.

"Will work for (the memory of) salami"

 

[veritas]

Okay, done some further digging. ppedUK - you are correct, it is a Cat B breaker. It thus has a short time delay setting which a Cat A does not have.

Maximum short time pickup, Isd, is 10 x In = 6.3kA.
Maximum short time delay, tsd, is 0.3s.
The maximum Inst setting for the XH630NE is 12 x In = 7.56kA.

With Ics = 50kA and Icw = 10kA for 0.3s leads me to deduce that it must be current limiting and this is what it says in the TERASAKI literature. So it can safely interrupt a prospective fault current of 50kA threephase, rms, but a throughfault is limited to 10kA which will need to be interrupted within 0.3s. This means it can be used for cascading applications as well, which indeed it is according to the TERASAKI literature.

 

[ScottyUK]

Hi veritas,

The breaker may well enter a current limiting region of its curve, but don't assume it will limit the current to 10kA. The rating is saying that the breaker can survive a through-fault of 10kA for 300ms, but that doesn't mean the fault current will be limited to 10kA. The breaker allows time-based discrimination to be applied at moderate fault levels, although at high fault levels it will trip immediately.

 

[ppedUK]

This situation catches out many LV switchboard manufacturers who think they can be "cost competitive" by using MCCBs instead of open construction ACBs in switchboard incomers. I have seen boards rated at 50kA/1s (according to their rating plate) and fitted with 20KA/0.5s (Icw) incomer MCCBs. Because the Icu/Ics rating is >50kA, they (unbelievably) assume that these are acceptable.

According to the manufacturers, current limiting in an MCCB is normally achieved by the "repulsion" force of the high current acting on the contacts, rapidly separating the contacts and cutting off the fault current somewhere up the sinewave, before it reaches its peak prospective value. If there is a time delay element in the breaker, the current through the breaker will rise to the maximum prospective S.C.C. at the load terminals, so in theory, an MCCB with an intentional time delay release cannot be current limiting unless it has some kind of in-built reactor or resistor. The way that they get around this (as you have spotted) is to fix the maximum instantaneous setting below the Icw rating, so that above that Inst. threshold the breaker trips instantaneously (thus current limiting, but not necessarily to 10kA), thereby protecting itself from damage. The disadvantage of this normally occurs when you try to grade this instantaneous characteristic with downstream protection. If the fault current on the downstream circuit is higher that the Inst setting on your major MCCB, then you have a race on your hands, as your MCCB will want to trip rapidly in it's instantaneous region. As you say, your breaker can safely interrupt up to 50kA, but it can only carry 10kA for 0.3s, it will not limit the current to 10kA but will trip instantaneously above 7.56kA!

Don't bother trying cascading in MCCB circuits unless you are really desperate, it can be a nightmare to prove unless you use manufacturers's tested and certified combinations.

Don't forget also that if an MCCB clears a fault above its Ics rating, but below its ultimate rating, it should to be replaced. In theory, it can be reset once after that type fault, but how many companies keep a record of how many times an MCCB has tripped and what current it cleared under fault? Many people overlook that one too!

MCCBs have their uses, but choose carefully.

 

[veritas]

pped - an excellent post, thanks. That is hwy I prefer to use Ics and Icu as the interrupting rating of the breaker.

Your explanation makes perfect sense. High currents > 7.56kA are not entertained and Inst trip occurs. So in theory no Icw > 7.56kA rating required. The 10kA Icw rating thus provides some margin of safety.

A point to clarify - the XH630NE belongs to the Tembreak circuit breaker series. The Tembreak, Tembreak Plus and Tembreak 2 are all current limiting as per the TERASAKI literature. To reconcile this with the discussion above, I surmise that the current limiting effect kicks in for currents above the max INST setting. This is for a breaker that has no Icw rating such as the XS800NJ. For one with an Icw rating, such as the XH630NE, I would presume that the current limitation comes into play somewhere > 10kA. Thus it can interrupt prospective current up to 50kA but has no capability to carry 50kA for any length of time.

The XS800NJ has no Icw rating and hence no short-time delay. Maximum INST = 10 * 800 = 8kA. Longest LTI trip time is around 16s at 8kA. To me this could be interpreted as a pseudo Icw rating of 8kA for 16s. Just a thought.