MCB Short Circuit Test

Undersized cables.

Copper cable of 25 sq mm

So what? The factors included in a BS7671-style calculation have not been declared so far in justifying that cross-sectional area. So the question "Where are the cable calculations?" comes to mind, as it would after any similar occurrence. After all, the cable has to be sized for its protection by the upstream circuit protective device(s), the information about which has been withheld from the forum, otherwise the phenomenon described in the original post is most likely to occur.

The cable sizing class is down the corridor, third door on the right.

Hi,

25 sq mm cable is as per IEC60898-1 2002.

Then review the installation with the individual and the calculations for the purposes of making the upstream cable protective device protect the cable that has <...melt[ed]...>, for the installation has failed to survive the event that it was intended to survive. Those calculations cannot be seen from here.

<unsubscribes>

IEC60898-1 is for circuit breakers, not cables.

Undersized cables.

Or a bad connection at the breaker.

I once tested some breakers which I suspected to be faulty in the field using a heavy loop of wire connected to the CB terminals looped through a current transformer. Using the CT in reverse, I applied a variable voltage from a variac to the terminals and monitored the current in the loop with a clamp-on ammeter.

The advantage of this method is that there is high current but very low voltage and therefore, low power in the loop. Any unwanted resistance just limits the current without the flashy fireworks.

http://cr4.globalspec.com/comment/1063688

What was the anticipated short circuit current? It sounds like you performed an arc flash test instead of a short circuit test.

Then the technique is fundamentally wrong.

Doing it repeatedly and expecting a different result is one of the first signs of madness.

The illusion is that this particular feature makes any appreciable difference. All that is happening is that the supply is capable of unloading, potentially, up to three times the energy into the fault. One shouldn't be surprised if the installation is not able to survive the experience.

Do something, or risk a measure of unpopularity with the facilty's fire insurance provider, something of which the <...government of India...> might not approve....

Umakant, every one here want to help you and all you need is answer some questions. Assuming that you are infallible makes you unhelpable. You are accusing a manufacturer of a very serious crime and to be right at all cost can get you into a despute. Visit the manufacturer and request a test to proof their device function correct under test conditions and you may learn something. The question is easy, before you set out to do the test what did you calculate the result should be. If you did not anticipate an answer to that why did you do the test, how will you know if the result is what it should be. If you did not know the correct answer in advance why did you do the test?

When short circuit test conducted on a device like MCB,the test Rig should be perfect.

There should have been protective devices to trip the system when the current goes beyond 10 kA. Also the test Rig and the cable leading to the device should be capable of withstanding the current flow of 10 kA for one sec.

In this case the 25 sq.mm copper cable will not with stand 10 kA for one sec.When you study xlpe 25 sq.mm cable characteristics this cable can carry 10 kA for 150 milli seconds only.Please check.If required the TCC can be shared.

https://archive.org/details/gov.in.is.iec.60898.1.2002

Further on this topic, you have indicated trip time of 63 ms.This is irrelevant.

This device is tested for short circuit withstand capacity of 10 kA for one sec.

The test should have been subject the device for flow of 10 kA for one second and after this ensure the device works satisfactorily.

In your case selection of leading cable 25 sq .mm is wrong , which will not withstand flow of 10 kA for one second.For 10 kA flow the minimum cross section copper cable required is 70 sq.mm.Please refer any cable data.

Cross sectional area of protective conductor can be calculated from the formula

S=root of (I square*t)/k

where S= cross section area of conductor

I = R M S value of Fault Current

t = operating time of the disconnecting device, in seconds;

k = is the factor dependent on the material of the protective conductor, the insulation and other parts and the initial and final temperatures.

Please refer IEC 60439-1 annexure B

The 70 sq.mm copper cable can not be terminated to the 63 A MCB.

The terminal of the MCB to be extended/modified for terminating 70 sq.mm Cable.

As per IEC60898-1 2002,Test E of table 7,short circuit withstand capacity of 10 kA for 0.1 s. As per table 5, cable cross section is 10 to 25 sq mm for rated current between 50A to 80A.

Requested to explain ,why melting of 25 sq mm cable occured in three phase short circuit test (400V) & passed in single phase test (230V)

When current flows in a cable the temperature of the cable will increase .

Similarly when 10 kA current flows in 25 sq.mm cable temperature will increase and insulation of the cable will fail and burst.

I do not understand the purpose of your short circuit test.

If the MCB is rated for 10 kA, means it has to withstand 10 kA for second.

If you are testing the MCB for short circuit, you have to pass 10 kA for one sec and verify its withstanding capability.

Ramvinod, I think he confuses Short Circuit current with controlled 10kA current. He do not answer questions and that make it a waste of time. He think because it is a breaker that must withstand 10kA the breaker will limit the current under short circuit conditions to 10kA. This is my take on it and as long as he do not answer questions we can not help him.

No. A circuit breaker does not regulate the amount of current that happens during an arc flash event.

Stop now before somebody gets hurt.

Quite.

So far the original poster has revealed nothing about the equipment being used to protect these cables, and there is, admittedly by the original poster, a mismatch between that equipment and the cables connecting it to the test rig. Which is why those cables exploded.

The plot thickens...

Umakant look at my post 14 where I suggest you visit manufacturer of MCB and learn from them.

How is it known that the short circuit current in this rig is limited to only 10kA (rhetorical question - NNTR)? The exploding cables and the calculations indicated necessary for sizing them correctly by others in this thread would suggest that any assumption that 10kA is a maximum short circuit current in this rig is certainly invalid.

Something that hasn't been explained is why this testing is in any way necessary. Manufacturers type-test their equipment in similar (though apparently better-designed!) rigs in order to establish the breaking rating before releasing them to market. Their reputation for supplying that equipment depends upon it. In this "government-approved laboratory" (WTF does that mean [rhetorical question - NNTR]?), it isn't the breaker that is suspect: it is actually the cables connecting it onto the mains as the original poster indicated in the original posting.

The original poster is withholding more than is being revealed, and quoting reference standards that are not particularly relevant isn't particularly helpful. One might suggest that the only standard applicable in the case of the supply cables to this rig is the relevant national electrical code for wiring, which is designed to protect the upstream cables under all conceivable conditions. One of those is the closing of the breaker onto a short circuit fault for testing purposes, which the cables were, admittedly by the original poster, unable to withstand. Therefore the cable sizing is still suspect.

So here we go again: British Standard 7671?

Experimentally:

• Isolate the power
• Remove the MCB from circuit and place it to one side
• Short-out the cables that were supplying the input side of the breaker
• Re-energise from a safe distance.

If the cables still go KABOOM! then it has got stuff-all to do with the breaker or IEC 60898-1 and everything to do with inadequate attention to the design of the rig in the area of cable sizing, irrespective of what the Indian Government, or anyone else for that matter, says.

Please ensure that all other CR4 readers are standing well clear before doing so, and come back to the forum, if survival has been successful, and report.

Any failure to return will confirm the suspicion that the cables are undersized, as indicated at #1⇑, leaving the CR4 community with one more silent poster.

Make sure your last connection switch is remotely controlled. If you don't then this can happen.

There are no <...connection switch...> installations here that could do that.

Don't be too sure of that. It was reported that this breaker may have been damaged prior to the arc flash but not visibly damaged enough this licensed electrician refused to flip the lever. He and the breaker were certainly badly damaged afterwards.

I almost forgot, this breaker was rated to interrupt a 10kA incident. To my knowledge the cables survived and the arc was quenched with a bang.