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MCB Short Circuit Test

02/07/2018 2:26 AM

Conducted 63A SP short circuit test for 10KA,in dual voltage (three phase),at rated short circuit capacity,as per IS/IEC 60898-1.Heavy blast & melting of incoming supply cable in all three single pole were observed.Trip time was around 60-65 ms.But poles operation was intact & continuity was there after the test.

Sample were pretested in magnetic trip test at 10 In,C curve.trip time foe all samples was between 15-20 ms

What can be the probable cause of above issue?

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Guru
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#1

Re: MCB Short circuit test

02/07/2018 2:55 AM

Undersized cables.

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#2
In reply to #1

Re: MCB Short circuit test

02/07/2018 3:24 AM

Copper cable of 25 sq mm

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#3
In reply to #2

Re: MCB Short circuit test

02/07/2018 3:28 AM

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.

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#4
In reply to #3

Re: MCB Short circuit test

02/07/2018 4:27 AM

Hi,

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

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#5
In reply to #4

Re: MCB Short circuit test

02/07/2018 5:22 AM

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>

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#6
In reply to #4

Re: MCB Short circuit test

02/07/2018 7:59 AM

IEC60898-1 is for circuit breakers, not cables.

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#8
In reply to #1

Re: MCB Short circuit test

02/07/2018 2:49 PM

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

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#7

Re: MCB Short circuit test

02/07/2018 8:11 AM

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

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#9

Re: MCB Short Circuit Test

02/07/2018 10:45 PM

Hi CR4 user Rixter,

I have tested the 63A MCB in two different (government of india approved) institutes & observed same issue i e blast & melting of cables in three phase only.It didn't observed in single phase.Same issue was observed in 20A DP MCB.

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#11
In reply to #9

Re: MCB Short Circuit Test

02/08/2018 3:32 AM

Then the technique is fundamentally wrong.

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

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#12
In reply to #11

Re: MCB Short Circuit Test

02/08/2018 8:14 AM

Dear CR4 user PWSlack,

Anticipating solution.I already mentioned that it observed in three phase only not in single phase.

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#15
In reply to #12

Re: MCB Short Circuit Test

02/08/2018 11:33 AM

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....

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#13
In reply to #9

Re: MCB Short Circuit Test

02/08/2018 9:08 AM

What we have here is a failure to communicate. I proposed that your short circuit test actually created an arc flash condition. To confirm or refute my supposition I asked for the current level you expected to occur in your test and you gave me partial information on the type of circuit breaker. I asked for the current you expected to see if that agreed with the amount of energy required to cause a blast and melting of copper.

I now suspect that you don't understand the procedure you are following. (Happens to us all from time to time.) Unfortunately for you, to help you we must understand what you actually did and not what you think you did. This can only happen if you answer our questions.

Good luck. Try not to kill anyone.

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#10

Re: MCB Short Circuit Test

02/07/2018 10:45 PM

Hi CR4 user Rixter,

I have tested the 63A MCB in two different (government of india approved) institutes & observed same issue i e blast & melting of cables in three phase only.It didn't observed in single phase.Same issue was observed in 20A DP MCB.

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#14
In reply to #10

Re: MCB Short Circuit Test

02/08/2018 10:45 AM

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?

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#16
In reply to #14

Re: MCB Short Circuit Test

02/08/2018 12:39 PM

Hallo CR4 User Oomborrie

Its development short circuit test of MCB.So whatever failure recd is only feedback ,so that some improvement will be done to make the device i MCB 100 % safe.

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#17
In reply to #16

Re: MCB Short Circuit Test

02/08/2018 11:56 PM

This may be a case of manufacturing defect or spurious supply. If you can tell me the make of the MCB (through CR4 mail privately if you do not want to name it publically)I may be in a position to give some guidance. In a similar case I got replaced around 200 MCB by the manufacturer in one of my projects. Ours was a very large new project with installation of many Substations and distribution systems. This was around 1989. The manufacturer was new in the market. If you can prove the case to manufacturer (incase they are reputed) they will replace it and will carry out the corrective measures.

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#18

Re: MCB Short Circuit Test

02/09/2018 11:06 PM

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.

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#19
In reply to #18

Re: MCB Short Circuit Test

02/09/2018 11:57 PM

Dear CR4 User Mr Ramvinod,

Pl find the screenshot of IEC60898-2002-1,where details of cables acc/to current rating is given for MCB testing. MCB 63A poles checked in UL benglore recently for 10kA short circuit test.

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#20
In reply to #19

Re: MCB Short Circuit Test

02/10/2018 12:25 AM

Where is the screen shot? please share.

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#30
In reply to #20

Re: MCB Short Circuit Test

02/14/2018 3:32 AM
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#21
In reply to #18

Re: MCB Short Circuit Test

02/10/2018 2:24 AM

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.

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#22
In reply to #21

Re: MCB Short Circuit Test

02/10/2018 5:44 AM

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)

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#23
In reply to #22

Re: MCB Short Circuit Test

02/10/2018 7:55 AM

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.

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#24
In reply to #23

Re: MCB Short Circuit Test

02/10/2018 12:53 PM

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.

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#25
In reply to #23

Re: MCB Short Circuit Test

02/10/2018 2:00 PM

Ramvinod,

It is "developement" short circuit testing of MCB & MCB withstand the test for 65 ms against 100 ms as per IEC 60898.But issue is test fails as cable melts or burns.It happens only in three phase short circuit test.

I think we r focusing only on melting of cable.Solution is required for failure of mcb in debelopment test due to this cable issue.

With due respect to all, a guidance from MCB design & testing expert,if any,is required.

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#26
In reply to #25

Re: MCB Short Circuit Test

02/10/2018 3:01 PM

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

Stop now before somebody gets hurt.

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#29
In reply to #26

Re: MCB Short Circuit Test

02/13/2018 5:36 AM

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...

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#27
In reply to #25

Re: MCB Short Circuit Test

02/10/2018 3:23 PM

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

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#28
In reply to #22

Re: MCB Short Circuit Test

02/13/2018 5:27 AM

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?

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#31

Re: MCB Short Circuit Test

02/14/2018 9:35 AM

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.

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#32
In reply to #31

Re: MCB Short Circuit Test

02/14/2018 10:34 AM

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

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Guru
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#33
In reply to #32

Re: MCB Short Circuit Test

02/14/2018 1:49 PM

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

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#34
In reply to #33

Re: MCB Short Circuit Test

02/14/2018 6:49 PM

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.

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