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Why Did This Cable Burn?

09/02/2015 6:40 AM

We have an imbalance 3 phase supply to a kitchen using 25sq.mm PVC cable which were burnt on yellow phase cable last Sunday. From our site measurement, the current reading are R=10.38A, Y=45.6A & B=16.9A. As what shown on cable catalogue, cable rating of 25sq.mm cable shall be 114A for 2 cable single phase and 104A for 4 cable 3 phase installation. The data indicated the cable rating shall be sufficient on existing load. Is there any reason why this happened ? Is there formula which I can use to analyze the situation from existing data ?

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#1

Re: Why this cable burnt ?

09/02/2015 6:47 AM

I don't have the answer but I do know that more information is needed. Was you cable in conduit? Were there other cables in the conduit? If so, what were the other cables and how much current were they carrying? In addition, how confident are you that your current measurements represent the load at the time the cable got too hot?

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#2
In reply to #1

Re: Why this cable burnt ?

09/02/2015 7:41 AM

Or possibly a poor (or loose) termination.

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#4
In reply to #1

Re: Why this cable burnt ?

09/02/2015 9:11 AM

The cables are exposed to air. Not in conduit or trunking

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#3

Re: Why Did This Cable Burn?

09/02/2015 8:28 AM

There are a couple of questions that might lead you to the answer.

Are you measuring peak or RMS voltage? (And then is the cable rating peak or RMS?)

Next, your load is so far out of balance that the earth is also carrying significant current. Earth wire will also be heating due to current flow. Plus I have a suspicion about earth drift for a system so far out of balance.

Add these to the previous questions about conduit and other impediments to heat dissipation and you will probably be getting close to the answer.

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#5

Re: Why Did This Cable Burn?

09/02/2015 9:24 AM

Where did it burn, at one or both of the terminations, or somewhere in the middle, how long is the run, is this a new installation, etc.? Something to investigate is whether the cable was properly handled; i.e., if it was kinked, crushed, or pulled to hard, then there might have been hidden internal damage that broke some strands or reduced their diameter sufficiently to cause a hot spot and consequential cascading failure due to the reduced current carrying capability.

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#16
In reply to #5

Re: Why Did This Cable Burn?

09/03/2015 2:33 AM

The cable burnt at one end near termination. The run is only 1 meter length in meter compartment. The installation was made about 15 months from project handing over.

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#18
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Re: Why Did This Cable Burn?

09/03/2015 3:12 AM

then I would take a long hard look at post 8 from PWSlack, I have to agree with him 100% based on what you've just stated.

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#20
In reply to #18

Re: Why Did This Cable Burn?

09/03/2015 4:48 AM

I also agree it is probably a loose/dirty connection.

The complete terminal may need to be replaced if it is highly oxidized from heat.

If it is aluminum wire,be certain to clean the wire and apply an anti-corrosive

compound to the wire before terminating it in the panel.

However the high current on one leg is unusual,indicating an unbalanced

load.

However,this ,in and of itself, would not cause the problem you described.

Check all conductor terminations while you are at it.

The yellow may simply have been the worst of them.

Other failures may occur soon.

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#26
In reply to #20

Re: Why Did This Cable Burn?

09/03/2015 9:22 AM

"The complete terminal may need to be replaced "

From a historical perspective, I believe personnel replacement is appropriate.

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#6

Re: Why Did This Cable Burn?

09/02/2015 9:53 AM

"Is there any reason why this happened?" Yes

"Is there formula which I can use to analyze the situation from existing data?" No.
If the cause isn't obvious to you, you need to hire a competent electrician.

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#7

Re: Why Did This Cable Burn?

09/02/2015 10:32 AM

Yes.

Possible reason(s) for cable failure:

If the applied voltage exceeds the rating of the cable the insulation will fail.

If moisture penetrates the cable a short and/or earth/ground fault will occur and damage or destroy the cable.

If the cable insulation was damaged during the installation or was/is faulty due to manufacturing defects, the cable will fail.

Solution:

A section of the faulted cable can be sent to a certified test facility for evaluation and testing to identify the root-cause of failure.

Application of any formula without factual cable testing DATA will only be a waste of time.

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#8

Re: Why Did This Cable Burn?

09/02/2015 1:29 PM

The cable burned because of excessive heat.

The excessive heat was caused by any, some or all of the following:

  • Cable incorrectly terminated
  • Cable termination correct though loose
  • Cable undersized for the current being carried, indicating incorrect pre- or modified post-selection or installation method
  • Damaged conductor
  • Cable protective device(s) did not disconnect an overload current in time, indicating either (a) faulty or modified device(s)
  • Cable was subjected to an external heat source to its detriment

In the UK, British Standard 7671 applies. Other countries have different standards within which competent local Electricians work. Consulting a competent local Electrician is highly recommended, and may well be a requirement imposed by the building's Insurers and/or local legislation.

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#9

Re: Why Did This Cable Burn?

09/02/2015 2:46 PM

In my opinion, a cable it is a resistant staff. No overload could happened you can be noted in time. If it burned that means short-time very high current=short-circuit.

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#10

Re: Why Did This Cable Burn?

09/02/2015 2:55 PM

Sounds like a bad connection.....clean it off, reconnect it, and check connection under load for temperature increase.....use amp probe on each leg....what is causing the high amp draw on this leg? ...You might need to balance the load, if possible...

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#11
In reply to #10

Re: Why Did This Cable Burn?

09/02/2015 3:02 PM

Each leg/phase will have a circuit protective device. If that device is unable to operate before the cable heats up through overload, then balancing the load could make the problem three times worse when it does and three times less likely to happen, in rough terms. It doesn't sound like a solution, as each leg/phase ought to be able to operate up to the maximum current allowed by its own protective device(s) without damage occurring to the cable; that is what the protective device(s) is/are there to do.

Using a contactless thermometer only verifies that there is a problem and, if so, its location, which the original poster has already done.

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#12
In reply to #11

Re: Why Did This Cable Burn?

09/02/2015 3:19 PM

The thermometer is used to verify that a good contact now exists....or not(might have damage)....balancing the load where possible is only common sense...The circuit breaker won't trip in this case because the loose connection was creating resistance to electrical flow and causing heat, which in turn burned the connection...an overcurrent event never happened....If there are no other contributing factors existing...

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#13

Re: Why Did This Cable Burn?

09/03/2015 12:08 AM

Quick question, you said cable, so can it be assumed that this is not in conduit?

What type of cable is it? Round or Flat?

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#14
In reply to #13

Re: Why Did This Cable Burn?

09/03/2015 12:13 AM

#4

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#15
In reply to #14

Re: Why Did This Cable Burn?

09/03/2015 1:31 AM

thanks.. missed that bit... I assumed correctly!! still wondering what sort of cable he's using!!

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#17
In reply to #13

Re: Why Did This Cable Burn?

09/03/2015 2:38 AM

25sq.mm PVC cable

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#19
In reply to #17

Re: Why Did This Cable Burn?

09/03/2015 4:45 AM

25sqmm refers to the overall area of the conductors. The cable is described as flat, round, armoured etc. Another designator will be the number of and area of each individual strand of conductor, e.g. 19/1.6 vs 100/0.25. Your answer to the questions will help to give a better answer.

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#23
In reply to #19

Re: Why Did This Cable Burn?

09/03/2015 6:45 AM

7wire/2.14mm, thickness insulation 1.2mm, overall dia 8.9mm

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#28
In reply to #23

Re: Why Did This Cable Burn?

09/04/2015 5:41 AM

Do these conductors look coppery or black?

Based on your sizing I would agree with most posts that it is a poor connection, caused by either loose screws or corroded wire or terminal.

I once burnt my fingers on a very hot ceramic fuse holder ( the type that has thin fuse wire) due to aged, blackened, brass connectors. My fingers sizzled as soon as i touched the ceramic. The heat from the fuse holder traveled up the wire melting the insulation. Sound familiar?

Jim

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#21

Re: Why Did This Cable Burn?

09/03/2015 6:18 AM

There appears to be a great deal of emphasis among replies being put on the load imbalance and how the cable is installed but, provided the load current figures are reasonably accurate and did not increase appreciably during the time that the cable became damaged, then neither the imbalance nor the method of installation can have anything to do with the problem as the current rating of the cable is sufficient to carry a significantly larger current than has been recorded even with much greater load imbalance and allowing for any de-rating factors that may be considered for any enclosure method.

The most likely cause should be considered to be a faulty termination closest to the damage.

I am confused by one post that states the earth conductor will be carrying excess current due to the imbalance, I fail to see any reason in an otherwise healthy installation for this being the case as the neutral conductor would normally properly fulfill this function.

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#22

Re: Why Did This Cable Burn?

09/03/2015 6:38 AM

I assume that you have 3 phases and neutral. Did you measure the neutral current? If yes, what was it?

Is the neutral wire the same size as the phases?

Also, a little bit of balancing would probably do more good than harm.....

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#24
In reply to #22

Re: Why Did This Cable Burn?

09/03/2015 6:48 AM

Neutral cable is same size. But the current not measured

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#25
In reply to #24

Re: Why Did This Cable Burn?

09/03/2015 6:59 AM

It appears you have a lot of single phase load(s) on the system,but considering the wire size,this would not cause overheating.

However,a loose termination at the end(s) would cause the yellow wire to heat up more than the others,due to the higher current through the conductor.

Perhaps they are all loose,or corroded,or both,and the yellow is only the first to show.

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#27

Re: Why Did This Cable Burn?

09/03/2015 9:48 PM

unbalanced load will not damage your cable as long as below cable capacity.

possibly not so good termination,loose termination or dirty contact. if termination ends at a fuse holder a faulty fuse holder or under sized fuse can create local heat.only 1 meter long,. replace and monitor dont forget the "end terminated to " installation.

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