Something wrong with my MCB, Outside fuse always blown; MCB not trip
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 Jun 15 2016, 10:45 PM
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#1
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Senior Member
5,867 posts Joined: Feb 2006 
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Same problem at one of my properties. No trip at all but outside fuse near meter blown 4 times over a period of 2 months. Found out later that one external cable extended to a birdhouse was bitten by squirrel/mouse thus causing live and neutral shorted from time to time.
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Jun 16 2016, 12:14 PM
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#2
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Senior Member
5,867 posts Joined: Feb 2006
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QUOTE(ozak @ Jun 16 2016, 09:00 AM) Not TS case. But aeiou228 case. The shorted cable has a dedicated MCB extended from the DB in house A and all the way to a birdhouse B. The cable shorted in between A and B.MCB won't trip if the short circuit happen before the MCB incoming. While aeiou228 case is after the MCB. Since the MCB have lower Amp rating than the fuse, it should trip 1st before the fuse.  Only realized cable shorted upon seeing a spark at the cable. I did ask my electrician why there is no tripping but was told when live and neutral shorted, the MCB wouldn't trip but the main fuse blown instead. Can any sifu care to explain ? |
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Jun 16 2016, 01:44 PM
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#3
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5,867 posts Joined: Feb 2006
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QUOTE(weikee @ Jun 16 2016, 12:35 PM) Every DB will have one main fuse or some call it main breaker. There are many possible how your house cable is extend, Thank you sir,01) Extension/tapping before the main fuse 02) Extension/tapping after the main fuse. 03) Extension/tapping after the main fuse and one MCB. <If all Circuit Breaker function as normal> and your Cable A & B are shorted extension 01 method, only the incoming fuse beside meter will burned extension 02 method, the main fuse will trip making entire house will have no electric extension 03 method, usually is the prefer if current are not high. The individual MCB will trip first. This is also good if you want to totally shutdown the supply from outside. You can test it out by trip the Main fuse in the DB Box. and if the birdhouse still have power mean is using method 01. These are the info that I can gether. 1) when cable A to B shorted, the was a big spark, main fuse blown off and house A and birdhouse B no electricity. 2) before knowing the actual cause of the problem. wireman came replaced the main fuse in the morning but evening fuse blown again. 3) wireman disconnected the birdhouse cable from MCB in the DB, no more blown fuse but birdhouse B no electricity. 4) upon checking no wiring problem inside birdhouse B, I asked wireman to reconnect the birdhouse cable to MCB. 5) it lasted for a few days before the main fuse blown again. Replaced the fuse again without disconnecting the MCB for the birdhouse for further investigation. 6) break through came when a spark happened before my every eyes. A and B is about 40-50m apart and the extension outdoor cable is supported by cable pole and trees. The spark happened at a tree. 7) wireman fixed the exposed cable and no more blown fuse ever since. So which extension method in this case? |
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Jun 16 2016, 06:07 PM
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#4
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Senior Member
5,867 posts Joined: Feb 2006
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QUOTE(ozak @ Jun 16 2016, 01:49 PM) MCB function is just like a fuse. Fuse blow, you need to change. MCB trip, just need to reset back. So the MCB should be able to trip. Since the MCB is 20A max trip. While the fuse is 63A blow. Should ask the electrician to test it out the MCB is function properly or not. Change it if problem. I do have many experience some plug point short. and trip the MCB. Main fuse doesn't blow. it should be in this way. Do have experience some rare case, where the main fuse blow only. Where the shortcircuit Amp instant short is too high. QUOTE(weikee @ Jun 16 2016, 02:10 PM) Can't give a firm answer base on your experience. #1 happen because incoming fuse are blow. That affect House A & B. Thank you sirs.If you happen to be in that house, just try trip the main house Master switch inside your house. Check if your birdhouse still have electric. This will confirm of the birdhouse is tap before or after the Master switch. If is tap after the master switch, and your house master switch did not trip, you need to replace it as per ozak suggest. If is before the master switch that is normal behavior. I'm very sure if the master switch in house A tripped, the birdhouse no electricity also. Happened few times already. Also, when wireman disconnected the birdhouse cable from the MCB inside the house A's DB, birdhouse house no electricity. When reconnected the cable to MCB, birdhouse got electricity. The power supply of birdhouse is definitely tapped from DB of house A. So in this case, the wireman was wrong to say that if live and neutral shorted (the cable between A and B), the MCB wouldn't trip but main fuse will blow instead ? |
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