Tripping mcb

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David morris

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Hello i have a fuse box in the shed which is feed from a fcu in the house i plugged my table saw into a portable rcd from a extension reel and it tripped however it did not trip the portable rcd ,the shed fuse or the fcu fuse instead it fused the mcb on the main board in the house could anyone tell me why this was please

 
Hello i have a fuse box in the shed which is feed from a fcu in the house i plugged my table saw into a portable rcd from a extension reel and it tripped however it did not trip the portable rcd ,the shed fuse or the fcu fuse instead it fused the mcb on the main board in the house could anyone tell me why this was please
Not specifically, but I can give you some pointers.  RCDs trip for earth faults, so you don't have that.

Fuses and MCBs trip for over current. 

Hence, either the cumulative current of your saw plus whatever was running in the house exceeded your MCB rating, without exceeding the fuses ratings, OR it was a time issue.

MCBs are a lot faster than fuses.  If you put a short circuit fault at the end of a string of fuses with an MCB at source there is every chance that the MCB will trip before any of the fuses blow.

Further thought, motors like a saw have a large starting surge, possibly several times normal running current, so is it on starting that the MCB trips?

 
Thank you for your reply i have used the saw many times before with no problem however as soon as i turned it on this time motor started to run then tripped and now wont start at all

 
I presume it didn’t trip? 
 

it may be a combination to the way the shed circuit is fed from existing circuit and your saw motor is perhaps wearing that you find the issues that you have. As pointed out by Geoff, the fault will be a low resistance or high current pull Likely from within the saw. How old is the saw, has it ever been serviced? 

 
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