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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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sequence of tripping CB
Hi folks,
in a rushed piece of diy, i wired a lamp cord switch incorrectly which resulted in a short between live and neutral. i plugged the lamp (it had a 2 pin fuseless plug) into a mk twin wall socket. i had tested it by flipping the switch on the wall socket. A big flash occurred behind the switch and the house lights went out. Upon dissasembly of the wall socket I found the switch contacts inside the socket fused together. Now what i found strange was that besides the 32A CB for the wall sockets, my RLCB and my main 50A CB tripped too. Why should more than 1 CB trip for a given incident? Isnt the wall socket circuit CB supposed to trip alone and keep the rest of the house from plunging into darkness? Thanks Sam |
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sequence of tripping CB
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#4
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sequence of tripping CB
"Andrew Gabriel" wrote in message ...
Now the next issue is that MCB's and fuses can only break the current flow at two points in a mains cycle, that's where the current passes through zero. Current doesn't stop flowing just because two metal contacts come apart, particularly at high current flow. An arc will form which will extinguish when the current flow momentarily stops as it passes through zero. (There are other ways to extinguish the arc, but not used at normal mains voltage in domestic situations.) Andrew, I think you'll find that all modern MCBs and most HBC fuses are so-called 'current-limiting' designs which do interrupt arc current well before it reaches its prospective peak. Compare the manufacturer's published I^2*t let-through figures with the I^2 integral over a full half-cycle at the full rated breaking capacity. MK technical supplement: "The arc drawn beteen the contacts is moved by magnetic forces into the multiple plate arc chamber where the arc is split, rapidly cooled and extinguished, the total operating time of the MCB is between 3 and 5 milliseconds." Electrium-Wylex catalogue: "The high speed current limiting action ensures that the MCB operates before the full prospective fault current is allowed to develop. Under fault conditions, damage can be sustained to the installation and associated equipment due to the amount of energy that passes before the current is completely interrupted. The total energy let-through depends on the value of current and the time for which it flows, and is denoted by the symbol I2t. The high speed current limiting action of MCBs ensures that the energy let-through and any subsequent damage is minimised. This reduced energy let-through assists greatly with both back-up and discrimination considerations." But you still don't get discrimination on a high level s/c fault of course - the trip mechanisms all fire together, then it's a race to see which arc-quench chamber cuts off the current first. -- Andy |
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sequence of tripping CB
In article ,
"Andy Wade" writes: "Andrew Gabriel" wrote in message ... Now the next issue is that MCB's and fuses can only break the current flow at two points in a mains cycle, that's where the current passes through zero. Current doesn't stop flowing just because two metal contacts come apart, particularly at high current flow. An arc will form which will extinguish when the current flow momentarily stops as it passes through zero. (There are other ways to extinguish the arc, but not used at normal mains voltage in domestic situations.) Andrew, I think you'll find that all modern MCBs and most HBC fuses are so-called 'current-limiting' designs which do interrupt arc current well before it reaches its prospective peak. Compare the manufacturer's published I^2*t let-through figures with the I^2 integral over a full half-cycle at the full rated breaking capacity. Oh, OK. Looking at the tripping times for MCB's they are allowed to be up to 1/2 a mains cycle regardless of fault current flowing. I guess there's certainly no harm if they do better. I dissected a larger CB some years ago (I think it was 200A, and I don't know what it came out of). That had an air chamber and plunger to extinguish the arc with an air blast. Initially I thought it was a damper, but it didn't damp the trip mechanism. Then I realised it generated a blast of air through the contacts and through a mica honeycomb, and the mica did look like it might have had an arc blown into it -- some vapourised metal in it. -- Andrew Gabriel |
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sequence of tripping CB
"Andrew Gabriel" wrote in message ...
I dissected a larger CB some years ago (I think it was 200A, and I don't know what it came out of). That had an air chamber and plunger to extinguish the arc with an air blast. Initially I thought it was a damper, but it didn't damp the trip mechanism. Then I realised it generated a blast of air through the contacts and through a mica honeycomb, and the mica did look like it might have had an arc blown into it -- some vapourised metal in it. The designers must have a lot of fun testing things like that during development... -- Andy |
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sequence of tripping CB
On 22 Mar 2004 22:35:36 GMT, (Andrew
Gabriel) wrote: I dissected a larger CB some years ago (I think it was 200A, and I don't know what it came out of). That had an air chamber and plunger to extinguish the arc with an air blast. Initially I thought it was a damper, but it didn't damp the trip mechanism. Then I realised it generated a blast of air through the contacts and through a mica honeycomb, and the mica did look like it might have had an arc blown into it -- some vapourised metal in it. Interesting - I always wondered how very large currents were switched on the power grid, even an electric kettle can produce a spark when switched on - so multiplying that by the equivalent of several tens of thousands electric kettles must mean the capacity for a very large spark as contacts meet and separate. I'd be interested in hearing more about this topic PoP --- If you need to contact me please submit your comments via the web form at http://www.anyoldtripe.co.uk. I'll probably still ignore you but at least I'll get the message..... |
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sequence of tripping CB
In article ,
PoP writes: On 22 Mar 2004 22:35:36 GMT, (Andrew Gabriel) wrote: Interesting - I always wondered how very large currents were switched on the power grid, even an electric kettle can produce a spark when switched on - so multiplying that by the equivalent of several tens of thousands electric kettles must mean the capacity for a very large spark as contacts meet and separate. With difficulty, see: http://phil.ipal.org/electric/switch-arc-1.mpg http://phil.ipal.org/electric/switch-arc-2.mpg Actually, for dealing with fault currents (which isn't what these switches are meant to do), air-blast and oil-blast breakers are used. Air-blast is similar to what I dissected but much bigger. Oil-blast is where the contacts are under oil -- the arc is quenched by the oil but also any oil vapour pressure generated by the arc is used to speed up the separation of the contacts. In both cases, the effect is quite explosive. I may be somewhat out of date on current technology though. -- Andrew Gabriel |
#9
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sequence of tripping CB
In article ,
PoP wrote: I'd be interested in hearing more about this topic There's also 'magnetic blowout', where a strong local magnet is used to deflect the arc sideways across the opening contacts, and so increase the length of the arc until it cannot sustain itself. Used more often when switching DC voltages. -- Tony Williams. |
#10
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sequence of tripping CB
In message ,
(Sam) wrote: Hi folks, Mr. Gabriel has already given a comprehensive reply, but I was a little worried by this bit: i plugged the lamp (it had a 2 pin fuseless plug) into a mk twin wall socket. Do you mean the sort of plug found on shavers? Did you use an adapter because if so, this should have been fused too - at 1 amp. If not, how did you fit the plug in? Is the lamp even suitable for UK use? Hwyl! M. -- Martin Angove: http://www.tridwr.demon.co.uk/ Two free issues: http://www.livtech.co.uk/ Living With Technology .... I'm not schizophrenic. It's this guy beside me! |
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