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a disconection time of not more than 0.4seconds for circuts not exceeding 32a and 5seconds for circuts exceeding 32a been asked a question why is this surely the higher the current it should have a lower disconection time how do i explain this to a young keen apprentice
 
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telectrix

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Mentor
Arms
Esteemed
a disconection time of not more than 0.4seconds for circuts not exceeding 32a and 5seconds for circuts exceeding 32a been asked a question why is this surely the higher the current it should have a lower disconection time how do i explain this to a young keen apprentice
People are less likely to come into contact with these circuits. so there is lessdanger.
The 5secs. is more to protect the installation
 
G

Guest55

Because a circuit lower than 32a is more likely to feed items that are hand held or portable , thus a quicker time.
above 32a usually big fixed loads or sub-mains.
 
S

StuSpiers

^ as above

Less likely to be numerose joints.

Bigger CSA/Less resistance
 

marcusone

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Arms
Thanks Tel.....and here was I typing up a long in depth answer. Sometimes less is more :)

Sent from my HTC One X using Tapatalk 2
 
G

Guest55

Tel beat me to it lol.
 
E

Engineer54

In reality an MCB can trip anywhere from 0.1 to 5 seconds at that breakers instantaneous fault current... Which is why Zs values are important, especially on non RCD protected circuits or supplies...
 
  • Thread Starter Thread Starter
  • #9
thanks for replys
 
E

Engineer54

Was that a veiled warming to RCDs, E54? :bucktooth:
Nothing wrong with RCD's at all, ....so long as you don't totally rely on them like so many here seems too. They are at the end of the day classed as ''Additional Protection'' (except on a TT installation) not a cure all device, or a get out of trouble protection device!!

Your installations Zs is your primary earth fault protection....
 
My interpretation is that you have to allow 5 secs disconnection time, in order to allow for time delayed discrimination in larger premises. Also, if every disconnection time was 0.4, then your submains would have to be larger to allow for the lower resistance, which would be needed to permit the extra fault current to flow, to meet the shorter disconnection times.
 
E

Engineer54

My interpretation is that you have to allow 5 secs disconnection time, in order to allow for time delayed discrimination in larger premises. Also, if every disconnection time was 0.4, then your submains would have to be larger to allow for the lower resistance, which would be needed to permit the extra fault current to flow, to meet the shorter disconnection times.
In the bigger picture of things yes...

But let's take a domestic with a 40A Type B MCB protecting an outbuildings sub-CU that has say one 16A and one 6A Type B MCB's (forget about any RCD's in this instance) All the MCB's involved in this installation will have the same time /current curves eg, ...0.1 to 5 seconds. So what is the going to determine, that the downstream breakers are going to operate before the supplying 40A breaker??
 

Risteard

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Arms
Esteemed
So what is the going to determine, that the downstream breakers are going to operate before the supplying 40A breaker??
Indeed, which is why in my view MCBs should not be connected in series as selectivity (discrimination) cannot reliably be achieved.
 
  • Thread Starter Thread Starter
  • #15
tel have you got an i phone
 
E

Engineer54

Indeed, which is why in my view MCBs should not be connected in series as selectivity (discrimination) cannot reliably be achieved.

Depends very much on the current flowing during the fault....
 

BigSi

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Arms
a disconection time of not more than 0.4seconds for circuts not exceeding 32a and 5seconds for circuts exceeding 32a been asked a question why is this surely the higher the current it should have a lower disconection time how do i explain this to a young keen apprentice
I recently had an NICEIC DI assessment, and I quoted those figures to the assessor during a test on a TN system, who didn’t know what I was talking about? :confused:
 

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