This made me wonder.
Generally, with two breakers of the same type in series, what ratio of In is considered necessary for discrimination?
Discrimination
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Generally, with two breakers of the same type in series, what ratio of In is considered necessary for discrimination?
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Depends on the current which is caused by the fault, as an MCB is a mech device this can cause a headache!!
From the square D tech info, for a fault up to 500A you will need a 63A MCCB to allow full discrimination of a 32A MCB.
In reality, using a larger C type MCB upstream of a B type can help.
edit should have read MCCB, not MCB
From the square D tech info, for a fault up to 500A you will need a 63A MCCB to allow full discrimination of a 32A MCB.
In reality, using a larger C type MCB upstream of a B type can help.
edit should have read MCCB, not MCB
I'm not sure its very easy to get MCBs to discriminate, bit like RCDs in the way they work, if you get a fault above the level at which they operatae, then they will operate quickly
If the fault that occured was above the level at which the upstream breaker operated at, then chances are they'd both trip, if the Z's of the final circuit was high enough that the fault currents were too low to operate the upstream breaker, then obviously thats one way of forcing discriminination.
Fuses are easier,just make sure the pre-arcing I²t of the upstream one is lower than the total I²t of the downstream one (ie. make sure that the let through energy of the smaller fuselink after the arc has cleared and the circuit opened, is insufficent to melt upstream fuselink)
Fuse-> mcb discrimination is tad harder than fuse->fuse, similar situation, the total let through energy is available for MCBs (but it varies in relation to fault level*)
*so does a fuse, but in a different way
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If the fault that occured was above the level at which the upstream breaker operated at, then chances are they'd both trip, if the Z's of the final circuit was high enough that the fault currents were too low to operate the upstream breaker, then obviously thats one way of forcing discriminination.
Fuses are easier,just make sure the pre-arcing I²t of the upstream one is lower than the total I²t of the downstream one (ie. make sure that the let through energy of the smaller fuselink after the arc has cleared and the circuit opened, is insufficent to melt upstream fuselink)
Fuse-> mcb discrimination is tad harder than fuse->fuse, similar situation, the total let through energy is available for MCBs (but it varies in relation to fault level*)
*so does a fuse, but in a different way
[/i]
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This is a very complex area. When you address discrimination you need to define the range over which you want to achieve it.
Discrimination over the range of, say, over load current to low value fault current is one thing. Discrimination for higher fault currents may not be achieved by the same combination of devices.
The only real answer is to consult the manufacturer or use software that has the whole range of characteristics for each device.
Some use a two to one rule of thumb but 1.6 to 1 is often OK.
In most domestic situations discrimination is not a 'safety' issue and as such is not important. An exception to this would be discrimination between RCDs (front end and downstream) and to some extent that depends how you choose to define the requirements of section 314 (and we all know what happens when regulations are loosely drafted).
Discrimination over the range of, say, over load current to low value fault current is one thing. Discrimination for higher fault currents may not be achieved by the same combination of devices.
The only real answer is to consult the manufacturer or use software that has the whole range of characteristics for each device.
Some use a two to one rule of thumb but 1.6 to 1 is often OK.
In most domestic situations discrimination is not a 'safety' issue and as such is not important. An exception to this would be discrimination between RCDs (front end and downstream) and to some extent that depends how you choose to define the requirements of section 314 (and we all know what happens when regulations are loosely drafted
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