Discuss d type breakers needing a low zs in the Electrical Forum area at ElectriciansForums.net

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Jake1579

im a 1st year apprentice thats why im asking this question, from what ive gathered the higher the current it takes to trip as in 10 times the rated current etc... for that you need a lower zs than if it was a b type why is that?

my thoughts on it is that becuase you have a higher current so youll need to keep the impedance low so that it can trip the protective device in that desired time but i thought the higher the current the resitance is lower ohms law so i dont really get it.
 
If you have a look at the time/current graphs for OCPD's in the BYB you will be able to see the relationship between current and the time it takes each type of OCPD to operate.You will find a much higher current is required to operate a type D mcb than a type B of the same rating for a 0.4s disconnection time. For a higher current to flow a lower resistance is required,simple ohms law....thus a lower maximum Zs is specified.
 
If I could have a quid for the number of times I have come across circuits that have Zs readings that were too high purely because C type mcb's have been swapped out for D's.
And the reason is usually down to start up currents tripping the latter.
In the scheme of things its not or ever will be the answer to badly designed circuits, but still the practice persists.
 
If I could have a quid for the number of times I have come across circuits that have Zs readings that were too high purely because C type mcb's have been swapped out for D's.
And the reason is usually down to start up currents tripping the latter.
In the scheme of things its not or ever will be the answer to badly designed circuits, but still the practice persists.

It is obviously bad practice to end up with a circuit which will not meet disconnection times. However it is sometimes the case that the use of an existing circuit changes from the original design which necessitates changing the type of mcb. Of course if that is the case then zs must be checked,but it may be that the zs only slightly exceeds the max for the device and that an RCD can take care of disconnection times.
Personally I see that as a practical solution to avoid an expensive rewiring of an otherwise perfectly sound circuit just because the zs exceeds the maximum for a type D or C by a fraction of an ohm.
 

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