well,
as you run a successful electrical business, then you should know how to solve this,
go back to the DNO and ask for a measured PFC
MCCB
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well,
as you run a successful electrical business, then you should know how to solve this,
go back to the DNO and ask for a measured PFC
:shakehead
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Well it wouldn't be
do you know what the network impedance is?
have you checked on the 'bullrush tables,' at what current the OCPD will lift at within an acceptable time?....that would be my first step
its simply a ridiculous thread
how much are your penny sweets?
frm2005,
DO not circumvent the swear filter else you will be moderated or banned.
I seem to recall you asking this question before, and you have also asked this on the IET forum I suspect.
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opcorn Guinness opcorn Guinness 
Oh Dear....
You might have run an electrical company for 15 years, but you have either forgotten the basics of have no idea what you are doing..
What you really mean is, is what value of EFLI is required to trip the device in the required time..
You say you have a fault current of 2000A, well, 230/2000 = 0.115 which i would suggest is miles lower than the EFLI required.... [BUT READ BELOW]
All you need to know, is can the device break the maximum fault current [PEFC and PSSC] at the point where it is installed, AND what is the minimum [maximum?? You know what i mean!!] value of EFLI required for it to meet the disconnection time required...
Max fault current it going to be about 23kA for three phase and about 16kA for single. Yours will be MILES less than this, [as you have noticed] so no problem there..
[THE BIT BELOW!!] If you are measuring a fault current at the device of 2ka, the the EFLI at that point is clearly low enough, BUT, you need to know what the EFLI will be AT THE END of the circuit the MCCB is protecting, NOT the start of the circuit...
Go back and have a think about it...
Just a question.... Do you have this problem with MCB's????? because there is absolutely no difference, other than one looks different and costs more!!! How do you normally design circuits...
john..
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As i understand, it the figures in the Ics column, are he current it can break and still be servicable [at least for a few times!!] and it WILL break the currents in the Icu column, but the device will now be stuffed...
john
View attachment 7985
is there not an argument that as any circuit breaker is passing say 63A max, then the fault current upstrem is irrelevent becuase to create a 25kVA short at the MCB would require a dead short live to neutral or earth within about 0.1mm of the MCB terminal which is physically impossible?
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Got to be fair!! That has to be, officially, just about the best answer i have ever seen on the forum!!!!
john....
john....
leesparkykent
Well-known member
Your original design that the consultant is making you change sounds ****e anyway.....mcb's protecting distribution circuits that's going to have mcb's at the other end :Applaud
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Our Mcb distribution board is feeding a domestic board full of rcbo's. It's student accommodation, our discrimination calculations all stack up. The bedsits maximum demand area arround 30amp. you wouldn't have three tiers of MCCB protection. It breaks down as follows from the hob for example
32amp RCBO (Bedsit consumer unit)
63amp MCB (apartment DB)
250amp MCCB (Block LV Panel)
400amp MCCB (Main LV panel)
800amp main incomer (supply)
As mentioned before the consultant is worried about the high fault current quoted by the DNO at 25ka, this isn't an issue in our opinion as the highest fault current we've calculated is around 6ka and that's 8 meters away from the main supply coming in which is from one of the panels feeding one of the blocks.
We only ever use MCCB boards to feed apartments if their demand is over 63amp, never had an issue on any other project we've ever done. Mainly down to cost if we fit what the consultant wants these would cost us arround 10k to do so.
32amp RCBO (Bedsit consumer unit)
63amp MCB (apartment DB)
250amp MCCB (Block LV Panel)
400amp MCCB (Main LV panel)
800amp main incomer (supply)
As mentioned before the consultant is worried about the high fault current quoted by the DNO at 25ka, this isn't an issue in our opinion as the highest fault current we've calculated is around 6ka and that's 8 meters away from the main supply coming in which is from one of the panels feeding one of the blocks.
We only ever use MCCB boards to feed apartments if their demand is over 63amp, never had an issue on any other project we've ever done. Mainly down to cost if we fit what the consultant wants these would cost us arround 10k to do so.
A 800A main incomer would usually have a dedicated 500kVA transformer. The DNO’s don’t like going above 600A fed off an area transformer. It would involve a dedicated feed from the feeder pillar.
2kA fault current is so wrong I believe the OP hasn’t done the loop test correctly.
2kA fault current is so wrong I believe the OP hasn’t done the loop test correctly.
Tony,...Of course thats at the transformer terminals, 2Ka might be realistic serveral tiers of distribution down the line up on thr fifth floor of the block the otherside of the courtyard!. He doesn't seem to making much sense as to where the 2ka figure is at! . As to loop testers, most of them will struggle to give a sensible result at those fault levels, even the LTW425 struggles at that level. An all in one test meter would be less usful than a fruit machine....
Op, I find it hard to beleive "our discrimination calculations all stack up". For your original design of a MCB feeding a sub board of MCBs. Thats unlikely.
What might be more likely is a design decision being taken on whether full descrimination is actually needed. One might consider that as these are small bedsits and only likely to have one lighting circuit anyway, then trying to design in discrimination to avoid a faulty appliance taking out the submain and causing loss of lighting is not worth it when a failed lamp would still cause the same darkness. A non maintained bulkhead in the bedsit along with the front desk/security chaps knowing where to go to reset a breaker might be a better idea.
Though the consulatant in this case seems to want full descrimination, what does the spec for the job say?
Op, I find it hard to beleive "our discrimination calculations all stack up". For your original design of a MCB feeding a sub board of MCBs. Thats unlikely.
What might be more likely is a design decision being taken on whether full descrimination is actually needed. One might consider that as these are small bedsits and only likely to have one lighting circuit anyway, then trying to design in discrimination to avoid a faulty appliance taking out the submain and causing loss of lighting is not worth it when a failed lamp would still cause the same darkness. A non maintained bulkhead in the bedsit along with the front desk/security chaps knowing where to go to reset a breaker might be a better idea.
Though the consulatant in this case seems to want full descrimination, what does the spec for the job say?
Yes 25kA is at the transformer terminals. Transformer tails are usually as short as possible leading to only a marginally lower prospective fault current at the incomer. Sub main feeders will reduce the PFC but not to the extent the OP alludes to.
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