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7.2.4 on p77, note that it doesn't reference any regulation though:
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It's also the thermal constraints part you need to comply with not just disconnection time. I'm not near my big blue book but it is in there. You have to use the 0.1s value of MCB and use it in the adiabatic to confirm.
Indeed, I think I read your post pre-edit so got the wrong end of the stickIt's also the thermal constraints part you need to comply with not just disconnection time. I'm not near my big blue book but it is in there. You have to use the 0.1s value of MCB and use it in the adiabatic to confirm.
Why would you use 0.1s for this calculation?It's also the thermal constraints part you need to comply with not just disconnection time. I'm not near my big blue book but it is in there. You have to use the 0.1s value of MCB and use it in the adiabatic to confirm.
Because you would be trying to meet the current to operate the magnetic part of the MCB which disconnects in 0.1s-5 on the tables. They show the values for 0.1s for you to use in calcs and state 5s (for exactly the same fault current) to show the ADS met incase someone can't work out that 0.1s is less time than 5 seconds. Funny they do that really.Why would you use 0.1s for this calculation?
And of course all other aspects of the regs still apply, the use of an rcd to achieve one aspect of the regs, doesn't mean other aspects are no longer required.
But we know the trip times in the example by the op is not within the instantaneous portion of the characteristics, so using that would be totally inappropriate.l
Because you would be trying to meet the current to operate the magnetic part of the MCB which disconnects in 0.1s-5 on the tables. They show the values for 0.1s for you to use in calcs and state 5s (for exactly the same fault current) to show the ADS met incase someone can't work out that 0.1s is less time than 5 seconds. Funny they do that really.
But we know the trip times in the example by the op is not within the instantaneous portion of the characteristics, so using that would be totally inappropriate.
If we use his 25m x 25mm^2 cable and say the original design (assuming Ze = 0.35) then the current would be around 560A, which would trip in 2 secs or so. That time and current should be used for the adiabatic calculations, which I guess would be around 4-6mm^2 minimum
With the 0.51 ohm Ze this would reduce the current to around 400A , and 20s trip time, again these figures in the adiabatic would give around 12-15mm^2 min cable.
But we know the trip times in the example by the op is not within the instantaneous portion of the characteristics, so using that would be totally inappropriate.
If we use his 25m x 25mm^2 cable and say the original design (assuming Ze = 0.35) then the current would be around 560A, which would trip in 2 secs or so. That time and current should be used for the adiabatic calculations, which I guess would be around 4-6mm^2 minimum
With the 0.51 ohm Ze this would reduce the current to around 400A , and 20s trip time, again these figures in the adiabatic would give around 12-15mm^2 min cable.
Sorry, yes you are absolutely right. I was generalising regarding final circuits and not his particular circumstance with a BS88. I had skimmed down through the posts.But we know the trip times in the example by the op is not within the instantaneous portion of the characteristics, so using that would be totally inappropriate.
If we use his 25m x 25mm^2 cable and say the original design (assuming Ze = 0.35) then the current would be around 560A, which would trip in 2 secs or so. That time and current should be used for the adiabatic calculations, which I guess would be around 4-6mm^2 minimum
With the 0.51 ohm Ze this would reduce the current to around 400A , and 20s trip time, again these figures in the adiabatic would give around 12-15mm^2 min cable.
Zs is only applicable to ads it isn't applicable to fault protection for L-N or L-L faults.Sorry, yes you are absolutely right. I was generalising regarding final circuits and not his particular circumstance with a BS88. I had skimmed down through the posts.
The point I was getting at is that is that L-N often does not get considered when people are applying RCD's to cover fault protection then they can't meet max Zs by OCPD.
Thats the problem. The regs only require Zs and don't get people to take Zn because the cpc =/< than the neutral so the fault current will =/> too and still disconnectZs is only applicable to ads it isn't applicable to fault protection for L-N or
Zs is only applicable to ads it isn't applicable to fault protection for L-N or L-L faults.
Correct, which is why I said earlier on that all aspects of the regs still need to be adhered to.If you meet ADS by OCPD then you meet thermal constraints. If you meetr ADS by relying on an RCD then you may well not meet thermal constraints for L-N by the OCPD which is still covering that.
Correct, which is why I said earlier on that all aspects of the regs still need to be adhered to.
I grant you that some people may not check various aspects of the regs, usually because "it's always been OK in the past" but I think that happens all the time, I come across so much work where the person installing hasn't thought about it one bit - basically it's never designed, just installed.
Not always. With MCBs, the let through energy tends to increase with increasing fault current. So the most onerous part of the circuit for thermal constraint is close to the origin, where ADS is assuredIf you meet ADS by OCPD then you meet thermal constraints.
Not always. With MCBs, the let through energy tends to increase with increasing fault current. So the most onerous part of the circuit for thermal constraint is close to the origin, where ADS is assured
I don't think there any additional considerations for using manufacturer's data, it just replaces the I2t in the equation AFAIK.Generally it does work out with MCB's if you are operating with pfc's withing the breaking capacity of the device. At least it always has in the real world scenarios I have applied it. There is also the other variable of manufacturers data where you can go with faster times than the 0.1s in the regs which I believe has additional considerations attached that I can't remember what they are?
I think the adiabatic appears only to be related to protective conductors in the regs as it's included in that section and the CPC is usually the smallest (or at least the same size).I don't think there any additional considerations for using manufacturer's data, it just replaces the I2t in the equation AFAIK.
Using, for example, data for a 32A Hager B curve, and using a 1mm2 CPC, it is possible to fall foul for fault currents of ~3kA or greater, and for higher fault currents for lower rated MCBs. So potentially a problem for old ring finals wired with 1mm CPCs, or other such circuits.
Interesting to note that the adiabatic equation only seems concerned with protective conductors, as far as the regs are concerned, as far as I can tell anyway
Thanks JulieYes, time delayed rcd on distribution circuits, standard rcd on final circuits - rcds as fault protection.
No need to convert to TT
leave it as TN-C-S and you can still use 0.4s / 5s but add rcd as fault protection where required.
So if the cable from the meter location to cu fails to operate in 5s due to fault, add a time delayed rcd.
If any final circuit fails to operate in 0.4s due to fault add a standard rcd.
If a circuit will operate within these times, then a rcd is not required for fault protection, but may be required for additional protection.
It was on the IET forum about why we should not use adiabatic for fault currents operating devices <0.1S or >5S. I couldn't remember but just found it again on there.I don't think there any additional considerations for using manufacturer's data, it just replaces the I2t in the equation AFAIK.
Using, for example, data for a 32A Hager B curve, and using a 1mm2 CPC, it is possible to fall foul for fault currents of ~3kA or greater, and for higher fault currents for lower rated MCBs. So potentially a problem for old ring finals wired with 1mm CPCs, or other such circuits.
Interesting to note that the adiabatic equation only seems concerned with protective conductors, as far as the regs are concerned, as far as I can tell anyway
Interesting to note that the adiabatic equation only seems concerned with protective conductors, as far as the regs are concerned, as far as I can tell anyway
My bad, it isn't only concerned with protective conductors:I think the adiabatic appears only to be related to protective conductors in the regs as it's included in that section and the CPC is usually the smallest (or at least the same size).
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