Adiabatic equation infinite loop

[johnduffell]

Also bear in mind the cpc size of the final circuit wouldn't affect the fault current because you're considering the case where the fault occurs directly after the mcb (ie worst case) so you would use zdb not zs

Just realised from wilkos post even that's wrong, it should not be calculated from the zdb, it should be the PFC which would be the maximum of the PSSC and the PEFC.
But still the final circuit would have nothing to do with the let through (energy per ohm)

 

[Strima]

What does the equation say when using 20mm conduit as the CPC?

 

[Zdb]

What does the equation say when using 20mm conduit as the CPC?

I'm not quite sure how to work that out TBH.

 

[Strima]

I'm not quite sure how to work that out TBH.

All you need is the resistance of the conduit per meter, you can then work out your R1+R2 etc, change the K value on the adiabatic.

IIRC the minimum CSA of 20mm galv is around the 53mm area so you have plenty to play with.

Let me see if I can find the information I had.

 

[johnduffell]

How many people on here think the zdb+r1+r2 ie the Zs of the circuit in question is used in the calculation of the cpc size for adiabatic?
I feel like I'm the only one picked up the fundamental error (and wrong side error) the op described, and I'm not even a sparkly.

 

[Wilko]

Hi - not sure I've understood correctly, but it may not be an issue in real life. Using the Wylex data from post #15 a B6 disconnects with I2T of about 10,000 on 10kA short circuit current (worst case, I should think). Plug that into adiabatic and 1mm2 is still ok. So not really something we have to think about every day ?

 

[Ian1981]

Th

How many people on here think the zdb+r1+r2 ie the Zs of the circuit in question is used in the calculation of the cpc size for adiabatic?
I feel like I'm the only one picked up the fundamental error (and wrong side error) the op described, and I'm not even a sparkly.

the zs of the circuit and resulting fault current is what is used as I in the equation

 

[Zdb]

How many people on here think the zdb+r1+r2 ie the Zs of the circuit in question is used in the calculation of the cpc size for adiabatic?
I feel like I'm the only one picked up the fundamental error (and wrong side error) the op described, and I'm not even a sparkly.

That doesn't really make any sense.

The Zs would be even lower due to parallel paths.

All you need is the resistance of the conduit per meter, you can then work out your R1+R2 etc, change the K value on the adiabatic.

IIRC the minimum CSA of 20mm galv is around the 53mm area so you have plenty to play with.

Let me see if I can find the information I had.

I'll look into it. If you do have the info that would be great.

 

[davesparks]

How many people on here think the zdb+r1+r2 ie the Zs of the circuit in question is used in the calculation of the cpc size for adiabatic?
I feel like I'm the only one picked up the fundamental error (and wrong side error) the op described, and I'm not even a sparkly.

Zs is used because a cpc which is suitable for conditions at the far end of the circuit can be assumed to be suitable at the supply end of the circuit.

Applying the adiabatic equation at the supply end as you suggest and at the load end results in a larger cpc from the load end calculation.

 

[johnduffell]

the zs of the circuit and resulting fault current is what is used as I in the equation

I'm saying that's wrong, as the worse case ie highest fault current will be when the fault happens close to the origin.
This is exemplified by the suggestions to increase the length of the circuit arbitrarily in order to reduce the necessary size of the cpc. If the fault happen 1m away from the cpc, the remaining 2m or 20m of the circuit are irrelevant.

That doesn't really make any sense.

The Zs would be even lower due to parallel paths.

Maybe it doesn't make sense at the moment, but by rating it Dumb, I'm not sure if that shows a good attitude to understanding whether you are making a serious and potentially dangerous mistake.i also notice it appears you are using cmin to calculate rather than the actual p(e)fc which is also incorrect.

Zs is used because a cpc which is suitable for conditions at the far end of the circuit can be assumed to be suitable at

I don't think that can be assumed, for example with a mechanical mcb the graph posted above clearly shows that not to be the case.

Applying the adiabatic equation at the supply end as you suggest and at the load end results in a larger cpc from the load end calculation.

Did not know that. Interesting, i will look at that. I would expect it to be either head end or the same.

 

[Strima]

If we sized the CPC for a fault at the beginning of the circuit then the CPC size would be excessively large for what is required.

I can see where you're coming from but feasibly you could end up with a CPC of 6mm for a 1mm lighting circuit if you calculate for the fault at the origin of the circuit..

 

[johnduffell]

If we sized the CPC for a fault at the beginning of the circuit then the CPC size would be excessively large for what is required.

I contend that it would be exactly what is required to protect for a fault at that point. The remainder of the circuit is not relevant to the fault current. If the cable insulation are damaged on exit from the DB, they need to be not thermally damaged by the fault current. As with any other fault before the furthest point. Hence the need to find the worst case

I can see where you're coming from but feasibly you could end up with a CPC of 6mm for a 1mm lighting circuit if you calculate for the fault at the origin of the circuit..

If that's what is needed then there are either missing protective devices or they are not properly matched. Or someone has calculated i2t from 0.1ms instead of using the manufacturers chart.
If you check the above chart, you should find that even for 1mm cpc on a 6A class 3 mcb would be fine at 16kA as long as it's backed up by a 100A cutout fuse.

 

[Ian1981]

I'm saying that's wrong, as the worse case ie highest fault current will be when the fault happens close to the origin.
This is exemplified by the suggestions to increase the length of the circuit arbitrarily in order to reduce the necessary size of the cpc. If the fault happen 1m away from the cpc, the remaining 2m or 20m of the circuit are irrelevant.

Maybe it doesn't make sense at the moment, but by rating it Dumb, I'm not sure if that shows a good attitude to understanding whether you are making a serious and potentially dangerous mistake.i also notice it appears you are using cmin to calculate rather than the actual p(e)fc which is also incorrect.

I don't think that can be assumed, for example with a mechanical mcb the graph posted above clearly shows that not to be the case.

Did not know that. Interesting, i will look at that. I would expect it to be either head end or the same.

It’s not wrong you need the fault current at the most onerous point in the circuit

 

[johnduffell]

the zs of the circuit and resulting fault current is what is used as I in the equation

I'm saying that's wrong, as the worse case ie highest fault current will be when the fault happens close to the origin.

It’s not wrong you need the fault current at the most onerous point in the circuit

Indeed, so for the purposes of the adiabatic equation, would you consider higher or lower current more onerous?

A higher I2t is certainly more onerous (as csa required is proportional to the square root of I2t).
At disconnection times less than 0.1s, it's clear from the I2t chart posted previously that onerous case is close to the origin.

At disconnection times between 0.1s and 5s we need to calculate I2t from the manufacturer's charts for disconnection time. That is the case I mentioned in my 6:36 post today I needed to look into (but didn't!)

For >5s we would indeed end up needing huge CPCs but that's because adiabatic is not relevant due to the adiabatic (no heat disapated) assumption not holding.

 

[johnduffell]

Zs is used because a cpc which is suitable for conditions at the far end of the circuit can be assumed to be suitable at the supply end of the circuit.

Applying the adiabatic equation at the supply end as you suggest and at the load end results in a larger cpc from the load end calculation.

So as promised I eventually looked into this, it seems like we were both right!
I found an interesting thread on IET to explain IET Forums - Understanding the adiabatic equation - https://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=65487&enterthread=y
The gist of it is for fuses the worst case is indeed as you state, where the I is lower the I2t will be higher, so you have to check the distant end.
However for MCBs the situation reverses when the magnetic trip comes into play - when the I is higher the I2t will be higher, so you have to check the head end instead.
Cheers, I learnt something today!

However, after all that, the OPs situation, as far as I can tell, he still needs to use the actual PEFC rather than the Cmin, and he still needs to use the head end and the tabulated I2t values due to the magnetic trip being relevant.
And also not forgetting that the other conductors are also subject to the same process - which may apply if PEFC and PSSC are not the same.