Adiabatic Equation - why only for CPC?

[charzree]

Why adiabatic equation is used only for sizing of CPC? Why not for phase conductors too?; As the fault current also flows through the phase conductors

 

[Ian1981]

It’s used when the cpc is at a smaller CSA to the live conductors up-to 16mm2
Or You can comply with table 54.7

 

[johnduffell]

Great question, it would certainly also apply to phase conductors, but that would only really be needed if the ccc of the cable is less than the reading of the ocpd, for example if you are wiring a 50A circuit with a door bell transformer at the end in 1mm singles.
Normally a circuit wouldn't be likely to not comply, actually to be honest even normal t&e is designed so it will always comply when the ccc of the cable is at least that of the ocpd. In domestic

 

[Deleted member 26818]

The adiabatic equation is used to determine the required CSA for a conductor intended for use at high currents for a short duration.

Higher current = shorter time = smaller CSA.

It’s no use for conductors intended to carry current for an extended duration.

 

[Richard Burns]

The application of protection to a live conductor is covered in BS7671 regulation 434.5.2: t=(S²k²)/I² this is a modified form of the adiabatic equation used to ensure any conductor is not damaged due to fault current.
As spinlondon states it is only applicable for short duration events, since the equation is adiabatic there can be no time for thermal transfer to the surroundings.

 

[johnduffell]

It’s no use for conductors intended to carry current for an extended duration.

True but just to be clear, if the conductor is carrying normal current beforehand that just means the starting temperature is higher, so rather then starting from 20c it would start from 70c. Then when the adiabatic ie sub 5s fault event happens, that part is covered by calculating the energy per metre deposited into the cable.

If there was a double fault situation where the was an overload that turned into a fault before it was cleared, we might be on shaky ground.

 

[MFS Electrical]

True but just to be clear, if the conductor is carrying normal current beforehand that just means the starting temperature is higher, so rather then starting from 20c it would start from 70c. Then when the adiabatic ie sub 5s fault event happens, that part is covered by calculating the energy per metre deposited into the cable.

If there was a double fault situation where the was an overload that turned into a fault before it was cleared, we might be on shaky ground.

Have you ever came across a value that was borderline? Most of the time where I am the pfc is so low you could get away with 4mm

 

[johnduffell]

Have you ever came across a value that was borderline? Most of the time where I am the pfc is so low you could get away with 4mm

I'm not a spark so i have limited practical experience, but given 100A is the largest bs88 commonly used in domestic my supposition is that it would be ok.
Of course all this only applies to tn-s, as if it's pme then that means that the incoming link to the MET and the bonding would be a current carrying conductor, as well as usually the main earth, all due to parallel paths.
This is interesting because for example your cpc to the boiler would be carrying some diverted neutral current via the water and gas supply. Under normal circumstances with good bonding that would be negligible.

 

[Michael Davis-McCoy]

You don't work out the cable size using the adiabatic equation. your design current lets you know what the protective device should be, the protective device lets you know what the cable size should be, bigger than. The volt drop will confirm if your cables is big enough. If not, the next size cable up. You then need to work out the circuit impedance. Then calculate the PSCC, with the nominal voltage. Then you need to check the associated graph for the protective device and the disconnection time. As you know the cable size and the value of k.
The transition of the adiabatic equation. Looking for t.
The answer t, is the limiting time for the cable to get up to temperature. As long as the disconnection time is lower than the limiting temperature, thus the cable is thermally protected.
It's not just for cables up to 16mm2.
You have 2 choices.
By selecting from table 54.7, or by calculation. The adiabatic equation.

 

[Rpa07]

@Michael Davis-McCoy , you sir need an ‘avatar’ to go with those qualifications and informative answering of queries. Get to it! and make it good.

 

[Rpa07]

And while we are at it - a lot of the rest of you too, need avatars. Come on it’s not difficult and it says something about you.
Don’t put a selfie in though as it’s weird! @Dillb

 

[johnduffell]

You don't work out the cable size using the adiabatic equation

There's no prescribed order for designing your circuits, as long as they comply in the end.

the protective device lets you know what the cable size should be, bigger than.

That wouldn't work if you have a fixed load not subject to overload, the nature of the load determines the necessary cable size along with adiabatic.
A lot of shower circuits are on a higher ocpd that you would use for the cable in other circumstances

 

[Michael Davis-McCoy]

Thank you very much Rpa07 for the kind words. It's the duty of the experienced electricians to help the younger generation, new electricians, non sparks, apprentices and also the diyer's. The objective is to prevent accidents and fires.
It's no good keeping all the information to yourself.
My avatar would only be a piece of cake, because it's a piece of cake if you know. I't wouldn't be good if I do that.
My objective is to help and assist where I can.

 

[Michael Davis-McCoy]

Addition to my post. It's the Earth fault current you need to find, when you'r looking for the cpc. I went off on a tangent as the thread did with the phase conductors and not the cpc.
My earlier post is right but it checking the PSCC and if the cable is thermally protected. your looking for the earth loop impedance, Ze + R1 + R2, then your fault current using the nominal voltage. Again looking at the graph associated with the protective device. Then to the adiabatic equation, now your looking for S, The value of k will be dependent on your cpc, copper or steel (swa as cpc), it then will give a minimum size. As long as the proposed cpc is bigger than the calculated one, S.