Cable Calculation Help - Adiabatic Equation

[Pretty Mouth]

There's also table B7 on P146 of the OSG, if you don't wish to calculate.

However, manufacturer's data is best. Here is an example from Hager. Use your fault current to find the (I²t), and run it through the adiabtic. You should get a much smaller CSA.

 

[iDragonBox]

Thanks for your help guys giving me much more detail then I get in the class room!

 

[Lucien Nunes]

I think your numerical question has been well answered but just referring back to:

Seems like I will have to make the circuit longer to increase the Zs value

You can't manipulate the Zs value by altering the length of the circuit to achieve some desired result regarding fault protection of that circuit itself, since the fault might occur anywhere along the length and the OCPD must be suitable regardless. OT: About the only time one might want to make a circuit longer artificially (and I have done this) is an extra foot or two on a short distribution circuit near the origin, to lower an unusually high fault level at the sub-DB to within the Ics rating of its MCBs.

In day-to-day work, sizing a small circuit's CPC rarely involves calculation. As per @timhoward's post No.8, for typical final circuits the default option (CPC same as live conductors or whatever size comes in the appropriate Twin & Earth) works just fine and attempts to calculate it might give an impractical result as you spotted. Your original solution of 1.75mmsq can't be applied in practice (even if the 0.1s disconnect time were appropriate) because if that size CPC is needed to prevent thermal damage then the same must be true of the live conductors. Calculation begins to bear fruit in situations where the use of an unusually small CPC is desirable for some reason (e.g. existing cable) but needs validation, or some input value like Zdb is a bit marginal, or on large circuits.

Thanks for your help guys giving me much more detail then I get in the class room!

You'll get good input because you're going about it the right way. Posting your own working clearly, then asking about the bit that (quite reasonably) puzzles you. I gave it a like because it sets a good example. You'd be amazed at the nonsense some students post on here, either a jumble of word-salad that we can't decipher, or no working at all but a blunt request to answer the question from scratch.

 

[pc1966]

@Pretty Mouth has it spot-on that you really need to get the manufacturer's data for breakers (MCB, RCBO or MCCB) as the usual current-time plots do not really give you a good idea of what happens once you enter the "instant" magnetic trip region. Some give you useful curves (like the Hager example) and others just give you a maximum value to base calculations on.

There are some generic limits that can be used as well, for example the On-Site Guide in Table B7 (page 146 of current 18th AM2 version), and usually any of the "standard circuits" in Table 7.1(ii) of the OSG are fine for domestic cases where you very rarely see above a couple of kA fault currents.

As @Lucien Nunes says it can sometimes be necessary to look at how you limit the worst-case fault current for various reasons, not just cable adiabatic but also the OCPD breaking limit, etc. However, trying to increase R1+R2 is really your last resort if you find a problem, a better option where practical is to look at OCPD that can safely break the worst-case fault and/or is able to limit the I2t let-through energy to a lower value as well.

Fuses can be your friend here! Many BS88 fuses can break over 50kA PFC (compared to 6-10kA for MCB and often 18-25kA for many MCCB, though some do go higher to 70kA and above) and they do very well at limiting I2t often to orders of magnitude lower than a similar capacity breaker.

They are one-time only devices of course, and need skilled staff for safe replacement, but otherwise are cheap, reliable and effective if those two limitations are acceptable.