CSA main earthing conductor calc

[gazdkw82]

I'm trying to understand the different ways I can calculate or find the correct size of main earting conductor of an installation. Iv found a formula but it's quite long winded. Is there an quicker process?

Here is what I have at the moment after doing some research and practice

 

[telectrix]

that's the adiabatic equation. your value of t is not necessarily correct as you need to look up in appx. 3 (tripping curves ) to get an accurate value for t given the pefc. if can generally be as low as 0.1 secs. try doing the equation on a scientific calculator, using brackets and the squared functions. it only takes 20 seconds.

 

[gazdkw82]

thanks for the reply teletrix.

would you consider this equation a long winded approach?


In a real life situation, would this adiabatic approach bethe most logical route to find the CSA of main protective conductor?

 

[telectrix]

the adiabatic is the way to calculate the minimum csa. generally, esp. in domestic we bung in 10mm or 16mm without calculation as , except in extreme circumstance, this will be oversized. however, in the industrial scenario, fitting oversized conductors may not be cost effective.

 

[gazdkw82]

so how would you decide if its 16 or 10?

the size of the meter tails?

 

[Rpa07]

The adiabatic is quick and easy if you have all the information you need. It can be used to win a discussion in a number of CSA related discussions, can save the customer money making you more money and will help you sleep better knowing that you've personally made sure protective devices will react to a fault in the given time.
Practicing will also make remembering easier next time around.
I'm still working from the BIg Green so 70c copper is 115 @ <300mm thermoplastic which gives a larger CSA for the CPC - if it has changed in the Big Yeller then I'll need to amend.


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[telectrix]

it's 115 for when the pc is part of a multicore cable . 143 with single cable.

 

[Rpa07]

Got you, thanks for that


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[telectrix]

basically because , in a multicore cable, the heat dissipation is poorer.

 

[Rpa07]

I was looking at table 43.1 whereas it's better explained in table 54.2/3/4. Thanks again.


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[Richard Burns]

The cross sectional area of a main earthing conductor can be calculated using the adiabatic equation or selected from table 54.7.
However the size is also constrained by whether the earthing arrangement is TNCS or not and
by whether the earthing conductor is buried in the ground (normally only relevant for TT systems).

If the system is TNCS then the main earthing conductor is also a bonding conductor and is subject to the minimum csa for bonding conductors in table 54.8.
For a NEUTRAL supply conductor (this is actually the size of the incoming cable neutral, rather than the tails, which can be hard to determine) <=35mm² then the EC must be >=10mm².

For a buried earthing conductor that is a mechanically protected and sheathed cable then the minimum is 2.5mm² or if it is not in conduit or other protection from mechanical damage then a minimum of 16mm².

Once you have covered those two items then you can use table 54.7 to select the size of the EC.
For a LINE conductor size (again incoming supply cable csa) between 16mm² and 35mm² the minimum csa of the EC is 16mm².

However using the table may well mean that your EC is over sized and if this is a relevant factor then using the adiabatic equation can permit a smaller csa to be used, saving money and resources.

Using the adiabatic equation may seem long winded but once you are familiar with it it is not too bad.
As Tel has said the time, t, is the time in which the protective device will operate at the fault current, I, so this does have to be looked up before using the equation.

In your example you have the correct time as an 80 A BS88-3 fuse will disconnect in a fraction more than 0.4s at 793A.
The table in the top right of each time current graph gives the fault currents required for different disconnection times and the 80A BS88-3 will disconnect in 0.4s if the fault current is 800A.

 

[gazdkw82]

Thanks for that. Really useful information