Adiabatic equation and bonding

[South Spark]

Can somebody have a look at this and tell me which tree I'm barking up?

Using the Adiabatic equation to determine the required size of main bonding and therefore the size of supplementary bonding.

Ze = 0.18

PFC = 1.3 kA

Fuse type 1361 IIa

S = square root (1300 x 1300 x 0.1) /115

= 3.57

Is that right? Seems like a very low value for main earth

 

[telectrix]

your calculation is spot on, mate. it,s because the earthing conductor only has to withstand the fault current for a 0.1 sec. duration.

 

[GaryM]

Looks good to me. You'll be surprised to see that when using the adiabatic equation that the earthing conductor could be as small at 6mm2, same for the protective bonding conductors, unless it's PME.

 

[telectrix]

in actual fact, as the main earthingf conductor is a separate cable, your value for k should be 143, which makes it even less.

 

[i=p/u]

page 244 brb says 1800 A for 0.1 secs, and interested to know how pme would affect this

 

[telectrix]

correct, for a 100A fuse, 1100A for 0.1sec. on a 80A. OP didn't specify the fuse rating.

 

[i=p/u]

whats affects on tn-c-s/pme have on the sum???

 

[Sparky Ninja]

Can somebody have a look at this and tell me which tree I'm barking up?

Using the Adiabatic equation to determine the required size of main bonding and therefore the size of supplementary bonding.

Ze = 0.18

PFC = 1.3 kA

Fuse type 1361 IIa

S = square root (1300 x 1300 x 0.1) /115

= 3.57

Is that right? Seems like a very low value for main earth

The adiabatic is used for sizing of the earth conductor and protective conductors, not bonding conductors.
Bonding conductor sizes are selected.

 

[telectrix]

none whatsoever. however, regs stipulate a 16mm earthing conductor for PME.

 

[i=p/u]

and, why cant you use 0.4 secs as this keeps it under the 50v and use 1200A in sum

 

[GaryM]

The adiabatic is used for sizing of the earth conductor and protective conductors, not bonding conductors.
Bonding conductor sizes are selected.

His wording isnt clear but he mentioned main earth at the bottom of his equation so his intention was selecting the size of bonding conductors in accordance with the calculated size of his earthing conductor.

 

[telectrix]

as in the 50% rule IMO.

 

[i=p/u]

isit not same size bonding conductors upto 25mm then can be 16mm

 

[GaryM]

isit not same size bonding conductors upto 25mm then can be 16mm

No.

544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation and not less than 6 mm2. The cross-sectional area need not exceed 25 mm2 if the bonding conductor is of copper or a cross-sectional area affording equivalent conductance in other metals.

PME supplies require a minimum of 10mm2 main protective bonding conductors if the supply neutral is less than 35mm2 though local distributor's network conditions may require a larger conductor.

 

[South Spark]

Nice one - thanks for all your help. FYI the fuse rating is 80A hence 0.1.

Of course the reason I'm asking is the house has existing 6mm bonding, which is in good nick and I needed to know if I had to upgrade to 10mm, which would have been a major pain in the arse.