Earthing Cable Size Calculation

• Sizing of protective earthing conductor - Electrical ...
  
• calculation of earth cable size - Entries - Forum - Industry ...
  
• Tables for Earth Sizing and Cables in Conduit | Sherriff ...
  
• Cable Sizing Calculation - Open Electrical

In order to calculate switchgear bus bar required cross section area you have to calculate the electromagnetic [electro-dynamic force withstand ] effect and also the thermal effect [thermal stability].

You may follow the IEC 60865-1and 2 Short-circuit currents -Calculation of effects.

In my opinion, in a case of rigid bar the dynamic effect could be the leading criteria.

On a cable thermal effect is the leading since the dynamic effect issue is easy to overcome here.

According to IEC 60865-1 -without considering vibration- if there is only one parallel bar of good resistance copper, in vertical position, fixed at 320 cm at both ends then 20kA [I"k] could be the maximum short-circuit current that this bar could withstand[approximately].

Thermal withstanding depends on duration[up to clearance of short-circuit].So, and 500 sqr.mm copper bar could withstand 70 kA within 1 sec[but not in this case].

7anoter4:- 320cm is a long way - unlikely in a switchboard to be unsupported over such a length.

Earth bars are normally subjected to an asymetric test i.e. 60% of board fault level. Did you account for that?

Thank you rogerggbr! It was a wrong calculation-I did not close the loop-it has to be 90 cm.

However, it is only a rough estimation since I don't know all actual dimensions and material. My intention was to explain where are the limits-more or less.

No, I considered full short-circuit subtransient current[I"k] in order to calculate maximum force.

What is the size of the incoming feeder?

Its an MSB with 400A main incomer, the fault current only to be taken to account for the calculation of earthing conductor, here it is 25kA, only to confirm that symmetrical fault or only earth fault current to be considered. if it is e/f to consider i hope 70sq.mm is ok.

If your incoming feeders are 70mm2 than that's fine.

but

70mm2 protected by a 400A breaker....cable seems on the small size unless it is very short. What will your service load be? Maybe a smaller breaker is required?

If you are working in India refer IS 3043 ( Indian Standards ) for all detailed information regarding Earthing. IS 3043 Clause 12.1 specifies the size of connecting cables for Earthing.

In my opinion, choosing the right grounding copper conductor cross-section area it is not a simple job. Let's start with the live conductors at first.

According to NEC Art. 240.4 Protection of Conductors. (B) Overcurrent Devices Rated 800 Amperes or Less.

"The next higher standard overcurrent device rating (above

the ampacity of the conductors being protected) shall be

permitted to be used, provided all of the following conditions

are met:

(1) The conductors being protected are not part of a branch

circuit supplying more than one receptacle for cord and-

plug-connected portable loads.

(2) The ampacity of the conductors does not correspond

with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed

800 amperes."

So, 380 A rated conductor may be protected by 400 A MSB.

According to NEC 2014 Table 310.15(B)(16) 500 MCM copper 75oC withstand 380 A.

From Table 250.102(C) grounding electrode conductor has to be 1/0 AWG minimum [53.5 mm^2].

In IEC World according to IEC 60364-5-52 3*240 mm^2 copper XLPE rated 396 A [underground].

According to IEC 60364-4-41 if line conductor cross-sectional area it is more than 35 mm^2 the protective conductor has to be S/2 [120 mm^2].But in any case the requirement of this formula will be taken into account.

The formula S=sqrt(I"k^2*t)/k

k= SQRT(K20*C*dens/alpha20*ln((1+alpha20*(Tc-20))/(1+alpha20*(Tb-20)))/10^6 [for excel convenience].For copper conductor:

K20cu=56*10^6[1/ohm.m] ;alpha20=0.0039[1/oC];dens=8900[kg/m^3]

Now the tricky point is to choose the right Tc [maximum conductor temperature],Tb=ambient temperature and t=clearance fault time

For a short time of 0.02 to 1 sec. the maximum temperature could be from 150 [pvc insulation],250 [XLPE insulation],1084[bare].

For more than 40 times rated short-circuit current the clearance time it is one cycle [according to most of circuit breaker trip characteristics].So if Isc=25 kA [25/0.4= 62.5 multip.] the time could be 1/50 [IEC World] then for 25 kA 0.02 sec. 150oC maximum 40oC ambient the minimum copper cross-section area will be 27.4 mm^2.

thank you, do u know any source to freee download IEC 60364-5-54?

Sorry, I don't know. However you may purchase it ,I think, only for 8$:

http://www.pdf-standard.com/--iec-c-pdf-standard-41/iec-603645542002-pdf-format-electronic-copy-p-pdf-standard-62568.html.

I did not try it.

thanks a lot