Grounding and Lightning protection as per NFPA 780 & 70.

The  main objective of this post is to creating both Lighting Protection plan & grounding system plan with a good knowledge of standard references.

NFPA 780 (Lightning Protection) BRIEF.

A) Air terminals height 0.6 m along edge (within 0.6 m from roof edge) with 7.6 m maximum spacing. Air terminals height 0.6 m with maximum spacing of 15 m. [4.8.2.1], [4.8.3].

B) 45 m maximum length of cross-run conductor permitted without a connection from the cross-run conductor to the main perimeter or down conductor.[4.8.3].

C) Two Down-conductors Minimum for any shaped structure with maximum spacing is 30m .If run coursed on or in reinforced concrete columns or on structural steel columns shall be connected to the reinforcing steel or the structural steel member at their upper and lower extremities with maximum 60 m vertical spacing.[4.9.10.1].

D) Bare looping  minimum size  is 58mm.sq  for class II AND 29mm.sq for class I, WHILE 25X3mm(or 70mmsq )typically used.

E) Ground rods shall be not less than 12.7 mm in diameter 2.4 m long and extend vertically not less than 3 m into the earth with minimum spacing 6 m.[4.13.23].

F) Ground Ring Electrode directly contact with the earth, consist at least 6m of bare copper conductor not smaller than 35mmsq. [NEC2011,250.52(A)(4)] with minimum depth 4.6m below grade shall be connected to steel columns around the perimeter of the structure at intervals averaging not more than 18 m.[4.16.4.1].

G) Loop Ground conductor is commonly used is 120 mm.sq.

H) Grounding Electrode Conductor as per shown NEC table 250.66.

I) Considered Sphere radius is 46.0 m.[4.7.4].

J) The following figures for typical plans of the systems.

Roof Floor Lighting Protection Plan (Typical)

 

Ground Floor Lighting System Plan (Typical)

 

Isometric of the Ordinary Building (Typical)

 

 

K) The following figures showing the Reference NEC for GROUNDING cable sizing.

The Equipment grounding to the ground loop/rod Based on the feeder size of the equipment.

 

Grounding of equipment running with feeder cable.

 

Showing different grounding conductor/Electrode.

 

Connection between Lightning Protection and equipment grounding.

 

 

Fast and Simple Short circuit current calculation

Quick Methods For Transformer

• Short circuit current Based on the infinite source (Rated current Method):
  Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
  Rated current(Ir)= S(KVA)*1000 / V / root(3) = 1000*1000 /400 / 1.732 = 1443A
  Short circuit current (Isc) = Rated current (Ir) / Zsc
  Short circuit current (Isc) = 1443 A / 0.05 = 28,900 A = 28.9 KA

• Short circuit current Based on the infinite source (Impedance Method):
  Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
  Impedance(Z) of the transformer = Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
  Short circuit current (Isc) = V / root(3) / Z
  Short circuit current (Isc) =  400 / 1.732 / 0.008 = 28,900 A = 28.9 KA

• Short circuit current Based on upstream 500MVAsc (Admittance Method):
  Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
  Transformer MVAsc= MVA / 1000 / Zsc=1000 KVA /1000/ 0.05 = 20 MVA
  1 / MVAsc = 1 / Transformer MVAsc + 1 / Upstream MVAsc
  1 / MVAsc = 1/20+1/500 = 0.052
  MVAsc = 1 / 0.052 = 19.23
  Short circuit current (Isc) = MVAsc / root(3) / V = 19.23 / 1.732 / 0.4 = 27,757 A = 27.8 KA

• Short circuit current Based on upstream 500MVAsc (Impedance Method):
  Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
  Impedance(Zt) of the transformer = Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
  Impedance(Zu) of the Upstream at MV side = V^2 / upstream MVAsc = 13.8^2 X 500 = 0.381 Ω
  Impedance(Zu’) of the Upstream at LV side= 0.381 x (400V/13.8KV)^2 = 0.00032 Ω
  Impedance(Z) of the Upstream and Transformer=0.008 Ω +  0.00032 Ω = 0.00832 Ω
  Short circuit current (Isc) = V / root(3) / Z
  Short circuit current (Isc) =  400 / 1.732 / 0.00832 = 27,757 A = 27.8 KA

• Short circuit current Based on upstream 250MVAsc (Impedance Method):
  Example: Sn=1000 KVA transformer, 3 PH, 13.8 KV / 400 V , Zsc=5%
  Impedance(Zt) of the transformer= Zsc x V^2 / Sn = 0.05 X 400^2 / 1000*1000 = 0.008 Ω
  Impedance(Zu) of the Upstream at MV side= V^2 / upstream MVAsc = 13.8^2 X 250 = 0.762 Ω
  Impedance(Zu’) of the Upstream at LV side= 0.381 x (400V/13.8KV)^2 = 0.00064 Ω
  Impedance(Z) of the Upstream and Transformer=0.008 Ω +  0.00064 Ω = 0.00864 Ω
  Short circuit current (Isc) = V / root(3) / Z
  Short circuit current (Isc) =  400 / 1.732 / 0.00864 = 26,729 A = 26.8 KA

Impedance Method For System