Automobile repair shop earthing and lightning protection design

Lightning protection and grounding design for a car service station

Task:

Facility: an automobile repair shop 30.4 x 15.3 x 5.8 m (L x W x H).

Soil: sandy. Soil resistivity: 500 Ohm*m.

It is required to perform calculation and develop a earthing and lightning protection design with the resistance of 10 Ohm. With soil resistivity ρ > 100 Ohm*m, said standards may be increased by 0.01 • ρ times, but not more than 10 times.

0,01 * ρ = 0,01 * 500 = 5

5 * 10 = 50 Ohm.

Solution:

In accordance with RD 34.21.122-87 the automobile repair shop is classified as per category III of lightning protection and shall be protected against direct lightning strikes and high potential penetration through the above ground (overhead) metallic infrastructure.

Protection zone B is selected in line with item 4 of Table 1 of RD 34.21.122-87 and expected lightning strikes per year.

Protection from high potential is performed by attaching metal columns of the automobile repair shop building to the ground terminal. Columns have a continuous electrical connection to each other.

The expected number of the lightning strikes into the object per year is determined by the formula:

Formula for calculation of number of lightning strikes to the facility

where h = 5,8 m is the highest object elevation;

L = 30,4 m is the object length;

S = 15,3 m is the object width;

n is lightning strike density per 1 km2 of the ground surface per year.

n = 6,7 · Тth/100, where Тth is the average thunderstorm duration in hours.

For Moscow Region the average thunderstorm duration is 20 to 40 hours.
Therefore:

Formula for calculation of number of lightning strikes to the facility per year

The expected number of the lightning strikes into the object per year is 0.00604 strikes per year.

The object protection against direct lightning strikes is made by means of ZANDZ 16m double vertical lightning arrester with the following overall dimensions:

h₀ = 0,92 * h = 0,92 * 16 = 14,7 m, where h = 16 m is the lightning rod height.

r₀ = 1,5 * h = 1,5 * 16 = 24 m.

r_x = 1,5 * (h - h_x/0,92) = 1,5 * (16 - 5,8/0,92) = 14,5 m.

h_c = h₀ - 0,14(L - h) = 14,72 - 0,14(38,7 - 16) = 11,5 m.

r_c = r₀ = 24 m.

r_cx = r₀(h_c - h_x)/h_c = 24(11,5 - 5,8)/11,5 = 11,9 m.

For the protective zone boundaries, refer to the drawings of this design.

The repair shop ground terminal consists of the electrolyte earthing kits ZZ-100-102 (by 2 pcs. per each lightning rod) and copper-plated strip 30х4 GL-11075, connected through the clamp ZZ-005-064 for conductor connection as per GOST 21130-75 (full specifications and references to the hardware description are given below).

A waterproofing shall be made for the bolted joints, which joints shall be accessible for the inspection at any moment.

The ground terminal is welded to the lightning rod.

The ground terminal is laid at the depth of 0.5 m from the ground surface and at the distance of 1 m from the lightning rod.

16m vertical lightning rod ZANDZ own structure (galvanized steel; with parts embedded under the foundation) is used as a conductor as per 3.2.2.5 of SO 153-34.21.122-2003. Electrical continuity between different parts is provided for a long term.

The ground terminal is arranged on the closest area out of the underground infrastructure protective zone, the location hereof is to be defined during construction.

For the external ground terminal arrangement refer to the drawings of this design.

The lightning protection and earthing installation shall be performed in accordance with EIC, Rev. 7, RD 34.21.122-87 "Guidelines for making the lightning protection of buildings and structures", SO 153-34.21.122-2003 "Guidelines for making the lightning protection of buildings, structures and industrial infrastructure", GOST 21130-75 "Electrotechnical products. Earthing clamps and earthing signs. Design and size".

Ground terminal resistance calculation:

The required grounding arrangements resistance is 10 Ohm

Allowed grounding device resistance is 50 Ohm.

The electrolyte earthing kit ZZ-100-102 includes:

earthing electrode – 1 pcs. (0.6 х 0.06 х 2.4 m);
filler around electrodes - 3 bags (0.1 cu. m).

Calculate the resistance of electrolyte kit ZZ-100-102 as follows:

Formula for calculation of resistance of the electrolytic grounding set ZZ-100-102

З - G

where C is a non-dimensional coefficient describing the electrolyte content in the surrounding soil;

ρ is soil resistivity, Ohm*m;

ρ_n is filler soil resistivity, Ohm*m;

l_s the ground terminal length, m;

r₀ is the ground terminal radius, m;

r is the pit excavation radius, m.

ρ = 500 Ohm*m;

ρ_н = 0.5-1 Ohm*m;

r = 0,1 m;

r₀ = 0,03 m;

l = 2,4 m;

C = 0,125;

З - G
Ом - Ohm

The design resistance of the ground terminal is 16.05 Ohm, which is below the allowed resistance of 50 Ohm.

List of required materials:

Item #	Fig.	Designation	Name	Quantity	Weight (kg per unit)
1		ZZ-005-064	ZANDZ Clamp for connecting conductors (up to 40 mm)	6	0,31
2		ZZ-006-000	ZANDZ Conductive grease	1	0,19
3		ZZ-007-030	ZANDZ Waterproof tape	6	0,442
4		ZZ-100-102	ZANDZ Electrolyte earthing kit	4	98
5		ZZ-201-016	ZANDZ 16m vertical lightning rod (galvanized steel; with parts embedded under the foundation)	2	-
6		GL-11402	GALMAR Inspection pit	6	2,6
7		GL-11075	GALMAR copper-plated steel strip 30х4 mm	34	0,98 (в метрах)