Furse Total Solution to Earthing & Lightning Protection

Total Solution to Earthing & Lightning Protection |

9AKK106354A3360

10/5

A series of zones is created within the structure according to

the level of threat posed by the LEMP with each zone to have

successively less exposure to the effects of lightning - for

example LPZ 0 (outside the structure) where the threat of

lightning currents and fields is most severe being more

onerous than LPZ 3 (within the structure) where the threat of

lightning is considerably reduced such that electronics can be

safely located within this zone.

Figure 1. illustrates the basic LPZ concept defined by

protection measures against LEMP as detailed in

IEC/ BS EN 62305-4. Equipment is protected against both

direct and indirect lightning strikes to the structure and

connected services, through the use of Surge Protection

Measures (SPM), formerly referred to as a LEMP Protection

Measures System (LPMS).

To achieve this reduction in LEMP severity, from conducted

surge currents and transient overvoltages, as well as radiated

magnetic field effects, successive zones use a combination

of shielding measures, bonding of incoming metallic services

such as water and gas and the use of coordinated SPDs

(further details can be found in the Furse Guide to

BS EN 62305 Protection Against Lightning).

Given that the live cores of metallic electrical services such as

mains power, data and telecom cables cannot be bonded

directly to earth wherever a line penetrates each LPZ, a

suitable SPD is therefore needed.

The SPDs characteristics at the boundary of each given zone

or installation location need to take account of the surge

energy they are to be subject to as well as ensure the

transient overvoltages are limited to safe levels for equipment

within the respective zone.

Table 1, below, details the standardized test waveforms with

peak currents used to test SPDs typically located at each

zone boundary.

Table 1:

Standardized test waveforms with peak currents used to test SPDs at each LPZ boundary

SPD location/LPZ boundary

LPZ 0/1

LPZ 1/2

LPZ 2/3

Typical SPD installation point

Service Entrance (e.g. Main distribution Sub-distribution board or telecom

Terminal Equipment (e.g. socket outlet)

board or telecom NTP)

PBX frame

Mains Test Class/SPD Type

(1)

I/1

II/2

III/3

Surge test waveform

10/350 current

8/20 current

Combination 8/20 current and 1.2/50 voltage

Typical peak test current (per mode)

25 kA

(2)

40 kA

3 kA (with 6 kV)

Signal/Telecom Test Category

(1)

(3)

Surge test waveform

10/350 current

Combination 8/20 current and 1.2/50 voltage Combination 8/20 current and 1.2/50 voltage

Typical peak test current (per mode)

2.5 kA

2 kA (with 4 kV)

0.5 kA (with 1 kV)

(1)

Tests to BS EN 61643 series

(2)

Peak current (per mode) for a 3 phase SPD to protect a TN-S mains system

(3)

Test category B2 10/700 voltage waveform (also within ITU-T standards) up to 4 kV peak

also permissible

Boundary

of LPZ 2

(shielded room)

Boundary

of LPZ 1

(LPS)

Antenna

Electrical

power line

Water pipe

Gas pipe

Telecoms

line

Mast or

railing

LPZ 2

LPZ 1

Critical

equipment

Equipment

SPD 1/2 - Overvoltage protection

Connected service directly bonded

SPD 0/1 - Lightning current protection

Equipment

LPZ

FIgure 1. Basic LPZ concept - IEC/ BS EN 62305-4