Safety: Shock, Earthing, Shielding and Lightning

Tip

Exam Focus

Tier-1 topic — NTC 2081 asked earthing/shielding importance + lightning devices [4+6], NTC 2082/2079 asked shock + lightning protection [5–10]. Always: list earthing objectives, name the full LPS chain (air terminal $\to$ down conductor $\to$ earth $\to$ bonding $\to$ SPDs), give earth-resistance targets ($\le1\,\Omega$ exchange, $\le5\,\Omega$ tower), quote the 30 mA RCD, and draw the site diagram.

Earthing (Grounding)

Earthing connects non-current-carrying metal parts (chassis, racks, towers, shields) to the mass of earth through a low-resistance conductor, so fault, leakage, static and lightning currents are safely discharged.

Objectives: (1) personnel shock safety; (2) equipment protection; (3) fast operation of fuses/breakers/ELCB; (4) lightning discharge; (5) common zero-potential reference (less EMI/hum/crosstalk); (6) static drain.

Caution

Key Formula — Earth-fault loop

$$I_f=\frac{U_0}{Z_s},$$

$U_0$ = phase-to-earth voltage, $Z_s$ = total fault-loop impedance. A lower $Z_s$ gives a larger $I_f$ and faster automatic disconnection by the overcurrent device.

Earth electrode in treated soil, and shield bonding (one end for low-frequency signals to avoid a ground loop; both ends for RF).

Type	Construction	Use
Plate	Cu/GI plate ($60\times60\,cm$) $\ge3\,m$ deep in charcoal+salt	Substations, high fault current
Pipe	GI pipe ($\approx40\,mm$, 2.5 m) with charcoal+salt	Most common; buildings, exchanges
Rod	Cu / Cu-bonded steel rods driven in	Rocky/sandy soil; quick install
Strip / wire	GI strip or wire in horizontal trenches	Line towers, hilly areas
Chemical	Electrode with conductive backfill	Sites needing a stable low $R$

Earthing construction types.

Function	Purpose
Protective (safety) earth	Equipment body/chassis earthing for shock safety
System / signal earth	Reference for DC power ($-48$ V return) and signals
Lightning earth	Dedicated down-conductor path from the air terminal
Ring (equipotential) earth	Buried ring bonding all electrodes together

Earthing by function at a telecom site.

Note

Key Point

Telecom earth-resistance targets: $\le1\,\Omega$ (exchange/data centre), $\le5\,\Omega$ (tower), $\le10\,\Omega$ (small installation). Factors: soil resistivity, moisture, temperature, salt content, electrode depth/area. Reduce: multiple parallel electrodes (spacing $\ge$ electrode length), charcoal/salt or bentonite backfill, greater depth and moisture. Measure: by the fall-of-potential (3-point) method.

Shielding

A grounded conductive barrier (braid, foil, enclosure) intercepts EMI/RFI and drains it to earth: it blocks external interference, prevents cross-talk between pairs, protects low-level signals and cuts radiated emissions. The shield must be bonded (a floating shield re-radiates noise): a low-frequency signal cable is usually bonded at one end to avoid a ground loop, whereas RF/coaxial shields need $360^\circ$ bonding at both ends. Shielding never replaces the protective-earth conductor.

	Earthing / bonding	Shielding
Purpose	Shock safety, fault/lightning path, equipotential	Reduce EMI/RFI, crosstalk, emissions
Normal current	Ideally none in the PE	May carry induced/common-mode current
Key feature	Continuous low-impedance electrode/loop	Continuous coverage, correct termination

Earthing versus shielding: different jobs, coordinated by equipotential bonding.

Lightning Protection

Lightning (10–200 kA, microseconds) damages telecom plant by direct strike, induced surge, ground-potential rise, and conducted surge on mains/lines/coax. Protection is a coordinated system that intercepts, conducts, disperses, equalises and limits.

Complete telecom-site protection: air terminal and down conductor to a ring earth, single equipotential earth (MEB) bonding rack/shields, and SPDs on incoming services.

Current path: air terminal (rolling-sphere zone) $\to$ short, straight down conductor $\to$ low-impedance earth termination (tower-leg electrodes $+$ buried ring) $\to$ equipotential bonding of tower/racks/trays/PE to a master earth bar $\to$ single-point cable entry (bond armour/shields) $\to$ coordinated power SPDs plus GDTs on line pairs and arrestors on RF feeders.

Note

Key Point — Why bonding leads must be short

A $1\,\mu\mathrm{H}$ one-metre lead at $di/dt=10\,kA/\mu\mathrm{s}$ develops $V=L\,di/dt=10\,kV$. The low-inductance path, not any single device, protects the equipment.

[!NOTE]

Key Point

Shock protection layers: insulation/barriers $\to$ protective earthing and bonding $\to$ automatic disconnection (fuse/MCB/MCCB, breaking capacity $>$ prospective fault current) $\to$ residual-current device ($I_\Delta=|I_L-I_N|$, 30 mA for personnel). An RCD does not replace earthing or overcurrent protection.

Surge Protectors, Fuses and Circuit Breakers

A surge protective device (SPD) sits in parallel with the load: high impedance normally, low impedance during a transient, diverting the impulse and clamping the downstream voltage. It must satisfy $U_c>U_{normal,max}$ and $U_p<U_{equipment,withstand}$.

Device	Principle	Features
MOV	Voltage-dependent ZnO clamp	Fast, high energy; degrades with surges
GDT	Gas ionises/arcs at spark-over	Very high current, slower; line pairs
TVS diode	Semiconductor precise clamp	Very fast (ps–ns), low energy; boards
Spark gap	Air/ceramic flashover	Heavy-duty power-entrance protection

SPD technologies (energy: GDT $>$ MOV $>$ TVS; speed: TVS $>$ MOV $>$ GDT).

Class	Location	Handles
I (Type 1)	Service entry / LPS	Direct lightning current ($10/350\,\mu$s)
II (Type 2)	Sub-distribution board	Induced surges ($8/20\,\mu$s)
III (Type 3)	At the equipment socket	Residual let-through voltage

SPD classes coordinated by installation zone (I $\to$ II $\to$ III).

Coordinated SPD protection: a Type 1 device at the service entry diverts partial direct-lightning current, a Type 2 device at the distribution board clamps induced surges, and a Type 3 device limits the residual let-through voltage at the equipment.

A fuse is a sacrificial series device whose element melts on overcurrent; its let-through stress is $I^2t=\int i^2(t)\,dt$. A circuit breaker is a resettable series device (thermal inverse-time overload $+$ magnetic instantaneous short-circuit trip).

Breaker	Principle / use
MCB	Thermal $+$ magnetic; final circuits ($\le125\,A$)
MCCB	Higher, adjustable ratings; distribution boards
ELCB / RCCB / RCD	Trips on residual (leakage) current — shock protection
ACB	Large LV switchboards
VCB / $\mathrm{SF}_6$	Vacuum / gas arc-quench — HV substations

Circuit-breaker types.

Feature	Fuse	Circuit breaker
Operation	Element melts (one-time $I^2t$)	Thermal/magnetic trip (resettable)
Restoration	Replace with correct type	Reset/reclose after inspection
Functions	Overcurrent only	Overload, short-circuit; $+$RCD: leakage
Breaking capacity	Very high (HRC)	Select rated Icu/Ics
Control	None	Trip indication, remote operation

Fuse vs circuit breaker. Both need breaking capacity $>$ prospective fault current.

Note

Key Point — Telecom site protection procedure

Air terminal $\to$ down conductor $\to$ dedicated lightning/ring earth ($\le1$–5 $\Omega$) $\to$ equipotential bonding to the master earth bar $\to$ single-point cable entry (shield bonding) $\to$ coordinated SPDs (Class I at AC entry, II at DB, III at equipment; GDTs on pairs, arrestors on RF) $\to$ fuses/MCB $+$ ELCB $\to$ shielded cabling, with periodic earth-resistance measurement. No single rod, breaker or SPD gives complete protection.

[!NOTE]

Example — Reducing earth resistance

A single rod gives 4 $\Omega$ but $\le1\,\Omega$ is required. Use $\approx4$ rods in parallel (spacing $\ge$ rod length; ideally $4/4=1\,\Omega$, in practice $\approx1.2$–1.5 $\Omega$ from mutual resistance) plus charcoal/salt or bentonite backfill and maintained moisture to lower soil resistivity further.