TT Earthing System - Must be RCD Protected

411.5 TT system

In this system, all exposed-conductive-parts and extraneous-conductive-parts of the installation must be connected to a common earth electrode. The neutral point of the supply system is normally earthed at a point outside the influence area of the installation earth electrode but need not be so. The impedance of the earth fault loop therefore consists mainly in the two earth electrodes (i.e., the source and installation electrodes) in series, so that the magnitude of the earth fault current is generally too small to operate overcurrent relays or fuses, and the use of a residual current operated device is essential.

"BS 7671, 411.5.l Every exposed-conductive-part which is to be protected by a single protective device shall be connected, via the main earthing terminal, to a common earth electrode. However, if two or more protective devices are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device.

• The neutral point or the midpoint of the power supply system shall be earthed.

• 411.5.2 one or more of the following types of protective devices shall be used, the former being preferred:

• (i) An RCD

• (ii) An overcurrent protective device

NOTE 1: An appropriate overcurrent protective device may be used for fault protection provided a Suitably low value of Zs is Permanently and reliably assured."

In simple terms, international regulations recommend the use of an RCD-protected TT earthing system. Let's see why?

Installation without RCD:

In the Case of No fault

Point A is at 230 Volts, Point C and D at 0 Volt potential. Because all potential has been absorbed by the Impedance / resistance of appliance. Very negligible Resistance of the Line and Neutral conductor say (0.5 ohms) in compared to Appliance load of 1 KW drawing 5 amps of current, i.e., 45-ohm.

This is ideal condition or absolutely fine to have point C and D at Physically and electrically at same level.

In the Case of Earth fault

Line conductor is in direct contact of metallic casing of the appliance. Earth Current will flow to the metal case through Circuit breaker to the earth rod then it will travel through the soil to reach Neutral Earth rod. Let's assume we have good earth rod connection with 60-ohm resistance only, anything less than 200-ohm is acceptable depending upon soil conditions (Dry Soil-More resistance) in different weathers.

Total Impedance will be 60.61 ohms in our example. We need to see voltage drop across each point. 

By applying ohms law, first we will get the value of current across the circuit.

=230/60.61 = 3.8A

By applying ohms law, 

3.8 x 0.5          = 1.9 Volt

3.8 x 0.01        = 0.038 Volt

3.8 x 0.1          = 0.38 Volt

3.8 x 60           = 228 Volt

Total = 230 Volt

Each part of circuit will share the voltage as per their proportion of resistance.

Because Soil is the greatest resistance, earth rod is at 228 volts. and so metal case, downstream terminals are at almost same voltage level.

But current is only 3.8 Amps. It will never trip the circuit breaker. It will only operate in overload current but not in fault current.

Installation with RCD:

In normal working condition without fault, 5 Amps flows along the line and into the appliance and 5 Amps flows back on the neutral. Thats always Good.

Now Appliance develops a fault, as we got in this example, Earth fault of 3.8 Amps current flows through the earth.

So, 5 Amps still flows in the system and 3.8 Amps developed earth fault current as well.

Total of 8.8A current flowing into line conductor.

while returning current split into 5A along neutral and 3.8A through the earth, this means imbalance between line and neutral. So current difference is more than 4 Amps.

And RCD with 0.03Amps Leakage current setting within default time, will definitely trip.

In this scenario, even a 6 Amps of B-type circuit breakers will not trip in this condition of 8.8Amps overall current.

Because In max will be 5x6, 30 Amps. So, 6Amps MCB will not give earth fault protection, only an RCD device can.

It is important to note that RCDs, or residual current devices, provide additional protection against electrical shock by detecting imbalances in the electrical current and quickly disconnecting the circuit. This is especially important in TT systems where the earth leakage may not be sufficient to trip an MCB but could still pose a risk of electrocution. The resistance of the human body is around 1000-2000 ohms and 50-100mA of current can cause death.

Another consideration of Touch Voltage:

411.5.3 Where an RCD is used for fault protection the following conditions shall be fulfilled:
(i) The disconnection time shall be that required by Regulation 411.3.2.2 or 411.3 2.4 and
(ii) Ra x IΔn ≤ 50 V
where:
RA is the sum of the resistances of the earth electrode and the protective conductor connecting it to the
exposed-conductive-parts (in ohms)
IΔn is the rated residual operating current of the RCD.

The requirements of this regulation are met if the earth fault loop impedance of the circuit protected by the RCD meets the requirements of Table 41.5
NOTE 1: Where selectivity between RCDs is necessary refer also to Regulation 536.4.1.4
NOTE 2: Where RA is not known, it may be replaced by Zs

For temporary supplies (to work sites, …) and agricultural and horticultural premises, the value of 50 V is replaced by 25 V.

Example:

The resistance of the earth electrode of substation neutral Rn is 10 Ω.
The resistance of the earth electrode of the installation RA is 20 Ω.
The earth-fault loop current Id = 231 V / (10+20) Ω = 7.7 A.
The fault voltage Uf = Id x RA = 7.7 x 20 = 154 V and therefore dangerous, but for touch voltage of 50 Volts, IΔn ≤ 50/20 = 2.5 A so that a standard 300 mA RCD will operate in about 30 ms without intentional time delay and will clear the fault where a fault voltage exceeding appears on an exposed-conductive-part.

But In case if RA > 200 Ω , IΔn ≤ 50/200 = 0.25 A

This Means, 300 mA tripping device will fail to protect.

So, Earthing resistivity test of Earth Electrode connected to metal case, or body earthing must have resistance less than 200 Ω in all-weather condition.

Note: - ELR with Shunt Trip Circuit Breaker or Audio-Visual Alarm Can also be used as per site condition.

The choice of sensitivity of the residual current device is a function of the resistance RA of the earth electrode for the installation, and is given in Fig.

The upper limit of resistance for an installation earthing electrode which must not be exceeded, for given sensitivity levels of RCDs at UL voltage limits of 50 V and 25 V

Here, Total Maximum Loop impedance, RA = 50/0.03 = 1667 Ω, which can be termed as "ABSOLUTE MAXIMUM LOOP IMPEDANCE".

Considering variation in resistance in different weather conditions, it should always be targeted within 100 Ω , so that touch voltage is under much safer and acceptable situation, 0.03A x 100 Ω = 3 Volts.