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?States that 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.

Is it possible to use a 30mA RCD sensitivity setting instead of 300mA?

Using a 30mA RCD instead of a 300mA RCD can offer better protection against smaller current leakages to earth. For example, if the touch voltage is 50V and the resistance is 20Ω, the RCD sensitivity (IΔn) should be 2.5A or less. A 30mA RCD has a lower sensitivity threshold than a 300mA RCD, which means it can trip faster in response to even smaller current leakages. This increased sensitivity can improve safety, particularly in situations where lower fault currents can still be dangerous. However, it's important to check your local electrical code for specific requirements and regulations regarding minimum sensitivity settings for RCDs, as well as the potential for nuisance tripping.

The choice of sensitivity for earth leakage protection devices based on the regulations outlined in IEC 60364:

1. Protection from Electric Shock by Direct Contact: Applications: General-purpose power sockets up to 20 A, appliances near water sources, portable outdoor appliances up to 32 A, lighting for exhibitions, outdoor lighting. Recommended Sensitivity: 30 mA. Installation: Typically set up in the final distribution switchboard or as a residual current device (RCD) protecting a circuit or a group of circuits.
2. Protection from Electric Shock by Indirect Contact: Application: The entire power distribution system (except for specific cases with Class II insulation or operating at Safety Extra Low Voltage). Recommended Sensitivity: 100 mA to 3000 mA, depending on the earthing system. Installation: RCDs or residual current circuit breakers are used in various locations, including the incoming feeder, sub distribution board, and main switchboard.
3. Protection from Fire Due to Current Leakage: High-Risk Premises: Explosion-prone areas (BE3), fire-prone areas (BE2), agricultural and horticultural buildings, equipment for fairs, exhibitions, and temporary outdoor installations. Recommended Sensitivity: 300 mA or 500 mA. Installation: RCDs or residual current circuit breakers are used at different points, including the final distribution switchboard, sub distribution board, main switchboard, and protecting individual circuits or groups of circuits within high-risk zones.
4. Special Case (Very High Sensitivity - 10 mA): Application: Certain specific applications, such as healthcare equipment for hospital beds, where there is a risk that someone could sustain a non-dangerous current (10 to 30 mA) without being able to get free. Note: Devices with this very high sensitivity may cause frequent tripping due to natural leakage currents of the installation.

The choice of sensitivity for earth leakage protection devices is determined by the specific function they need to perform, whether it's protecting against direct electric shock, indirect contact, or fire due to current leakage. The sensitivity levels are recommended based on international standards, and they vary depending on the application and risk level. It's essential to select the appropriate sensitivity to ensure the safety of electrical installations and equipment in different scenarios.

The maximum breaking times for terminating circuits in TT Earthing Systems based on the given information:

Maximum Breaking Time for Terminating Circuits (ms):

For TT Earthing System, the maximum breaking time for terminating circuits depends on the network phase/neutral voltage as follows:

• Network Phase/Neutral Voltage: 50...120V Maximum Breaking Time: 300 ms
• Network Phase/Neutral Voltage: 120...230V Maximum Breaking Time: 200 ms
• Network Phase/Neutral Voltage: 230...400V Maximum Breaking Time: 70 ms
• Network Phase/Neutral Voltage: > 400V Maximum Breaking Time: 40 ms

Additional Notes:

• For distribution circuits, a breaking time of no more than 5 seconds is permitted to ensure discrimination with the devices installed on the terminating circuits. However, this time should be reduced to the essential minimum for better safety.

For example, residual current device (RCD) with a sensitivity of 30mA, the maximum response time varies based on the fault current as follows:

• 15 mA: No tripping
• 30 mA: Maximum response time of 300 ms
• 60 mA: Maximum response time of 150 ms
• 150 mA: Maximum response time of 40 ms

Maximum Disconnection Times IET regulation BS 7671 (Table 41.1):

In a TT Earthing System, the maximum disconnection times for different voltage ranges are specified as follows:

• AC 50V < Uo < 120V: Maximum disconnection time of 0.3 seconds.
• AC 120V < Uo < 230V: Maximum disconnection time of 0.2 seconds.
• AC 230V < Uo < 400V: Maximum disconnection time of 0.07 seconds.
• AC Uo > 400V: Maximum disconnection time of 0.04 seconds.

These maximum disconnection times are crucial for ensuring safety and protection against electric shock in TT Earthing Systems, and they depend on the voltage levels involved.