TT system - Practical aspects
Protection against indirect contact
General case
Protection against indirect contact is assured by RCDs, the sensitivity IΔn of which complies with the condition [math]\displaystyle{ I\Delta n\le\frac{50V}{R_A} }[/math] (1)
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 Figure F28.
| (1) 25 V for work-site installations, agricultural establishments, etc. |
| IΔn | Maximum resistance of the earth electrode | |
| (50 V) | (25 V) | |
| 3 A | 16 Ω | 8 Ω |
| 1 A | 50 Ω | 25 Ω |
| 500 mA | 100 Ω | 50 Ω |
| 300 mA | 166 Ω | 83 Ω |
| 30 mA | 1666 Ω | 833 Ω |
Fig. F28: 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
Case of distribution circuits (see Fig. F29)
IEC 60364-4-41 and a number of national standards recognize a maximum tripping time of 1 second in installation distribution circuits (as opposed to final circuits). This allows a degree of selective discrimination to be achieved:
- At level A: RCD time-delayed, e.g. “S” type
- At level B: RCD instantaneous
Fig. F29: Distribution circuits
Case where the exposed conductive parts of an appliance, or group of appliances, are connected to a separate earth electrode (see Fig. F30)
Protection against indirect contact by a RCD at the circuit-breaker level protecting each group or separately-earthed individual appliance.
In each case, the sensitivity must be compatible with the resistance of the earth electrode concerned.
Fig. F30: Separate earth electrode
High-sensitivity RCDs
(see Fig. F31)
According to IEC 60364-4-41, high sensitivity RCDs ( ≤ 30 mA) must be used for protection of socket outlets with rated current ≤ 20 A in all locations. The use of such RCDs is also recommended in the following cases:
- Socket-outlet circuits in wet locations at all current ratings
- Socket-outlet circuits in temporary installations
- Circuits supplying laundry rooms and swimming pools
- Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs
Fig. F31: Circuit supplying socket-outlets
In high fire risk locations
(see Fig. F32)
RCD protection at the circuit-breaker controlling all supplies to the area at risk is necessary in some locations, and mandatory in many countries.
The sensitivity of the RCD must be ≤ 500 mA, but a 300 mA sensitivity is recommended.
Fig. F32: Fire-risk location
Protection when exposed conductive parts are not connected to earth
(see Fig. F33)
(In the case of an existing installation where the location is dry and provision of an earthing connection is not possible, or in the event that a protective earth wire becomes broken).
RCDs of high sensitivity (≤ 30 mA) will afford both protection against indirect-contact hazards, and the additional protection against the dangers of direct-contact.
Fig. F33: Unearthed exposed conductive parts (A)
