TT system - Practical aspects

From Electrical Installation Guide

Specified maximum disconnecting time

The tripping times of RCDs are generally lower than those required in most national standards; this feature facilitates their use and allows the adoption of an effective selective protection.

IEC 60364-4-41 specifies the maximum operating time of protective devices used in TT system for the protection against indirect contact:

  • For all final circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment, the maximum disconnecting time will not exceed the values indicated in Fig. F13
  • For all other circuits, the maximum disconnection time is fixed to 1 s. This limit enables selectivity between RCDs when installed on distribution circuits.
Fig. F13 – Maximum disconnecting times in TT system for final circuits not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment
Uo[a] (V AC) T (s)
50 < Uo ≤ 120 0.3
120 < Uo ≤ 230 0.2
230 < Uo ≤ 400 0.07
Uo > 400 0.04
  1. ^ Uo is the nominal phase to earth voltage

RCD is a general term for all devices operating on the residual-current principle.

RCCB (Residual Current Circuit Breaker) as defined in IEC 61008 series is a specific class of RCD.

General type and S type (Selective) of IEC 61008 have tripping time/current characteristics as shown in Fig. F14. These characteristics allow a certain degree of selective tripping between the several combinations of ratings and types, as shown later in Coordination of residual current protective devices. Industrial type RCD according to IEC 60947-2 provide more possibilities of selectivity due to their flexibility of time-delaying.

Fig. F14 – Maximum operating time of RCDs (in seconds)
x IΔn 1 2 5 > 5
Domestic Instantaneous 0.3 0.15 0.04 0.04
Type S 0.5 0.2 0.15 0.15
Industrial Instantaneous 0.3 0.15 0.04 0.04
Time-delay : 0.06 s 0.5 0.2 0.15 0.15
Time-delay (other) According to manufacturer

Case of distribution circuits

(see Fig. F15)

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 selectivity to be achieved:

  • At level A: Time-delayed RCD, e.g. “S” type
  • At level B: Instantaneous RCD.
Fig. F15 – Distribution circuits

Case where the exposed conductive parts of an appliance, or group of appliances, are connected to a separate earth electrode

(see Fig. F16)

Fault Protection is provided 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. F16 – Separate earth electrode

Protection when exposed conductive parts are not connected to earth

(see Fig. F17)

(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 fault protection (against indirect contact hazards), and the additional protection against the dangers of contact with live parts (direct contact).

Fig. F17 – Unearthed exposed conductive parts (A)