Technical Application Papers No.3: Distribution systems and protection against indirect contact and earth fault

Overview

The paper introduces TT, TN (TN-C, TN-S, TN-C-S) and IT earthing systems, covers the physiological effects of current, and gives touch-voltage conditions for each system. Below 50 V touch voltage the tolerance time is indefinite; at 50 V it is 5 s. The TT condition is RA x Ia <= 50 V (25 V in particular locations), with Ia the 5 s operating current or IΔn for an RCD. At 50 V the maximum earth-electrode resistance by RCD rating is 1666 ohm for 0.03 A, 166 ohm for 0.3 A, 16 ohm for 3 A and 1.6 ohm for 30 A.

The TN condition is Zs x Ia <= U0, where Zs is the fault-loop impedance. Final-circuit disconnection times in TN are 0.8 s for U0 120 V, 0.4 s for 230 V, 0.2 s for 400 V and 0.1 s above 400 V, with up to 5 s for distribution circuits and fixed equipment. The paper also covers how RCDs and the G (earth-fault) function of electronic trip units are applied, and selectivity between earth-fault protections. With function G set at 0.2 x In the 5 s trip current is about 24 A, allowing about 2.1 ohm of earth resistance.

In IT systems the first-fault current flows only through line capacitances, so it need not be disconnected; a second fault must be cleared, and the system then behaves as TT or TN depending on how exposed parts are earthed. Annexes address DC supply systems, direct contact and the neutral/protective conductor. The paper builds the IEC 60364 logic; local-code additions need separate checking.

Key points

1. Below 50 V touch voltage the tolerance time is indefinite; at 50 V it is 5 s.
2. TT condition: RA x Ia <= 50 V (25 V in particular locations); Ia is the 5 s operating current or IΔn for an RCD.
3. Maximum earth-electrode resistance at 50 V: 0.03 A 1666 ohm; 0.3 A 166; 3 A 16; 30 A 1.6 ohm.
4. TN condition: Zs x Ia <= U0, Zs being fault-loop impedance.
5. TN final-circuit disconnection times: 120 V 0.8 s; 230 V 0.4 s; 400 V 0.2 s; above 400 V 0.1 s; up to 5 s for distribution circuits and fixed equipment.
6. G function at 0.2 x In: 5 s trip current about 24 A, allowing about 2.1 ohm earth resistance.
7. In IT systems the first fault flows only through line capacitances and need not be disconnected; a second fault must be cleared.

Using it in design

• Keep the earthing system as a project setting {TN-S, TN-C-S, TT, IT}.
• For each final circuit compute fault-loop impedance and check Zs x Ia <= U0 against the U0-dependent disconnection time (5 s for distribution circuits).
• For TT check RA x IΔn <= 50 V and show the electrode-resistance limits; suggest the G function or an RCD when cable length makes the loop-impedance check fail.
• For IT (for example some critical supplies) require insulation monitoring and second-fault disconnection checks.

Who should read it

Electrical designers choosing earthing systems and checking automatic disconnection in LV distribution.

Related standards

• IEC 60364-4-41
• IEC 60479-1
• IEC 60947-2
• IEC 61008
• IEC 61009

Publication details

Authors

ABB

Publisher

ABB

Year

2006

Document number

1SDC007102G0201 (QT3)

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation