TT circuits require a disconnection time of 0.2 seconds

by Steve on May 22, 2009


TT circuits require a disconnection time of 0.2 seconds.  (BS7671 Table 41.1).


How can this be achieved when the Max Loop Imp Tables in BS7671 only deal with 0.4 and 5 seconds?




The note below Table 41.1 states that:

TT disconnection time of 0.2 secs up to 32A & 1 sec over 32A (411.3.2.4) is achieved by:

  1. an Overcurrent Protective Device and

  2. Equipotential Bonding (connected to all extraneous metallic parts).

Then the disconnection times for TN systems can be used ie. 0.4 seconds up to 32A & 5 secs over 32A (411.3.2.3)


On-Site Guide Section 3

Disconnection Times for TT circuits.

The required disconnection times for TT systems can (except in the most exceptional circumstances) only be achieved by protecting every circuit with an RCD.


Source information:

BS7671:2008 411.3.2.2

On-Site Guide Section 3

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